THE ORIGIN AND THE FORMS OF MOUNTAINS
=A mountain defined.=—As ordinarily understood, mountains are
elevations upon the earth’s surface which rise above the general level
of the country. Their summits need not be at great heights above the
sea, but it is essential that they project above the average level
of the surrounding country by at least a quarter of a mile. Lower
elevations are described as hills. On the other hand, the elevation of
a plateau like the “High Plains” of the western United States may be as
much as a mile, but the vast expanse of nearly level surface precludes
the use of the term “mountain.” The word is thus applied to a feature
of the earth and not merely to an elevated tract.
In a collective sense, though more often in the plural form, the
term is properly applied to groups of similar features which have a
common origin in local uplift of the land. The origin of mountains
used in this sense of mountain complexes is thus connected with some
essentially local uplift of the earth’s surface. This may take place
by the processes of folding and superincumbent fault displacement,
by volcanic extravasations or ejections, or by a deeper seated and
essentially hydrostatic elevation of rock beds over molten rock
material.
The existing _forms_ of mountains, as we are to see, are largely shaped
by the erosional processes which are set in operation by the uplift
itself, though often completed long subsequent to it.
=The festoons of mountain arcs.=—From our earliest studies of school
geographies, we have become familiar with the arrangement of the more
important mountains in long chains or systems. Comparatively few
persons have given any further attention to the arrangement of the
chains, though over large areas of the earth’s surface the distribution
of mountain ranges is deeply significant. The map of Asia in particular
presents a series of great sweeping arcs or crescents which are grouped
as though hung upon the map in festoons with knots or vertexes to
separate neighboring groups (Fig. 474, p. 438, and Fig. 472).
[Illustration:
FIG. 472.—The great multiple mountain arc of Sewestan, British India
(after de Saint Martin and Schrader).]
The significance of these mountain groupings in the evolution of the
earth’s surface has been pointed out by the great Viennese geologist
Suess, to whom we are indebted for focusing upon the plan of the
earth an amount of attention which before had been largely given to
the preparation of hypothetical sections of strata which were largely
buried from sight beneath the earth’s surface. Broadly speaking,
the mountain arcs may be said to be grouped about those shields of
older rock which geological studies have shown to be the oldest land
masses upon the globe. Within the northern hemisphere these original
continents are represented by the areas of crystalline rock centered
over Hudson Bay, the Baltic Sea, and an area in northeastern Siberia
known to geologists as Angara Land. In our study of the figure of
the earth (Chapter II) it was found that these shields represent the
truncated angles of the rounded tetrahedral form toward which the
planet is tending (Fig. 3, p. 12).
=Theories of origin of the mountain arcs.=—The mountain arcs, when
studied in detail, are found to be composed of closely folded rock
strata, the flexures of which are generally so overturned that their
axial planes dip toward the center of the arc (Fig. 473). It was the
view of Suess that these arcs are to be explained by a pushing outward
of the rock strata from the center of the arc toward its periphery,
thus causing a wrinkling of the surface strata and an overriding of the
surrounding formations, which upon this hypothesis opposed a greater
resistance to the sliding movement. The folding together of the strata
due to the sliding naturally involves a very considerable diminution
of the surface area presented by the strata (Fig. 22, p. 42). In the
case of the Alpine chains it has been estimated that a flat land area,
four hundred to eight hundred miles across, has by the folding process
been reduced to a width of only about one hundred miles, or from a
fourth to an eighth of its former width.
[Illustration:
FIG. 473.—_a_, diagram to illustrate the Suess’ theory of the origin
of mountain arcs; _b_, the author’s modification of this view.]
The weakness of Professor Suess’ theory lies in the fact that such
compression as it implies is assumed to be due to an _outward_
movement of the relatively small area of the earth’s outer shell
which is included _within_ the arc. It must be obvious that such a
movement, being from a center toward three sides at once, would for
this circumscribed area involve enormous proportionate reduction in
superficial area of the strata and could only result in a hiatus near
the center of the arc. No such gap is to be found, and one would,
moreover, be difficult to account for upon any plausible hypothesis.
On the other hand, the general contraction of the planet as a whole,
involving as it does reduction of surface over large areas, is a
well-recognized fact; and if it be true that the shields formed by the
older continents are less subject to contraction than the remaining
portions of the surface, it is easy to understand why the earth’s outer
skin should be wrinkled by _underfolding_ and thrusting about these
continental margins. The contrast of this view with that of Professor
Suess is expressed in the diagrams of Fig. 473.
[Illustration:
FIG. 474.—Festoons of mountain arcs about the borders of the Pacific
Ocean—Pacific type of coast (based upon Suess).]
We may illustrate this conception by a stretched sheet of rubber cloth
such as is in common use by dentists, upon which a thin layer of hot
Canada balsam has been spread. This substance congeals upon cooling to
near-normal temperatures, and if a small local area of the balsam layer
be chilled and the tension upon the rubber then released, the viscous
balsam of the unchilled portion of the layer is thrown into wrinkles
about the cooled and more resistant areas. These more resistant
portions of the stratum may thus represent the ancient continental
shields of our planet.
=The Atlantic and Pacific coasts contrasted.=—In his studies of
mountain arcs in their relation to the plan of the earth, Professor
Suess has shown how the arrangements of the mountain chains about the
two larger oceans represent two strongly contrasted types. Whereas
about the Pacific margin the mountain arcs are, as it were, strung in
festoons which trend parallel to and are convex toward the coast, or
else lie in fringing garlands of islands in the same attitude (Fig.
474); the mountain chains about the Atlantic become sharply truncated
as they reach the coast, and thus indicate that the basin of this ocean
has been produced by an inthrow or depression between great marginal
displacements in some period subsequent to the formation of the
mountains.
[Illustration:
FIG. 475.—The interrupted system of the Armorican Mountains common to
western Europe and eastern North America (after Arldt).]
Thus the mountain folds of the Appalachian system are in Newfoundland
cut off abruptly at the coast line, and the same beds, similarly
truncated, are encountered again across the expanse of ocean in the
folds at the coast of western Europe (Fig. 475). In discontinuous
remnants this ancient mountain chain may be traced in an east and
west direction across western and central Europe. We have thus here
to do with a single mountain system which extends from central Europe
to northern Alabama, out of which a great link has been taken by the
subsequent sinking in of the basin of the Atlantic Ocean.
[Illustration:
FIG. 476.—Schematic representation of a “zone of diverse displacement”
in the Great Basin of the western United States (after Powell).]
=The block type of mountain.=—The inclusion of most elevations in
mountain chains and arcs is one of the most obvious facts to any one
who has examined world atlases with this subject in mind. Such chains
are almost invariably composed of folded rocks, thus indicating that
erosion has removed great superincumbent masses of strata since the
crustal compression produced the folds at considerable depths below the
then surface.
There are, however, large elevated tracts upon the earth’s surface
which are intersected by deep valleys, but where no arrangement of the
elevated portions within chains or ranges is to be detected. In such
cases the distribution of mountain and valley may bear a resemblance to
a mosaic of disturbed parts which stand at different levels (Fig. 476).
[Illustration: FIG. 477.—Section of an East African block mountain
(after J. W. Gregory).]
Such block mountain districts are to be found in many parts of the
earth’s surface, but notably within the Great Basin of the western
United States, and in the land area which borders the Indian Ocean
upon the west and northwest. In contrast with the mountain arcs, so
strikingly exemplified by the continent of Asia as a whole, its extreme
southwestern portion is made up of an alternation of plateau and
rift valley separated from each other by great displacements. Though
modified to some extent by erosion, the elevations seem generally to
represent the displaced crust blocks which in mutual adjustments have
been left at the highest levels (Fig. 477). The valley of the Jordan,
with the mountains of Lebanon rising above it, is near the northern
extremity of this faulted mountain region (Fig. 434, p. 404), while
the Great Rift valley, crossing east Central Africa, and the many
neighboring rifts to the east and west, are graven in lines so deep
that an observer upon a neighboring planet might perhaps detect them.
It is not necessary in all cases to assume that the block mountains of
a faulted district represent the blocks which in the adjustments were
left the highest. Erosion in the course of time accomplishes marvels of
transformation, and it may result that heavy masses of more resistant
rock eventually project the highest, even though they may represent the
downthrown blocks in the fault mosaic (Fig. 43, p. 60).
[Illustration: FIG. 478.—Tilted crust blocks in the Queantoweap
valley.]
Where in addition to undergoing changes of level the earth blocks
have been tilted, the features long since described from our western
interior basin as “Basin Range structure” are developed. Here the
upper surface of the disturbed earth blocks betrays the evidence of a
definite tilt in some one direction (Fig. 478, and Fig. 431, p. 402).
=Mountains of outflow or upheap.=—An important type of mountain,
generally described as volcanic, may be due either to the outflow of
lava at the earth’s surface, or to accumulations of separated fragments
of lava, first thrown into the air, and then deposited by gravity or
admixed with water as volcanic mud. Such mountains, both before and
after modification by erosion, assume the strikingly characteristic
forms which have been fully discussed in Chapters IX and X. The
dominant types are the lava dome and the puy, the cinder cone, and
the more complex composite cone. Excepting only the surface produced
by the few great fissure eruptions and the semivolcanic mesa type, the
individual mountains of volcanic origin develop features with notably
circular bases.
[Illustration:
FIG. 479.—Pen drawing of the laccolite of the Carriso Mountain by W.
H. Holmes, which shows the jagged surface of the igneous rock core and
the sloping tables which still remain of the roof of sedimentary rocks
(after Cross).]
[Illustration:
FIG. 480.—Map of laccolitic mountains. A portion of the Judith
Mountains, Montana. The intrusive igneous rock is shown in black (after
Weed).]
=Domed mountains of uplift—laccolites.=—At a considerable number
of widely separated localities upon the earth’s surface, mountainous
regions are encountered, the central areas or cores of which are
composed of intrusive igneous rock such as granite, and about this
core the sediments dip away in all directions as though they had once
formed a continuous roof above it and had been forced into this dome by
hydrostatic pressure of the once viscous material beneath (Fig. 152, p.
143, and Figs. 479 and 480). Examples of such domed mountains of uplift
were first described by Gilbert from the Henry Mountains of Utah, but
instances are furnished by many elevated tracts, especially within
the western United States. Such mountains are known as _laccolites_,
but when one margin at least of the igneous core corresponds to a
displacement, the mountain is described as a _bysmalite_ (Fig. 481).
[Illustration: FIG. 481.—Ideal sections of laccolite and bysmalite.]
When subjected to long-continued erosion, the generally fissured
granitic core of the laccolite weathers in a wholly different manner
from the bedded sediments which surround and still in part mount
over it. The former usually presents a more or less jagged surface
which contrasts sharply with the gently sloping tables of the latter
(Fig. 479). About the high granite core of the mountain, the several
strata of the uptilted formations present each a steep slope toward
this higher land, and a gentler slope in the opposite direction.
Such unsymmetrical ridges which surround the mountain area are often
referred to as “hog backs” (plate 12 B). The arrangement of the
strata in the hog backs thus presents an overlapping series like the
shingles upon a roof, except that the overlapping is here from the
bottom instead of the top, and the exposed ends thus face toward the
crest. Unlike a shingle roof the hog backs do not shed the water which
descends to them from the higher levels, but, on the contrary, they
cause it to flow in troughs parallel to the base of the slope except
where outlets are found through them.
=Mountains carved from plateaus.=—In the mountain types thus far
discussed, the local uplifting of the land has itself developed
features which in the aggregate may be referred to as mountains, even
though the characters of the original surface are soon destroyed by
erosive processes of one sort or the other. Erosive processes are,
however, quite competent to produce mountain forms from a featureless
plateau, and particularly through the incision by streams of running
water, the best studied process of mountain sculpture (see Chapters
XI-XIII). This process of throwing valleys about an elevated section
of the earth’s surface, and so carving out mountains, is sometimes
described as _circumvallation_; and if the term “mountain” be applied
in its ordinary sense to describe an individual feature, it is clear
that most mountains have been formed in this way.
To discuss the characteristic shapes of such mountains would be largely
to review the contents of this book, and especially those portions
which discuss the character profiles resulting from the action of each
sculpturing or molding agent. The work of frost and other weathering
agencies, of running water, of mountain and of continental glacier,
would all have to be considered in order to evolve the history of each
mountain.
In addition to discovering the agents which were chiefly responsible
for the shaping of the mountain, we may, further, in many cases
determine at what stage the work of one agent has been succeeded by
that of another, and at least at what stage of its complete cycle of
activity the latest agent is now at work.
[Illustration:
FIG. 482.—The gabled façade so largely developed in desert landscapes
and sharply contrasted with the recurring curves in the landscapes of
humid districts (from a painting of the Grand Cañon of the Colorado by
Moran).]
=The climatic conditions of the mountain sculpture.=—Since the
different geological agencies operate either in a different manner
or with differences in vigor according to the varying climatic
conditions, the mountains of arid regions may in most cases be readily
differentiated from those of the more habitable humid sections of
country. In broad lines these differences may be summed up in the
greater prevalence of the curving line within the landscapes of humid
districts. This may be largely ascribed to the influence of the
mat of vegetation, which protects the rock surface from more rapid
mechanical degeneration, and arrests the sliding movements within
the already loosened rock débris. In place of the reversed curves
of the lines of beauty, so generally observed in the landscapes of
well-watered regions, the desert lands present ever a repetition of the
vertical cliff alternating with a sort of many gabled façade which is
occasionally due to truncation of mountain spurs by the waves of former
lakes, but far more often the outlines of débris cones built up beneath
each prominent joint of the cliff walls (Fig. 482).
=The effect of the resistant stratum.=—In a striking manner mountain
landscapes may disclose the influence of the diversified rock materials
and of the rock structures as well. After prolonged erosion there
is likely to be little correspondence between the positions of the
anticlinal folds and the crests of the higher mountains. Such mountains
are, in fact, much more likely to rise over synclines than upon the
site of anticlines. The traveler who enters the Alps by any of the
several railways, or who journeys by steamer over the beautiful lake
of Lucerne, has a most favorable opportunity to study the position
of the rock folds in the mountain sections that are unrolled in
succession before him. Rarely indeed will he find a definite anticline
in correspondence with a mountain peak, for the layers which are most
resistant have developed the peaks, and it is because the outer layers
of the anticlines open by local tension (see Fig. 26, p. 45) that
they were first cut away by erosion, so that the hard layers within
the synclines are likely to constitute the peaks within the existing
surface.
[Illustration:
FIG. 483.—The Mythen, composed of Jurassic and Cretaceous sediments,
and resting upon softer Tertiary formations. View from a balloon (after
a photograph by C. Schmidt).]
When, as sometimes happens, an older and likewise more resistant bed
has been folded back upon younger and softer formations, an isolated
remnant may be found “unrooted” to its base, upon which it appears as
though floating within a billowy sea of the softer formations (Fig.
483).
=The mark of the rift in the eroded mountains.=—Applying the term
“mountain” in its collective sense for a circumscribed area of uplifted
crust, whether represented to-day by a folded or a faulted complex,
a lava mass, or a granite dome; the period of uplift has marked the
beginning of the activity of sculpturing agencies. By these the mass
is pared down as it is shaped into a more or less intricate design of
component and essentially repeating units. In the vernacular the word
“mountain” is applied to these units into which the larger mountain
mass is subdivided.
[Illustration:
FIG. 484.—The battlement type of erosion mountains. Die Drei Zinnen
(Three Battlements) in the Dolomites (after Marr).]
It has been one of the main objects of this work to point out that the
peculiar shapes of these elementary mountains are each characteristic
of the erosive agents which produced them, and that each surface has
marks which may be recognized in those lines of profile which recur
within the landscape—the character profiles. In the subdivision
of the larger mass—the _genetical_ mountain—to form the numerous
smaller masses—the _erosional_ or _circumvallational_ mountains—there
is disclosed a pattern of fractures which has guided the erosional
agents in their incisional operations (see Chapter XVII). In high
altitudes, where the action of frost is so potent in prying at the
wider fractures, this subdivision of the mass may be revealed by the
sculpturing of squared towers or battlements (Fig. 484).
[Illustration: FIG. 485.—Symmetrically formed low islands repeated in
ranks upon Temagami Lake, Ontario.]
For other examples in which the sculptured surface is largely the
handiwork of a single erosional agent, as over vast areas in the
Canadian wilderness, the revelation of the fracture design is no less
apparent. Here a series of crystalline rocks underlie broad expanses
of territory and are without noteworthy variations of hardness and
almost bare of surface débris. Sculptured beneath a mantling ice sheet,
excavation has naturally been concentrated above the more widely
gaping fissures of the joint-fault system, doubtless already marked out
in the river network which the glacier overrode. The result has been a
division of the surface into a series of low, oval ridges or hummocks,
which over vast areas are repeated with monotonous regularity. Wherever
the lower levels have been flooded, symmetrical low islands of nearly
uniform elevation rise from the expanse of water and may be counted
by thousands. Though the smaller islands have notably regular shore
lines, the larger ones disclose their composition from smaller units
by the breaking of their shores into similar bays spaced with regular
intervals (Fig. 485, and Figs. 243 and 245, p. 229).
The ever repeating fracture design of the earth’s crust is not
restricted to the mountain masses which it has broken up, and the
unity of which it has done so much to conceal. It extends far outside
the margin of these masses, and is in fact common to whole continents
and perhaps even to the planet as a whole. The part played by this
design of fractures in the control of the sculpture of landscapes it
would be hard to overestimate. Through its influence the striking
features molded by one agent have been merged in the contrasted shapes
developed by another. It is the great outline blender in the creation
of nature’s masterpieces of form and color. Thus the lines of this
mysterious fracture network, though stamped in indelible characters
upon our landscapes, are generally lost in the ensemble effect and may
long remain undiscovered. Like a moss-grown inscription upon a slab of
marble, though veiled, it may yet be deciphered; and if the veil be
withdrawn, the runic characters are disclosed, and one of nature’s laws
lies open before us.
READING REFERENCES FOR CHAPTER XXXI
Mountain arcs or festoons:—
ED. SUESS. The Face of the Earth, vol. 2, 1906, pp. 201-207; vol. 4,
1909, pp. 498-542.
Block mountains:—
G. K. GILBERT. Surveys West of the 100th Meridian (Wheeler), vol. 3,
Geology, Washington, 1875, Pt. 1, pp. 19 _et seq._, 48.
J. W. POWELL. Report on the Geology of the Eastern Portion of the
Uinta Mountains and a Region of Country Adjacent thereto, U. S. Geol.
and Geogr. Surv. Ter., II Div. Washington, 1876, pp. 218.
JOHN W. GREGORY. The Great Rift Valley. London, 1896, pp. 422.
Laccolites and bysmalites:—
G. K. GILBERT. Report on the Geology of the Henry Mountains, U. S.
Geol. and Geogr. Surv. Ter., 1877, pp. 18-98.
WHITMAN CROSS. The Laccolitic Mountain Groups of Colorado, Utah, and
Arizona, 14th Ann. Rept. U. S. Geol. Surv., 1895, pp. 157-241, pls.
7-16.
W. H. WEED and L. V. PIRSSON. Geology and Mineral Resources of the
Judith Mountains of Montana, 18th Ann. Rept. U. S. Geol. Surv., Pt.
iii, 1898, pp. 485-556, pl. 75.
W. H. WEED. Geology of the Little Belt Mountains, Montana, etc., 20th
Ann. Rept. U. S. Geol. Surv., Pt. iii, 1900, pp. 387-400.
VERA DE DERWIES. Recherches géologiques et pétrographiques sur les
loccolithes des environs de Piatigorsk (Caucase du Nord). Geneva,
1905, pp. 84, pls. 3.
R. A. DALY. The Mechanics of Igneous Intrusion, Am. Jour. Sci. (4),
vol. 15, 1903, pp. 269-278; vol. 16, 1903, pp. 107-126.
JOSEPH BARRELL. Geology of the Marysville Mining District, Montana. A
study of Igneous Intrusion and Contact Metamorphism. Prof. Pap. 57, U.
S. Geol. Surv., 1907, pp. 151-178.
Climatic condition in relation to land sculpture:—
C. E. DUTTON. Tertiary History of the Grand Canyon District, Mon. 2,
U. S. Geol. Surv., 1882, pp. 264, pls. 42.
APPENDIX A
THE QUICK DETERMINATION OF THE COMMON MINERALS
Before one may gain a knowledge of rocks or the architecture of
their arrangement within the earth’s crust, it is quite essential
that some familiarity should be acquired with the appearance and
properties of the commonest minerals, and particularly those which
enter as essential constituents into the more abundant rocks. To be a
competent mineralogist, one must have a rather extended knowledge both
of inorganic chemistry and of the science of crystallography, which,
fascinating as it is to study, involves some technical knowledge of
mathematics and much laboratory experience. Though necessary to any one
who contemplates making a career as a geologist, this special study is
not essential to a cultural course like the present one. The attempt
will here be made to bring together a body of fact, from the study of
which the student may quickly learn to recognize the commonest minerals
in their usual varieties. The tests he is to apply are mainly physical,
and in place of an elaborate discussion of crystal symmetry, pictures
only can be supplied.
To the beginner the usual textbook of mineralogy is difficult to
read intelligently, for the reason that for each mineral species it
sets before him a catalogue of each physical property in its turn,
with little indication of those data which in the individual case
have special diagnostic value. None the less, however, the student
is advised to consider the several properties of each mineral in a
definite order, and the following may serve as well as any: crystal or
other form, cleavage, fracture, luster, color, streak, transparency,
tenacity, hardness, magnetism, and specific gravity. In endeavoring to
connect the specific values of these properties with individual mineral
species, the chemical composition and the manner of occurrence are
not to be forgotten. It is well for the student to be supplied with a
small pocket lens and with a pocket knife the blade of which has been
magnetized.
=Crystal form.=—Some mineral species generally occur in more or less
definite crystals—are bounded by definite plane surfaces developed
when the mineral was formed; others in groups of interfering crystals
or aggregates, in which case the mineral is said to be crystalline;
while still others are rarely found crystallized at all. Thus in
a given case crystal form may, or may not, be important for the
diagnosis of the substance. If a mineral species is usually to be
found in crystals, the student should be aware of the fact, and if
possible should have a mental picture of the common crystal shape or
shapes. Without an extended knowledge of crystallography, this must be
supplied him by drawings. Since crystals of most species are apt to be
distorted, owing to the fact that some planes within the same group
appear upon the crystal with a larger development than others, it is
convenient to remember that markings, such as lines or etchings upon
the crystal faces, are the same throughout the same group of planes,
and in the text figures such groups of planes are indicated by the use
of a common letter. For crystalline aggregates such terms as fibrous,
radiating, massive, or granular have their usual meanings.
=Cleavage.=—It is characteristic of most crystals that they break
or _cleave_ along certain directions so as to leave plane or nearly
plane surfaces, and the luster of the cleaved surface measures the
perfection of the cleavage property. It is important always to note
how many such directions of cleavage are present, and, roughly at
least, at what angles they intersect—whether they are perpendicular
to each other or inclined at some other angle. Further, it should be
noted whether a given cleavage is _perfect_, that is, easy, which will
be indicated by the thinness of the plates which can be secured. An
extremely perfect cleavage is possessed by the mineral mica, whose
plates are thinner than the thinnest paper. In the case of imperfect or
interrupted cleavage, the fracture surfaces are not plane throughout,
but interrupted, the surface “jumping” from one plane to a neighboring
parallel one. It is especially important to note whether, in the case
of several cleavages possessed by a crystal, all have the same degree
of perfection, or whether they exhibit differences.
=Fracture.=—In minerals with poorly developed cleavage, the fracture
surface is described as _fracture_. Fracture is thus perfect in
proportion as cleavage is imperfect. The fracture is described as
conchoidal when it shows waving spherical surfaces like broken glass.
For fine aggregates the fracture is described as even, uneven, earthy,
etc., names which are generally intelligible.
=Luster.=—This term is applied especially to the manner in which light
is reflected from mineral surfaces. The most important distinction is
made between those minerals which have a _metallic_ luster and those
which have not, the former being always opaque. Other characteristic
lusters are adamantine (like oiled glass), vitreous (glassy), resinous,
waxy, etc.
=Color.=—For minerals which possess metallic luster the color is
always practically the same, and hence it becomes a valuable diagnostic
property. Of minerals which have nonmetallic luster, the color may be
always the same and hence characteristic, but in the case of many
minerals it ranges between wide limits and sometimes runs almost the
entire gamut of hues, yet without appreciable changes in the chemical
composition of the mineral.
=Streak.=—This term is applied to the color of the mineral powder, and
is usually fairly constant, even when the surface color of different
specimens may vary within wide limits. In the case of fairly soft
minerals the streak is best examined by making a mark on a piece of
unglazed porcelain (streak stone).
=Transparency= (=diaphaneity=).—The terms “transparent”,
“translucent”, “subtranslucent”, and “opaque” are used to describe
decreasing grades of permeability by light rays. Through transparent
bodies print may be read, while translucent bodies allow the light to
be transmitted in considerable quantity through them, though without
rendering the image of objects.
=Tenacity.=—This comprehensive term includes such properties as
brittleness, flexibility, elasticity, malleability, etc.
=Hardness.=—Quite erroneous notions are held concerning the meaning
of this very common word, which properly implies a resistance offered
to abrasion. It is one of the most valuable properties for the quick
determination of minerals, since minerals range from diamond upon the
one hand—the hardest of substances—to talc and graphite, which are
so soft as to be deeply scratched by the thumb nail. For practical
purposes it is sufficient to make use of a rough scale of hardness made
up from common or well-known minerals. If we exclude the gem minerals,
this scale need include but seven numbers, which are: talc, 1; gypsum,
2; calcite, 3; fluor spar, 4; apatite, 5; feldspar, 6; and quartz, 7.
A given mineral is softer than a mineral in the scale when it can be
visibly scratched by a scale mineral, but will not leave a scratch when
the conditions are reversed. If each will scratch the other with equal
readiness, the two minerals have the same hardness.
Since it may often be desirable to test mineral hardness when no scale
is at hand, the following substitutes may be made use of: 1, greasy
feel and easily scratched by the thumb nail; 2, takes a scratch from
the thumb nail, but much less readily; 3, scratched by a copper coin
and very easily by a pocket knife; 4, scratched without difficulty by
a knife; 5, scratched with difficulty by a knife, but easily by window
glass; 6, scratched by window glass; 7, scratches window glass with
readiness, but a grain of sand may be substituted to represent quartz
in the scale.
=Magnetism.=—Though nearly all minerals which contain important
quantities of the elements iron, cobalt, or nickel may be attracted to
a strong electromagnet, there are but two common minerals, and these
of widely different appearance, whose powder is lifted by a common
magnet. Others are, however, lifted after strong heating in the air
(_ignition_), and this is a valuable test.
=Specific gravity.=—Rough tests of relative weight, or specific
gravity, may be made by lifting fair-sized specimens in the hand.
Better determinations require the use of a spring balance.
=Treatment with acid.=—The carbonate minerals react with warm and
dilute mineral acid so as to give a boiling effect (effervescence),
since carbonic acid gas escapes into the air in the process.
PROPERTIES OF THE COMMON MINERALS
The more important common minerals fall into two classes according as
they have large economic importance as ores, or enter in an important
way into the composition of rocks.
I. The Minerals of Economic Importance
=Hematite.=—The sesquioxide of iron, Fe_{2}O_{3}, and by far the most
important ore of iron. Rarely in good crystals, but sometimes in thin
opaque scales bearing some resemblance to mica and known as micaceous
or specular iron ore. At other times in nodules built up from radial
needles (needle ore); in hard masses mixed with fine quartz grains
(hard hematite); or in soft reddish brown earth (soft hematite).
Color, black to cherry red. The powdered mineral always cherry red or
reddish brown, and easily lifted by the magnet after ignition. Hardness
5.5-6.5; specific gravity 5.
=Magnetite.=—The magnetic oxide of iron, Fe_{3}O_{4}, often in
crystals like Fig. 486, ^{1-2}. Black and opaque with a metallic
luster. Streak black. Lifted by a magnet and sometimes itself capable
of lifting filings of soft iron (lodestone). Hardness 5.5-6.5. Specific
gravity 5.
=Limonite.=—The most abundant and most valuable of the hydrated iron
ores, 2 Fe_{2}O_{3}. 3 H_{2}O. Chemical composition the same as iron
rust, with which in the earthy form it is identical. Never in crystals,
but often in mammillary or rounded pendant forms resembling icicles,
or sometimes clusters of grapes. Its yellow (rust) streak is its best
diagnostic property. Ignited it gives off water and becomes magnetic.
The streak and its notably lower specific gravity distinguish it from
certain forms of hematite which it outwardly resembles. Hardness 5-5.5.
Specific gravity 3.6-4.
=Pyrite, iron pyrites, or “fool’s gold.”=—The sulphide of iron,
FeS_{2}. The most widely distributed sulphide mineral and now a chief
source of the great chemical reagent, sulphuric acid or vitriol.
Often, but not always, in crystals (Fig. 486, ^{3-5}) which have
peculiar striæ upon their faces. At other times the mineral is found
massive or in radiated needles. Bright metallic luster with the color
of new brass, though often tarnished or altered upon the surface to
limonite. Hard and brittle, and so distinguished from gold, which is
soft and malleable and of the color of the paler old brass (which
contained a larger percentage of zinc). Gold is, further, about four
times as heavy as pyrite. Hardness 6-6.5. Specific gravity 5.
=Chalcopyrite, copper pyrites.=—A mixed sulphide of copper and iron.
If in crystals, like Fig. 486, ^6; otherwise massive or compact.
Luster metallic. Color orange-yellow, often with local blue and green
iridescence like a pigeon’s throat. Distinguished from pyrite by the
deeper color and lower hardness, and from gold, particularly, by its
brittleness and lower specific gravity. Hardness 3.5-4. Specific
gravity 4.
=Galenite, galena.=—Sulphide of lead, PbS. The chief ore of lead, and,
from admixture of a silver mineral, of silver as well. Usually found in
crystals (Fig. 486, ^7). Always cleaves into blocks bounded by six very
perfect rectangular faces which, when freshly broken, show a bright
silvery luster and quickly tarnish to a peculiarly “leaden” surface.
Very heavy. Color and streak lead-gray. Hardness 2.5. Specific gravity
7.5.
=Sphalerite, zinc blende.=—Sulphide of zinc, ZnS, usually with
considerable admixture of sulphide of iron. The great ore of zinc.
Not infrequently in crystals (Fig. 486, ^{8-9}), but more often in
cleavable crystalline aggregates. The cleavage in fine aggregates is
sometimes difficult to make out, but in coarse-grained masses it is
seen to be equally and highly perfect in six different directions,
so that a symmetrical twelve-faced form may sometimes be broken out
(dodecahedron). Luster like that of rosin (rosin jack), though when
with large iron admixture the color may approach black (black jack).
The lighter colored varieties are translucent. Hardness 3.5-4. Specific
gravity 4.
=Malachite.=—Hydrated (basic) copper carbonate. The green copper ore
and the common surface alteration product of other copper minerals.
Usually has a microscopic structure made up of fine needle-like
crystals, but generally massive in various imitative shapes not unlike
those of the iron ores. Sometimes earthy. Its color is bright green,
and it is usually found in association with other characteristic copper
ores, such as chalcopyrite and azurite. When relatively pure and in
large masses, it is a beautiful ornamental stone. Effervesces with
acid. Hardness 3.5-4. Specific gravity 4.
[Illustration: FIG. 486.—Forms of Crystals: 1-2, magnetite; 3-5,
pyrite; 6, chalcopyrite; 7, galenite; 8-9, sphalerite; 10-13, calcite.]
=Azurite.=—Hydrated (basic) copper carbonate, less hydrated than
malachite, and known as the blue carbonate of copper. Generally in very
minute and quite complex crystals, but also in imitative shapes similar
to those of malachite, and at other times earthy. Slightly lighter in
weight than malachite, from which it is easily distinguished, as from
most other minerals, by its bright azure blue color and its somewhat
lighter blue streak. Effervesces with nitric acid. Hardness 3.5-4.
Specific gravity 3.7-3.8.
=Calcite.=—Calcium carbonate, CaCO_{3}. Almost always in crystals
(Fig. 486, ^{10-13}), or in confused crystal aggregates, though
rarely fibrous or dull and earthy. Some of the forms of the crystals
are described as “dog-tooth spar”, others as “nail-head spar”, while
still others are modified hexagonal prisms. There is a beautifully
perfect cleavage of the mineral along three directions which make
angles of about 105° with each other, so that under the hammer the
substance breaks into blocks which are shaped like the crystal of Fig.
486, ^{10}. Usually white or gray, but occasionally faintly tinted.
Streak white. Effervesces with cold and dilute mineral acids. An
associate of many ores and the chief mineral of limestone. A similar
mineral—dolomite—contains in addition magnesium carbonate, has
simpler crystals (like the drawing of Fig. 486, ^{10}, but often with
rounded faces), and effervesces only when the acid is warmed. Hardness
3. Specific gravity 2.7.
=Gypsum.=—Hydrated calcium sulphate, CaSO_{4}.2 H_{2}O, and the
source of plaster of Paris. Often in simple crystals (Fig. 487, ^1)
or else “swallow tail”, like Fig. 487, ^2; in which case the mineral
is generally either transparent or translucent and is described as
selenite. Such crystals show a cleavage approaching in perfection that
of the micas, but, unlike the mica laminæ, those produced by cleavage
in gypsum though flexible are not elastic. There are also fibrous forms
of gypsum (satin spar), a fine-grained form (alabaster), and the impure
earthy form (rock gypsum). Very soft, light in weight, and difficultly
fusible. Color usually white, gray, or pale yellow. Hardness 2.
Specific gravity 2.3.
=Copper glance.=—A sulphide of copper, Cu_{2}S. Not usually well
crystallized, but generally massive and associated or variously admixed
with other copper ores such as chalcopyrite, malachite, etc. Fracture
conchoidal, luster metallic, color and streak blackish lead-gray,
though often tarnished blue or green from surface alterations to the
copper carbonates. Softer and heavier than chalcopyrite. Blowpipe or
chemical tests are necessary for its identification. Hardness 2.5-3.
Specific gravity 5.5-5.8.
=Cerussite.=—The white or carbonate lead ore, PbCO_{3}, and an
important ore of silver as well. Often in crystals of considerable
complexity, though Fig. 487, ^{3-4}, shows some common shapes. Often
granular, massive, or earthy (gray carbonate ore). Very brittle and
with conchoidal fracture. The luster is adamantine or like that of
oiled glass. Color generally white or gray. Very heavy, the heaviest
of light colored and nonmetallic minerals. Dissolves in nitric acid
with effervescence. Hardness 3-3.5. Specific gravity 6.5.
=Siderite.=—The carbonate or “spathic” ore of iron, FeCO_{3}. Either
in crystals resembling in form Fig. 486, ^{10}, but with rounded faces,
or cleavable massive to finely granular and earthy. The crystalline
varieties cleave easily into smaller blocks of the same form as those
of calcite. Color usually gray or brown and streak white. On strongly
igniting, the white powder becomes black and magnetic. Lighter in both
color and weight than the other iron ores, and unlike them siderite
effervesces with acid. Distinguished from calcite by its higher
specific gravity and its change upon being ignited. Hardness 3.5-4.
Specific gravity 3.9.
=Smithsonite.=—Carbonate of zinc, ZnCO_{3}, and an important ore of
that metal. Seldom found in crystals except as a replacement of calcite
crystals, in which case it shows the forms characteristic of the latter
mineral. Usually kidney-shaped, stalactitic, or else in incrustations
upon other minerals. Sometimes granular or earthy. Brittle. Luster
vitreous, color white or greenish gray, though often stained yellow
with iron rust. Streak white except when the mineral is stained with
iron. Effervesces with warm acid. Hardness 5. Specific gravity 4.4.
=Pyrolusite.=—Black oxide of manganese, MnO_{2}, though generally
impure from admixture with other manganese oxides. Usually in intricate
aggregates which may be columnar, fibrous, mammillary, earthy, etc.
Opaque, with color and streak both black. Soft and easily soils
the fingers. With hydrochloric acid gives off the choking fumes of
chlorine. Hardness 2-2.5. Specific gravity 4.8.
II. The Minerals important as Rock Makers
These minerals are in most cases complex silicates of one or more of
a certain number of metals such as aluminium, calcium, magnesium,
iron, sodium, potassium, or hydroxyl (OH). For their identification an
examination of the physical properties is usually sufficient, whereas
of the typical ore minerals already considered, additional chemical
tests may be necessary.
=Feldspars.=—A group of similar alumino-silicates of potassium,
sodium, and calcium. The most important of all rock-making minerals.
Although with wide variation in chemical composition, the feldspars are
yet broadly divided into two classes; the one striated, and the other
an unstriated potash or orthoclase variety. The pocket lens is usually
necessary in order to make out the striations upon the crystal or
cleavage surfaces. When formed in veins, feldspar appears in crystals
(Fig. 487, ^{5-6}), but as a rock constituent the mutual interference
of crystals prevents the development of bounding faces. Two cleavage
directions, nearly or quite perpendicular to each other, are notably
different in their perfection. Hard enough to scratch glass, but easily
scratched by sand. Color pink (usually orthoclase or microline), white
(often albite) to gray. Sometimes with beautiful “pigeon’s throat”
effect of iridescence (labradorite). Low specific gravity. Hardness 6.
Specific gravity 2.5-2.8.
[Illustration:
FIG. 487.—Forms of Crystals: 1-2, gypsum; 3-4, cerussite; 5-6,
feldspar; 7, quartz; 8, pyroxene (cross section); 9, hornblende
(cross section); 10, garnet; 11, nephelite; 12-14, staurolite; 15-16,
tourmaline (cross sections); 17, olivine.]
=Quartz.=—Oxide of silicon or silica, SiO_{2}. Both an important vein
mineral associated with the ores and a rock maker. In the former case
particularly, often in crystals of notably simple forms (Fig. 487, ^7).
Few minerals which are not gems are so hard. Remarkable freedom from
cleavage so that the mineral breaks much like window glass—conchoidal
fracture. Wide range in both transparency and color. Transparent and
colorless crystalline variety (rock crystal), brown translucent (smoky
quartz), turbid white (milky quartz), and various colored varieties
(carnelian, jasper, jet, etc.). Insoluble in acids and infusible.
Hardness 7. Specific gravity 2.6.
=Micas.=—Like the feldspars a group of complex silicates, but here
chiefly of potassium, magnesium, iron, and hydroxyl. Abundant as rock
makers, the micas are all characterized by the thinnest and toughest of
elastic cleavage plates, such as are generally known as isinglass. When
a needle is driven sharply through a thin scale of mica, a six-rayed
puncture star forms about the needle point. The darker common variety
of mica is rich in iron and magnesium and is called biotite, and the
lighter colored alkaline variety, muscovite. Hardness 2.5-3.1. Specific
gravity 2.7-3.1.
=Chlorite.=—Generally an intricate mixture of more or less similar
microscopic crystals having varying and rather complex chemical
compositions and related to the micas, but all characterized by
a peculiar leaf green color. These minerals are a common product
of hydration weathering in rocks which are rich in magnesium and
iron—especially those that contain biotite, pyroxene, or hornblende
(see below). Hardness 1-2.5. Specific gravity 2.5-3.
=Pyroxenes.=—An important group of related rock-making minerals all
of which are silicates of the bases magnesium, calcium, aluminium,
iron, and manganese. Quite generally developed either in columnar or
needle-like crystals which are uniformly shaped in cross section like
Fig. 487, ^8. Two rather imperfect cleavages are directed parallel
to the longer axis of the crystal and nearly at right angles to each
other. The colors of all but the lime varieties are dark and generally
green, dark brown, bronze, or black. The lime varieties are white,
gray, or pale green. A dark colored and common iron variety is known as
augite. Streak generally either white or lightly tinted. Hardness 5-6.
Specific gravity 3.2-3.6.
=Amphiboles.=—A group of minerals of the same chemical composition
as the pyroxenes, with which also in most physical properties they
agree. The principal distinction is found in the shape of the cross
section and in the cleavage (Fig. 487, ^9). Whereas the cross sections
of pyroxenes are generally eight sided, those of the amphiboles have
six sides, and whereas the cleavage directions of pyroxenes are nearly
at right angles to each other (87°), the similar but much more perfect
cleavage directions of the amphiboles are inclined at an obtuse angle
(124½°). Owing to the obliquity of the amphibole cleavage, fractured
surfaces of the mineral appear splintery, which is not in the same
measure true of the pyroxenes. A fibrous variety of amphibole, and
occasionally other varieties of the mineral, is a not uncommon product
of weathering of pyroxenes. Other physical properties of the amphiboles
are in the main almost identical with those of the pyroxenes.
=Garnet.=—Complex alumino-silicates or ferro-silicates of calcium,
magnesium, iron, or manganese, or several of these combined. Nearly
always in crystals, and usually found in mica schist (see below).
The crystals usually have twelve similar faces, each a lozenge
(dodecahedron), or else twenty-four similar faces, or the two forms
combined (Fig. 487, ^{10}). Brittle. From any but the gem minerals garnet
is easily distinguished by its hardness, which in different varieties
ranges from somewhat below to somewhat above that of quartz. The luster
is vitreous, and the color runs the gamut of reds, browns, and greens,
but with the common hue dark red to black. Streak white. Hardness
6.5-7.5. Specific gravity 3.1-4.3.
=Nephelite= (=nephelene=).—An alumino-silicate of sodium and
potassium. In certain special provinces this mineral is developed in
abundance as an essential constituent of igneous rocks, but elsewhere
practically unknown. The rare crystals are hexagonal prisms (Fig.
487, ^{11}), but the mineral is most easily determined by its general
resemblance to feldspar, but with the differences of cleavage, luster,
and reaction with acid. Whereas the feldspars have two cleavages,
either nearly or quite perpendicular to each other and of different
degrees of perfection, nephelite has three equal cleavages inclined
60° and 120° to each other and of less perfection than either feldspar
cleavage. The luster of nephelite is perhaps the best clew to its
identity, since this is greasy and simulated by but few minerals.
The fine powder of the mineral treated for some time with strong
hydrochloric acid forms a perfect jelly of silicic acid, whereas
the feldspars do not. Though itself gray or white and unobtrusive,
nephelite is usually associated with brightly colored minerals, which
are often the first clew to its presence in a rock. Hardness 5.5-6.
Specific gravity 2.5-2.6.
=Talc= (=soapstone=).—A silicate of magnesium and hydroxyl which is
an important alteration product through weathering of certain pyroxene
rocks especially. Usually a foliated mass, this product is occasionally
fibrous or even granular. Talc is one of the softest of minerals,
having a greasy feel and being easily scratched with the thumb nail.
The luster of the foliated varieties is apt to be pearly, and the
color apple-green to white, though sometimes stained brown from oxide
of iron. The streak of the mineral is white except when stained by
iron. Although the rocks which are composed mainly of talc (soapstone)
are exceedingly soft, they are very tough and remarkably resistant.
Hardness 1-1.5. Specific gravity 2.7-2.8.
=Serpentine.=—Like talc, serpentine is a silicate of magnesium and
hydroxyl, and an important product of the breaking down of magnesium
minerals in the process of weathering. The mineral is usually found
as a fine web of microscopic needle-like fibers, and is best roughly
diagnosed by its color and its associated minerals. Like talc it is
usually developed within those igneous rocks from which feldspar is
lacking, but where either pyroxene or olivine is found in abundance or
was previous to alteration. The characteristic color of serpentine is
leek-green. The rock largely composed of serpentine is called by the
same name, and being exceedingly tough and unchanging is, in spite of
its softness, a valuable building and ornamental stone. A red magnesium
garnet is apt to be associated with such serpentine masses. Hardness
2.5-4, because of impurities. Specific gravity 2.5-2.6.
=Staurolite.=—A silicate of aluminium, iron, and hydroxyl. Found
in metamorphic rocks usually in association with garnet. Always in
crystals bounded by simple forms generally crossed, as shown in Fig.
487, ^{12-14}. The color is dark reddish brown, and the streak is
colorless to grayish. The hardness is exceptional and higher than that
of quartz. Hardness 7-7.5. Specific gravity 3.6-3.7.
=Tourmaline.=—An exceptionally complex silicate of boron and aluminium
as well as iron, magnesium, and the alkalies. Found in metamorphic
rocks and always crystallized. The crystals are columns or needles
whose cross section is the best guide to their identity, since this
is a modified triangle unlike that of any other mineral (Fig. 487,
^{15-16}). Additional diagnostic properties are the characteristic
striations which run lengthwise of the crystals upon prism faces, and
the lack of any cleavage (difference from hornblende). The hardness is
also a valuable property, since this is greater than that of quartz.
The mineral is brittle and the fracture subconchoidal. The range in
color is as great as, or greater than, that of garnet, though the
common forms are jet black. Streak uncolored. Hardness 7-7.5. Specific
gravity 3-3.2.
=Olivine.=—A silicate of magnesium and iron and a rock-making mineral
found only in those igneous rocks which have little or no feldspar. It
easily suffers alteration by weathering and passes into serpentine,
and in fact is seldom found except when at least partially altered to
the fibrous webs of that mineral. The form of the unaltered crystals
within the rocks is shown in Fig. 487, ^{17}, and, cut in sections, the
mineral appears in more or less elongated hexagons. The hardness of
the unaltered mineral is about that of quartz. It has rather imperfect
cleavages in two rectangular directions, and is usually translucent,
with a vitreous luster and a color which is olive-green when not
stained brown by oxide of iron. Streak uncolored. Hardness 6.5-7.
Specific gravity 3.2-3.3.
APPENDIX B
SHORT DESCRIPTIONS OF SOME COMMON ROCKS
In Chapter IV the classification and the structure of rocks have been
briefly discussed. Below are added brief descriptions of the more
important common rocks. For rocks as for minerals it is, however,
essential that a collection of well-chosen specimens be studied for
purposes of comparison. A small pocket lens is a valuable aid in making
out the component minerals and the textures of the finer grained rocks.
1. Intrusive Rocks
=Granite.=—Of granitic texture, though sometimes porphyritic as well.
The most abundant mineral constituent is a pink or white feldspar,
usually without visible striations, with which there is usually in
subordinate quantity a white striated feldspar. Next in importance to
the feldspar is quartz, which because of its lack of cleavage shows a
peculiar gray surface resembling wet sugar. In addition to feldspar
and quartz there is generally, though not universally, a dark colored
mineral, either mica or hornblende. The mica is usually biotite, though
often associated with muscovite.
=Syenite.=—Like granite, but without quartz, with more striated
feldspar, and generally also the rock has a darker average tint.
While biotite is the commonest dark colored constituent of granite,
hornblende is more apt to take its place in syenite. Less common than
granite, to which it is closely related in origin and in composition.
=Gabbro.=—A dark colored rock of granitic texture composed of striated
feldspar with broad cleavage surfaces and usually an abundance of
pyroxene. In contrast to the feldspars of granite, those of gabbroes
are often dull and colored grayish yellow or greenish. The pyroxene
is often in part changed to fibrous amphibole. Magnetite may be an
abundant accessory mineral.
=Diabase.=—In color dark like gabbro, and of similar constitution. In
diabase, however, the feldspar crystals, instead of being broad and of
irregularly interrupted outline, are relatively long (“lath-shaped”),
and the pyroxene acts as a filler of the residual space between them.
=Peridotite.=—A heavy and dark colored rock of granitic texture which
is nearly or quite devoid of feldspar but contains olivine. When
altered, as it generally is, it is largely a mass of serpentine, talc,
and chlorite, surrounding cores, it may be, of still unaltered pyroxene
and olivine. Magnetite is an abundant constituent, and a red garnet is
apt to be present.
2. Extrusive Rocks
=Obsidian.=—A rock glass rich in silica. It is usually black and
breaks with a perfect conchoidal fracture. It often passes over through
insensible gradations into pumice, which differs only in its vesicular
structure. As regards chemical composition, obsidian and pumice are not
notably different from rhyolite (below).
=Rhyolite.=—A light colored rock of porphyritic texture, often also
with fluxion or spherulitic textures, or both combined. The porphyritic
appearance is given the rock by large crystals of a glassy, unstriated
feldspar and crystals of quartz. Rhyolite is a very siliceous lava
containing rather more silica than granite, to which of the intrusive
rocks it is most closely related, and from which it differs in its
texture and in the manner of its occurrence in nature. Whereas
granite is found in great batholites, laccolites, and bysmalites,
and consolidated in most cases beneath the earth’s surface, rhyolite
generally occurs in sheets, flows, or dikes, and consolidated either
above or in fissures near to the surface.
=Trachyte.=—Similar to rhyolite, but usually with a peculiar gray
aspect from the greater abundance of feldspar crystals. The rock
is less siliceous than rhyolite, contains no quartz crystals, and
approaches a feldspar in its average composition.
=Andesite.=—Similar to rhyolite in appearance and in origin, but more
basic and correspondingly dark in color. The porphyritic crystals
are of lath-shaped, striated feldspar, with which are associated
crystals of either biotite or hornblende or both. A fluxion texture is
particularly characteristic of this type of extrusive rock.
=Basalt.=—A dark colored or black basic rock of porphyritic texture
which differs but little from diabase. It may show under the lens fine
lath-shaped crystals of striated feldspar associated with crystals
of augite, but more frequently the rock is dense and without visible
mineral constituents. It is particularly likely to occur divided up
into columns six inches to a foot in diameter and known as basaltic
columns. Especially fine examples are known from the Giant’s Causeway
and other localities in the western British Isles.
3. Sedimentary Rocks of Mechanical Origin
=Conglomerate= (“=pudding stone=”).—A rock made up from pebbles which
are cemented together with sand and finer materials. The pebbles are
usually worn by work of the waves upon a shore, and may vary in size
from a pea to large bowlders. They may consist of almost any hard
mineral or rock, though the sand about them is largely quartz.
=Sandstone.=—A rock composed of sand cemented together either by
calcareous, siliceous, or ferruginous materials. Sandstones are
described as friable when their surface grains are easily rubbed off,
or as compact when they are more firmly cemented. Sandstones are often
distinctly banded and are sometimes variously stained with oxide of
iron. Those sandstones which have been formed upon a seacoast are known
as marine sandstones, while those derived from accumulations collected
by the wind in deserts are distinguished as continental deposits.
Sandstones form much thicker formations than conglomerates, the latter
usually constituting a basal layer only of the sandstone formation
(basal conglomerate).
=Shale.=—A consolidated mud stone which is probably the most abundant
rock formation. In large part clay admixed in varying proportions with
extremely fine sandy grains.
4. Sedimentary Rocks of Chemical Precipitation
=Calcareous tufa= (=travertine=).—Not to be confused with tuff,
which is a fragmental extrusive or volcanic rock. Calcareous tufa is
formed when waters which contain carbonic acid gas and lime carbonate
in solution, give off the gas and with it the power to hold the lime
in solution. Such a liberation of the gas may occur when the stream
is dashed into spray above a cascade, and the lime is then deposited
about the site of the falls. Travertine is generally porous and formed
of more or less concentric layers or incrustations. A remarkable
illustration is furnished by the travertine deposits of Tivoli and
other localities near Rome, since here the material supplies a valuable
building stone.
=Oölitic limestone= (=oolite=).—This rock is made up of spherical
nodules and so has the appearance of fish roe. Broken apart, each grain
reveals in its center a core of siliceous sand about which carbonate
of lime has been deposited in concentric layers. It is thought that
waters charged with carbonate of lime, in issuing from a river near
a sea beach, coat the sand grains of the latter with successive thin
films of lime carbonate due to the rhythmic ebb and flow of the tides,
evaporation of the adhering water taking place when the sands are
exposed at low tide.
5. Sedimentary Rocks of Organic Origin
=Limestone.=—A generally white or gray rock composed of carbonate of
lime with varying proportions of clay, silica, and other impurities.
The lime carbonate is usually derived from the hard parts of marine
organisms, and the argillaceous and siliceous impurities from the finer
land-derived sediments which descend with them to the bottom.
=Dolomite= (=dolomitic or magnesium limestone=).—Differs from
limestone in containing varying proportions of the mineral dolomite
(_ante_, p. 455), which is made up of equal parts of calcium and
magnesium carbonates. Difficult to distinguish from limestone unless a
chemical test is made for magnesium, though it may be said in general
that dolomite is less soluble in cold mineral acids.
=Peat.=—An accumulation of decomposed vegetable matter within
small lakes and in lagoons separated from larger ones (_ante_, p.
429). Peat represents the first stage in the formation of coal
from vegetable matter, and differs from the coals by its larger
proportion of contained water. Because of this water its fuel value is
correspondingly small. It is usually dark brown or black and reveals
something of the structure of the plants out of which it was formed.
6. Metamorphic Rocks
=Gneiss.=—A generally more or less banded (gneissic) metamorphic rock
with a mineral constitution similar to granite, and often developed
by metamorphic processes from that rock. It may at other times, by
processes not essentially different, be derived from sedimentary
formations. It usually contains as important constituents unstriated
feldspar and quartz, but in addition it may include a striated
feldspar, biotite, muscovite, or hornblende, or several of these
combined. In proportion as mica or hornblende is abundant, it has a
marked banded texture, but it differs from mica schist (see below) not
only in the presence of its feldspar, but in the smaller proportion
of mica. Biotite gneiss, hornblende gneiss, etc., are terms used to
designate varieties in which one or the other of the dark colored
constituents predominate.
=Mica schist.=—A metamorphic rock without feldspar and mainly composed
of quartz and light colored mica (muscovite). The abundant mica lends
to the rock its characteristic schistose texture, which differs from
the usual gneissic texture. In some cases the mica is wrapped about
the grains of quartz, but at other times it forms a series of almost
continuous membranes separating layers of quartz.
=Sericite schist.=—A variety of schist which is characterized by
an abundance of a peculiar silvery mica rich in the element group
hydroxyl. The mica scales are often microscopic and wrought into an
intricate web with the quartz constituent.
=Talc schist.=—A schist made up largely of talc, but with varying
proportions of quartz, magnetite, etc. From the abundance of the talc
it is usually pale green or white.
=Chlorite schist.=—A greenish, fine-grained metamorphic rock in which
chlorite is the principal mineral, but in which magnetite is a quite
characteristic accessory constituent.
=Staurolitic garnetiferous mica schist.=—A mica schist in which garnet
and staurolite are so abundant as to be essential constituents.
=Clay slate.=—A metamorphosed mud stone or shale. In the process of
metamorphism the rock has been hardened, given a slaty cleavage, and
innumerable minute scales of mica have developed to produce a silky
luster upon the cleavage faces. The color may be gray, green, purple,
or black.
=Quartzite.=—A metamorphosed sandstone in which the sand grains have
become enlarged by accretion of silica. Whereas a sandstone fractures
about its constituent grains, a break in quartzite is continued through
the grains and the cement alike. In contrast to sandstones, the
quartzites derived from them are usually lighter in color and often
nearly white.
=Marble= (=crystalline limestone=).—The result of metamorphism upon
limestones. Usually white in color but sometimes gray, blue gray, or
yellow, and sometimes variously broken or brecciated and stained with
iron oxide. Effervesces with cold dilute acid.
=Coals.=—Under the head of peat the first stage in the formation of
coals from vegetable matter has been briefly described. Lignite, or
brown coal, represents a further stage and one in which the vegetable
structure is still recognizable. It is usually brownish black or
black in color and contains a considerable proportion of water. With
increased pressure or dynamic metamorphism, further percentages of the
volatile constituents are eliminated, and when from seventy-five to
ninety per cent of carbon remains, the material burns with a yellow
flame and is known as bituminous coal. This is the great fuel for
the production of steam. A continuation of the metamorphic processes
carries off a further proportion of the volatile matter and leaves a
dense, hard, black substance with sometimes as much as ninety-five
per cent of carbon. This is the so-called “hard coal” or anthracite
generally used for fuel in our houses, for which purpose it is so well
adapted because it burns with a production of much heat and almost
without smoke.
APPENDIX C
THE PREPARATION OF TOPOGRAPHICAL MAPS
=Topographical maps a library of physiography.=—For the satisfactory
working out in detail of the geology of any region of complex
structure, an accurate topographical map is prerequisite. This is so
much the more true because nearly all complexly folded or faulted rock
masses are to be found in mountainous, or at least in hilly regions.
The making of the topographical map must, therefore, precede that of
the geological map, and in modern usage the latter is a topographical
and a geological map combined in one.
Within certain narrow limits, predictions concerning the geological
history of a province may often be made by an expert geologist from
examination of an accurate topographical map. Just as in forecasting
the weather upon the basis of the usual weather maps, such predictions
can sometimes be made with entire confidence in their accuracy,
while at other times a guess only may be hazarded. The great value
of the modern topographical map is becoming, however, universally
acknowledged, and every highly civilized nation has either completed
or has in preparation sectional topographical maps of its domain on
such a scale as is warranted by its financial condition and its state
of development. Thus there is now being accumulated a vast library of
geographical and to some extent geological information, of which the
student of geology must be prepared to make use.
=The nature of a contour map.=—More and more the contour map is
replacing the earlier and less scientific methods of representing
topography on the large scale sectional maps, and hence this type only
need here be considered. In the contour map, the relief of the land
is represented by a series of curving lines, each the intersection of
a particular horizontal plane with the land surface, and the several
planes separated by uniform differences of elevation. This altitude
interval is known as the contour interval. Its choice is a matter
of considerable importance, for though regions of relatively simple
topography may be adequately represented upon a map of large contour
interval, say one hundred feet, another district may require an
interval as short as five feet. A contour map with this interval may be
conceived to have been made by flooding the region which it represents
and preparing maps of the shore lines for each rise of five feet of the
water surface, and superimposing the several maps thus derived with
accurate registration one above the other. Wherever the land slopes
are steep, the shore lines of the several maps will be crowded closely
together and give the effect of a relatively dark local shade; where,
upon the other hand, the surface is relatively flat, the several shores
will be widely spaced and the effect will be to produce a white area
upon the map. Thus in contour maps dark tones indicate steep gradients
and pale tones a flatness of surface.
=The selection of scale and contour interval.=—With the use of
the small scale in the contour map, the tones of the map will be
correspondingly dark, though the relative differences in tone will
remain the same. With the use of a closer contour interval the tones
will deepen throughout. The adjustment of scale and contour interval
to any given region is a matter requiring experience in topographical
mapping, and in addition a knowledge of the geological significance of
topographic features. Unfortunately, the element of expense and the
special commercial objects held in view, conspire to select scales
and contour intervals which are often little adapted to the districts
surveyed.
=The method of preparing a topographical map.=—Having fixed upon the
scale and the contour interval which is to be employed, the task of
the topographical surveyor is next to fix accurately the positions and
the elevations of a sufficient number of points to _control_ the map,
and then to hang, as it were, upon these points as attachments the
design represented by the relief. Were the surface of the ground to
be represented by a flexible fabric, the map maker might raise from a
flat base a series of stout posts of the heights and in the positions
which he has determined, and upon these supports arrange the slopes of
the fabric much as drapery is adjusted. The determination of the exact
positions and the elevations of his control stations is, therefore,
a process coldly precise and formal; whereas in the shaping of the
surfaces his attention should be fixed more upon correctly reproducing
the shapes than upon fixing accurately the position of every point.
As a matter of fact, the position of the average point will be most
accurately fixed when the shapes of the features are most clearly
comprehended. To some extent, therefore, the topographer should be
familiar with the geological significance of the earth features which
he is representing.
=Laboratory exercises in the preparation of topographical maps.=—The
principles which underlie the surveyor’s method for preparing a
topographical map may be learned in the laboratory by the use of
models and the simple device shown in plate 24 A and B. To represent
the section of country to be mapped a model in plaster of Paris is
substituted, and this is placed within a rectangular tank to which
locating carriages and altitude gauges are attached that allow the
student to fix the position and the elevation of any point upon the
surface of the model.
┌──────────────────────────────────────────────────────────────────┐
│ PLATE 24. │
│ │
│ [Illustration: _A._ Apparatus for exercise in the preparation of │
│ topographic maps.] │
│ │
│ [Illustration: _B._ The same apparatus in use for testing the │
│ contours of a map.] │
│ │
│ [Illustration: _C._ Modeling apparatus in use.] │
└──────────────────────────────────────────────────────────────────┘
Upon each model the student “locates”, or fixes, the position of a
sufficient number of points for the control of his map, entering upon
an appropriate map base for each position the altitude which was read
from the gauges. Now _with the map always before him_ he “sketches in”
the forms of the surface by means of contour lines. For this purpose
it is often desirable to fix roughly the direction of the steepest
slope at a number of places, and noting the differences in elevation
between control stations, divide up the distance in accordance with the
curves of slope and start the contours at right angles to the slope.
Afterwards such sections are connected by sketching in with the model
always in view for control (Fig. 488).
[Illustration: FIG. 488.—A student’s map prepared from a model by the
use of the contour apparatus represented in plate 24 A.]
=The verification of the map.=—The map prepared, its accuracy may be
tested by a simple method which is denied the topographer who has to
do with the actual surface of the ground. The locating carriages and
altitude gauges are removed from the tank, which is next filled with
water and leveled by means of guide marks upon the interior. A few
drops of milk or of ordinary clothes blueing are added to the water to
render it opaque, and it is then drawn off at the faucet in successive
installments, so that the surface drops by layers corresponding in
thickness to the contour interval of the map, plate 24 B. As each layer
is withdrawn, that contour of the map to which the shore line should
correspond is carefully examined and corrected. By such corrections
the nature of the first errors made is soon appreciated, and the
method of procedure is thus more easily acquired. At the same time the
significance of the design of the map is more quickly learned than by a
mere examination of the standard government maps.
The work above outlined calls for waterproofed models of suitable form
and size, and a series, each of which sets forth some typical feature
or series of features, has been designed by Mr. Irving D. Scott.[2]
=The preparation of physiographic models.=—The apparatus used
to prepare the topographic map is adapted also for preparing a
physiographic model from a standard topographical map. For this purpose
the method is essentially reversed, though the tank is replaced to
advantage by a light metal frame elevated upon one side so as to permit
a free use of the hands in modeling the clay.
The material used in preparing the model is artists’ modeling clay[3]
which has a base of beef suet, and hence does not dry out and crack as
does ordinary clay. Its form is, therefore, retained indefinitely, and
it may be used again and again. Most maps must be enlarged in modeling,
and the simplest way is often to photographically or by pantograph
enlarge the map to the scale of the model. The map prepared, it is
covered by a thin celluloid plate which has cut upon it a series of
crossed lines spaced in inches and larger subdivisions to correspond to
those of the locating carriages (plate 24 C).
The enlargement of the map is not essential to experienced workers,
and the standard map may be covered in similar manner by a transparent
plate with “checkerboard” design, the squares of which bear some simple
relation in size to the larger divisions of the locating carriages
(Plate 24 C, rear).
The method of preparing the model is comparatively simple. Beginning at
any point upon the map, the intersection of a heavy contour line with
one of the guide lines of the celluloid “position plate” is carefully
noted. Both the position and the elevation of this point are fixed by
the point of the altitude gauge of the modeling frame, and the clay
built up beneath it to that height. With the fingers the clay is now
roughly shaped in various directions from this point, the altitude
gauge is advanced by the locating carriage so as to correspond in
position to the intersection of the next heavy contour line with the
same guide line of the position plate, and the elevation for this point
similarly adjusted upon the model. As before, the surface of the clay
is roughly shaped in advance and upon the sides so as to conform to
the indications of the map; and this process is repeated until the
work is finished. Corrections for intermediate positions may be
carried to any desired degree of refinement which the scale and the
accuracy of the map permit. Models which are larger than the area of
the modeling frame are prepared by making a square foot at a time by
the above described process, and then moving the frame forward and
adjusting in a new position by means of the sharp pins in the legs
of the apparatus.
READING REFERENCES
WILLIAM H. HOBBS, New Laboratory Methods for Instruction in Geography,
Journal of Geography, vol. 7, 1909, pp. 97-104. Also Scot. Geogr.
Mag., vol. 24, 1908, pp. 643-652. The Modeling of Physiographic Forms
in the Laboratory, _ibid._, vol. 8, 1910, pp. 225-228.
APPENDIX D
LABORATORY MODELS FOR STUDY IN THE INTERPRETATION OF GEOLOGICAL MAPS
[Illustration: FIG. 489.—Models to represent outcrops of rock.]
The laboratory models which have been described on page 63, and are
used to represent outcrops in the study of geological maps, are shown
in Fig. 489. The drum-shaped blocks serve to represent massive rocks
which occur in irregularly shaped masses such as batholites and flows.
The long, narrow strips are for intrusive rocks in the form of dikes,
while the larger blocks provided with a swivel joint are used for
outcrops of sedimentary rocks, and after adjustment they give the dip
and strike of the exposure. The wing bolts used in their construction
should be of bronze, because of the effect of iron upon the compass.
For the same reason tables should not be placed near iron beams or
columns. All these blocks can be made by an ordinary carpenter, and
should be available in sufficient numbers to arrange problems like
those of Figs. 47, 48, and 490. With a view to supplying suggestions
for other problems of the same general nature, the three additional
field maps of Fig. 491 have been introduced.
[Illustration: FIG. 490.—Special laboratory table set with a problem
in geological mapping which is solved in Figs. 47 and 48.]
[Illustration: FIG. 491.—Three field maps to be used as suggestions in
arranging laboratory tables for problems in the preparation of areal
geological maps.]
The list of questions given below is intended to indicate the nature of
some of the problems which the student should be asked to solve in the
preparation of each map. The numbers in parentheses refer to pages in
this book where further information is given:—
STRATIGRAPHICAL
1. Of the formations represented what ones are sedimentary and what
igneous (Chap. IV, App. B)?
2. Which formations, if any, are separated by unconformities (51-53)?
3. What is the order of age of the sedimentary formations (65)?
4. What are the _exposed_ thicknesses of each of these formations
(48-49)?
5. Do any of these values represent full thickness of the formation,
and if so, which ones?
6. What is the age in terms of the sedimentary formations of each of
the igneous rock masses (65)?
7. Which igneous rocks, if any, occur in batholites (143, 441)? Which,
if any, in dikes (140)?
STRUCTURAL
8. What formations, if any, have monoclinal dip (42)?
9. Indicate upon the map by dashed lines the crests of all anticlines
and the trough lines of synclines.
10. Indicate by arrows the direction of pitch of all plunging
anticlines and synclines wherever disclosed by changes of dip and
strike (43).
11. Indicate the approximate position of all faults whose position
is disclosed (58-61), and, if possible, state which limb is the one
downthrown.
12. Prepare suitable geological sections.
READING REFERENCE
WILLIAM H. HOBBS. Apparatus for Instruction in Geography and
Structural Geology. III. The Interpretation of Geologic Maps. School
Science and Mathematics, vol. 9, 1909, pp. 644-653.
APPENDIX E
SUGGESTED ITINERARIES FOR PILGRIMAGES TO STUDY EARTH FEATURES
The chief value of the laboratory studies discussed in the preceding
appendices is as a preparation for observations made in the field—the
laboratory _par excellence_ of the geologist. The pilgrimages whose
itineraries are here suggested have been planned especially for
impressing by observation the lessons of this book. Such journeys are
best interrupted at a relatively small number of localities which,
because already studied in some detail, are specially adapted to serve
as centers for local excursions. These localities will in most cases
be the great scenic places to which tourists resort, or the seats of
universities near which specially detailed explorations have been often
made.
Within the United States a few local geological guides have been
published, and the Geologic Folios published by the United States
Geological Survey are already available for a number of such centers.
For one long geological pilgrimage we are fortunate in having a
carefully prepared guide, namely, from New York to the Yellowstone
National Park and back, with a side trip to the Grand Cañon of the
Colorado. Except for the side trip this route, in large measure,
corresponds with one here chosen, and for the return journey especially
the student is referred to it for information (Geological Guide Book of
the Rocky Mountain Excursion, edited by Samuel Franklin Emmons. Comte
Rendu de la Congrés Géologique Internationale, 5me Session, Washington,
1891, 1893, pp. 253-487, map and plates 13, figs. 32).
Our journey is begun at New York City, which is built about the deeply
submerged channels of an estuary choked with glacial deposits, though
the channel may be followed as a deep cañon across the continental
shelf to its margin (252,[4] pl. 17 B). New York City is also upon the
margin of the glaciated area, the outer terminal moraine of which is
well represented on Long Island (298). Across the Hudson in New Jersey
is the great Coastal Plain which meets the oldland in a well-defined
margin (159, 246, 247). A local geological guide of the vicinity of the
metropolis has been written by Gratacap (Geology of the City of New
York, Greater New York. Brentanos, New York, 1904, pp. 119, pls. and
map).
Traveling by the New York Central Railway, we follow up the Mohawk
outlet of the glacial lakes Iroquois and Algonquin (334), first
skirting upon the east the great sills of intrusive basalt known as the
Palisades, with their markedly columnar jointing and intersections by
numerous faults. Above Peekskill we enter the picturesque narrows of
the river (174), cut in the hard crystalline rocks of the Highlands.
Entering the Mohawk Valley, we pass Syracuse with limestone caverns
and well-oriented joints widened by solution through the agency of the
descending ground water (181, pl. 6 B). A branch line to the southwest
reaches the vicinity of Cayuga Lake and Ithaca, where are well-oriented
joints which have controlled the drainage directions, and there is also
a typical strath (55, 87, 428).
To Niagara Falls at least a day should be allotted for the “gorge ride”
by trolley car, thus making the complete circuit of the brink of the
gorge with interruptions and local studies at all important points
(352-366, pl. 23 A). From Niagara Falls over the Michigan Central
Railway we reach Detroit on the present outlet of the upper Great Lakes
as well as of the later Lake Algonquin (334). From this city as a
center a trip is made by electric railway to Ypsilanti and Ann Arbor,
across the bottoms of the early glacial lakes from the first Maumee to
Warren (330-333). The strong Whittlesey beach is encountered at the
little station of Ridge Road, and one of the Maumee beaches on Summer
Street in Ypsilanti. The city of Ypsilanti is built upon a terrace
(165) of the Huron River, and another terrace in the same series is
crossed by the electric line. In an excursion of a few miles down the
river, passing meanders (164-165) and ox-bow lakes (165, 415), is
found an interesting case of stream capture near the little village of
Rawsonville (175. See Isaiah Bowman, Jour. Geol., Vol. 12, 1904, pp.
326-334).
Continuing our journey from Ypsilanti over a high moraine (312), Ann
Arbor is reached, built upon the level plain of outwash with fosses
sometimes separating it from the moraine (281, 314). Upon the campus of
the university are great bowlders of jasper conglomerate and jaspilite,
which were transported from the north by the continental glacier (305).
Across the river from the Michigan Central station and behind the
little church is a delta formed in one of the glacial lakes Maumee and
here opened in section (168). West of the city is a great valley which
was the former course of the Huron River when thus diverted by the
continental glacier lying to the eastward of Ann Arbor—border drainage
(see Ann Arbor folio by the U. S. G. S., and, further, R. C. Allen and
I. D. Scott, An Aid to Geological Field Studies in the Vicinity of Ann
Arbor, George Wahr, publisher, Ann Arbor).
Returning to Detroit (M. C. Ry.), the great Sibley quarries in
limestone near Trenton may be visited. They display perfect jointing,
numerous fossils, and especially well-glaciated surfaces interrupted
by deep troughs and showing striæ of several glaciations (304). From
Detroit the journey is continued by steamer to Mackinac Island in the
strait connecting Lakes Michigan and Huron, passing on the way through
the peculiar delta of the St. Clair River (431), and coming in view of
the notched headlands, which are a monument to the post-glacial uplift
of the glaciated area (250, 341). A day is spent at Mackinac Island
and St. Ignace in order to study with some care these uplifted strands
of the late glacial lakes (341-344). Chicago may now be reached either
by steamer or by rail, and in its vicinity we may see the elevated
beaches and the ancient outlet of Lake Chicago (331-332, 347, pl. 22
A. See Chicago Folio, U. S. G. S.). By the Chicago and Northwestern
Railway the area of recessional moraines and intermediate outwash
plains, and later that of the drumlins, are crossed in journeying to
Madison, Wisconsin. By examination of the maps on pages 308 and 317
in connection with the larger scale atlas sheets of the United States
Geological Survey (Janesville, Evansville, and Madison sheets), this
car journey can be made most instructive in gaining familiarity with
the characteristic glacial features, and this study is continued to
special advantage in excursions about Madison as a center (316-317,
407). This is the more true since at numerous localities in the
vicinity of Madison the well-striated glacier pavement is exposed for
comparison of the striæ as regards direction with the axes of the
several types of glacial features.
An especially instructive excursion may be made by carriage in a single
day to the “driftless area” some twelve miles west of the city. Before
reaching it we cross in alternation a series of recessional terminal
moraines (pl. 17 C) and outwash plains, and near Cross Plains encounter
the partially dissected upland with its arborescent drainage and
even sky line (298, 300-301, 312-313, pl. 16 A and B). Typical shore
formations (233, 241, 242) are studied to advantage about Lake Mendota
in a walking trip to and beyond Picnic Point, where are found the best
ice ramparts (431-434. See Buckley, Trans. Wis. Acad. Sci., Vol. 13,
pp. 141-162, pls. 18).
Our journey is now continued over the Chicago and Northwestern Railway
to Devils Lake near Baraboo, where we cross a salient of the driftless
area, within which lies Devils Lake, imprisoned in a former valley of
the Wisconsin River, since diverted to another course as a result of
the glacial invasion (312-313). The valley here is a former narrows in
hard quartzite (466), which towers above the lake in unstable chimneys
(300), such as the Devils Tower, but such remnants are not found on
the other side of the moraine, being there replaced by rounded rock
shoulders. Just north of the lake the marginal moraine which blocks
the valley is so characteristic as to merit special study (pl. 17
C). Only a few miles northward along the railway from Devils Lake is
Ableman, where, exposed in a high cliff, the hard purple quartzite
with beautiful ripple marks to reveal its plane of sedimentation
(pl. 11 A) dips vertically, and is overlain by horizontally bedded
yellow sandstone. The marked angular unconformity which is thus
displayed is further made evident by a basal layer of conglomerate
(463) in the sandstone (51-53). Here also are deposits of loess along
the river, which display their vertical joint surfaces (207). An
excellent geological guide to this interesting district and that of
the neighboring “Dalles” of the Wisconsin River has been written by
Salisbury and Atwood (The Geography of the Region about Devils Lake and
the Dalles of the Wisconsin, etc., Bull. 5, Wis. Geol. and Nat. Hist.
Surv., 1900, pp. 151, pls. 38, figs. 47).
If we have taken a conveyance at Devils Lake for Ableman, we may
continue in the same manner to Kilbourn, where begin the picturesque
Dalles of the Wisconsin River—here a young gorge cut in sandstone,
because the Wisconsin was diverted from its old valley to border
drainage at the edge of the driftless area (300, 321). The side cañons
of the river, through their abrupt zigzags, reveal the control of their
courses by the joint system (224). In the journey up the rapids by
steamer to inspect the Dalles, we observe many beautiful examples of
cross bedding in the sandstone (37).
From Kilbourn we continue our journey to Minneapolis over the Chicago,
Milwaukee, and St. Paul Railway, and near Camp Douglas are over a
peneplain, out of which rise prominent monadnocks (171). At La Crosse
the Mississippi River is reached, flowing beneath bluffs of sandstone
which are capped by loess (207). The meanderings and the numerous
cut-offs of the Mississippi may be observed to the left (415). Lake
Pepin is a side-delta lake blocked by the deposits of the Chippewa
River (419).
From Minneapolis an excursion is made to Fort Snelling to view the
young gorge of the Mississippi, cut by the Falls of St. Anthony for a
distance of about eight miles in manner similar to that of the seven
miles of Niagara gorge (354), and to compare this narrow gorge with
the broad valley of the Warren River which drained Lake Agassiz (327).
Somewhat farther up the Warren River are examples of saucer lakes (416).
From Minneapolis the journey may be continued by the Great Northern
Railway to Livingston, Montana, thus crossing between the stations of
Muscoda and Buffalo the bed of Lake Agassiz and its marginal beaches
(325-328. For local geology of Minnesota consult C. W. Hall, Geology of
Minnesota, Vol. 1, Minneapolis, 1903).
The Yellowstone Park is entered from Livingston (Livingston Geological
Folio, U. S. G. S.) and departure from it made at the relatively
new Union Pacific terminal at the southwest margin. The regular
trip through the Park includes visits to the several geyser basins
(191-194), Obsidian Cliff (33, 463), the Cañon of the Yellowstone, etc.
Good climbers can make a side trip from near the Mammoth Hot Springs
to the top of Quadrant Mountain, the remnant of a “biscuit cut” upland
(372), and there study the nivation process (368, Yellowstone National
Park Folio, U. S. G. S.).
The trip from the Park to Salt Lake City, over the Union Pacific
Railway, passes through the Red Rock Pass, the former outlet of Lake
Bonneville (423), into the desert of the Great Basin (Chaps. XV and
XVI). Great Salt Lake is a saline lake or sink with an interesting
record of climatic changes (198, 401). The front of the Wasatch Range,
in view and easily reached from Salt Lake City, is deeply scored by
the horizontal shore terraces of Lake Bonneville (198, 199), and
these terraces are extended at every reëntrant by barrier beaches of
great perfection. In the Pleistocene period mountain glaciers in part
occupied the valleys of this range, though they did not always extend
as far as the mountain front. Big Cottonwood Cañon, which realizes this
condition, and the neighboring Little Cottonwood Cañon, from whose
front its glacier spread into an expanded foot (264), thus show for
comparison in a single view the V and the low U sections respectively
(172, 376). Here are also alluvial fans (213) and recent faults which
intersect them.
From Salt Lake City the return to New York may be made by the Denver
and Rio Grande Railway across deserts and through the Royal Gorge,
the cañon of the Arkansas River. A full itinerary of the points of
geological interest along this route, and continued to Chicago,
Washington, and New York, is supplied in much detail in the guide of
the geological excursion to the Rocky Mountains above cited. This
the traveling geologist should not fail to study. Some references to
points along this journey will be found on preceding pages of this
book (219-220, High Plains; 170, Allegheny Plateau in West Virginia;
176, water gap of Harper’s Ferry; 176-177, 184-186, side trip up the
Shenandoah Valley to Luray Caverns and Snickers Gap; 251, Chesapeake
Bay).
Instead of returning directly from Salt Lake City, the traveler,
if he has sufficient time at his disposal, may extend his journey
southwestward across the Great Basin to Los Angeles. A branch line
from this route leaves the Vegas Valley and passes within reach of the
famous Death Valley (201) to Tonopah (79) and the Owens Valley (77-78,
92), where are many surface faults dating from the earthquake of 1872
and other less recent disturbances. Returning to the junction point,
the route continues across the Colorado and Mohave deserts to Los
Angeles. From Los Angeles as a center the exceptionally interesting
terraces, caves, and stacks of an uplifted coast are to be seen
to best advantage near Pt. Harford (Chap. XIX). The islands of San
Clemente and Santa Catalina may also be reached from Los Angeles (239,
248, 249, 250, 256, 257, pls. 5 B, 7 A, 12 A). The return to the East,
if made by the Santa Fe Railway, permits of a visit to the Grand Cañon
(174, 443) from the station of Williams. From that point eastward the
geology of the route is fully covered in Emmons’ Guide to the Rocky
Mountain Excursion already cited.
* * * * *
For the benefit of those who are privileged to travel in Europe, and
the number increases yearly, a pilgrimage is suggested which may easily
be made to correspond with plans laid out on the basis of historical,
artistic, and scenic points of interest. The only popular guide of a
general nature written for geologists traveling abroad appears to be
a brief but valuable little paper by Professor Lane (The Geological
Tourist in Europe, Popular Science Monthly, Vol. 33, 1888, pp.
216-229). The publishing house of Gebrüder Bornträger in Berlin is
now publishing a quite valuable series of geological guides dealing
with special districts and written by well-known authorities (Sammlung
Geologischer Führer). Of this series some thirteen numbers have already
been issued. Many other valuable local guides of a geological nature
are the Livrets Guides of the International Geological and Geographical
Congresses, and the similar pamphlets supplied in connection with
annual meetings of national or provincial geological societies.
Passengers on steamships sailing from the harbor of New York pass
out over a deeply submerged cañon (252) largely filled with glacial
deposits, through the Narrows (174), and in sight of Sandy Hook, a
modified spit (238, 240). To the left are seen the great morainic
accumulations at the border of the glaciated area on Long Island (298).
In the course of the trans-Atlantic voyage a much-rounded iceberg may
be encountered (291), though this is much more apt to occur upon the
northern routes from Quebec, and late in the season. Upon entering the
English Channel the land on both coasts rises in steep cliffs, where
are found all the common shore features well developed (Chap. XVIII).
The German steamships pass in sight of Heligoland, that last remnant of
wave erosion (236).
While traveling in Europe, the student should consult a map of the
glaciated area (299), and so learn to recognize its peculiarities, and
carefully mark its marginal moraine (311) and other strongly marked
features.
If the British Isles are visited and the more rugged areas are
selected, one may study the cirques and other characteristic features
due to the presence of mountain glaciers about Snowdon (Chap. XXVI).
More mature stages of the same processes are to be found in the
Scottish Highlands and the Inner Hebrides, but especially upon the
Island of Skye (Fig. 492). A very valuable aid to excursions in
this district is Baddeley’s Scotland (part I, Dulau, London) and
Sir Archibald Geikie’s Explanatory Notes to accompany Bartholomew’s
Geological Map of Scotland (map and notes in cover, Edinburgh, 1892,
pp. 23).
[Illustration: FIG. 492.—Sketch map of Western Scotland and the
Inner Hebrides to show location of some points of special geological
interest.]
It is from Oban, the “Charing Cross of the Highlands”, that one should
start out upon the summer steamers in order to reach both Skye and
Staffa, the latter with fine basaltic columns (463), and Fingal’s
Cave. In sailing to Skye one passes upon either shore of the narrow
fjords many relics left in the dissection of volcanoes (139-143 and
Sir A. Geikie, Ancient Volcanoes of Great Britain, Vol. II); also
rocky islands and skerries marking submergence (252), and the coast
terraces which register a later uplift (250). Skye is a complex of
many intrusive and volcanic rocks of such markedly different colors
as to appear as tints in the landscape. In the Cuchillin Hills of dark
green rises the massive gabbro (462) cut by cirques into the jagged
pinnacles of horns and comb ridges (373); while lower down and to the
east are rounded domes of rhyolite (463) abraded beneath the glaciers
and of a delicate salmon tint. Still lower and to the westward are
flat mesas composed of horizontal layers of black basalt under a rich
carpeting of the brightest verdure. Eastward across the channel are
seen the purplish walls of an ancient sandstone. The jagged gabbro
core of the island thus represents a fretted upland (372) and is now
the training ground of the Alpinist (Abraham, Rock Climbing in Skye,
Longmans, London, 1908), while nestled in one of the bottoms of a
U-valley is Loch Coruisk, a typical rock-basin lake (412), its shores
of hard rock planed and scored.
From Skye we may go to study the remarkable thrusts (45) on the north
shore of Loch Maree, a marked lineament, and one directed at right
angles to that on the course of the Caledonian Canal connecting
Loch Linne with Loch Ness. This northeast wall of Loch Maree is a
strikingly rectilinear fault represented by an escarpment, up which
we climb to find at the top the crushed and fluted thrust planes of
movement dipping southeastward at a flat angle. Here also are beautiful
rock-basin lakes, lying in hollows molded beneath the continental
glacier. On our way from Skye we have passed up Loch Carron, a sea loch
or fjord (252), and along the strath at its head known as Strathcarron
(428).
Returning now to Oban, it is but a short trip by steamer up Loch Linne
to Fort William along the striking lineament (226) which continues to
Loch Ness and beyond (Fig. 492), and thence by rail to Glen Roy and the
neighboring glens of Lochaber (322-325).
From Paris as a starting point, we may visit in a most picturesque
region the beautifully preserved craters of extinct volcanoes in the
Auvergne of Central France (105, 124, 145), which district is entered
from Clermont-Ferrand. Here are found the characteristic puys, steep
lava domes of viscous lava (105), which figured largely in the early
controversies of geologists concerning the origin of rocks.
[Illustration: FIG. 493.—Outline map of a geological pilgrimage across
the continent of Europe.]
The rest of our pilgrimage will be so planned as to enter the noble
river Rhine at its mouth (Fig. 493), ascend its course to its
birthplace in the snows of Switzerland, and after further exploration
of the features of this fretted upland, traverse northern and central
Italy so as to make our departure for America by the southern route.
Entering then upon this course in the Low Countries, we have first
the opportunity of observing the characteristics of a great delta
with natural levees artificially strengthened as dikes (165-168).
Here also are found dunes of beach material which has been raised by
the wind into a great rampart near the shore (209-211). Such a wall
of dune sand is well displayed at the bathing resort at Scheveningen
near the Hague (421). The flood plain of the Rhine (162-165) may be
studied in a journey up the river to the university town of Bonn, from
whence a day’s excursion should be devoted to the relics of volcanoes
known as the Seven Mountains (H. von Dechen, Geognostischer Führer in
das Siebengebirge, Bonn, 1861). As a preparation for this trip and
others in the volcanic Eifel higher up the river, a visit should be
made to the mineral and rock collections of the Poppelsdorfer Schloss
at the University. In the volcanic Eifel are found some of the most
interesting of crater lakes (405), the largest being Lake Laach
with its somewhat peculiar volcanic ejectamenta and its picturesque
abbey (see von Dechen, Geognostischer Führer zu der Vulkanreihe der
Vorder-Eifel, etc., Bonn, 1886. Consult also Lane, A Geological Tourist
in Europe, _l.c._).
Continuing our course up the river from Bonn, we soon enter the gorge
of the Rhine cut in an uplifted peneplain (169, 171, 174). From
Coblenz, where the Moselle enters the Rhine, a side trip may be made up
this tributary river past Zell with its entrenched meanders (173) to
the ancient Roman city of Treves. Above Bingen on the Rhine we leave
behind us the narrow gorge and rapid current of the river and continue
over the broad floor at the bottom of a rift valley (403), lying
between the forest of Odin and the Black Forest on the east and the
“Blue Alsatian Mountains” far away to the west. At the margins of this
plain are beds of loess with their characteristic joint structures and
inclusions (207), and in the higher hills on either hand a wealth of
intrusive igneous rocks.
At the entrance of the Neckar River to this broad plain is nestled the
picturesque castle and university town of Heidelberg, a convenient
center for excursions (Julius Ruska, Geologische Streifzüge in
Heidelbergs Umgebung, etc., Nägele, Leipzig, 1908, pp. 208, map).
At Strassburg (Schwarzwaldstrasse 12) is located the German Chief
Station for Earthquake Study, with a particularly large set of modern
seismographs. In the university cabinet is also one of the largest and
most representative mineral collections in Europe. For excursions in
the neighborhood consult Benecke, Sammlung Geognostische Führer, Vol.
5, Elsass, 1900.
From Strassburg we may go by the Black Forest Railway to the Hegau with
its volcanic plugs (140), each surmounted by a picturesque castle.
We enter next the broadly extended piedmont apron site, above which
Lake Constance still remains as a border lake (399). Outwash aprons
(314), moraines (311), and drumlins (317) are each in turn encountered.
Still continuing our course up the Rhine from Bregenz, we enter the
fretted upland (372) of the Alps, mountains composed of great folds and
thrusts about a core of intrusive rock (Rothpletz, Sammlung Geologische
Führer, Vol. 10, 1902, Thrusts in the Alps between Lake Constance and
the Engadine). Some fourteen miles above Chur we pass the terrace
produced by successive landslides (414), known far and wide as the
Flimser Bergstürz. The further assent of the cascade stairway of this
glacier-carved valley brings us to the Furka Pass, from which point
magnificent views of the fretted upland are obtained. At the Känzli, a
mile from the hotel, one may view the névé of the Rhone Glacier, which
may also be easily visited.
We have now followed a great river from its mouth in the sands of
Holland to its source in the snows of the higher Alps. Passing over
the divide and descending to Gletsch, we may observe the lower end,
or foot, of the Rhone glacier and the crevasses and séracs (391) on
the steep descent of this radiating glacier (383, 386). The response
which glaciers make to climatic changes is here well illustrated by the
recession of the glacier front from near the hotel (its position in the
’50s of the nineteenth century) to its present position about a mile
farther up the valley.
The characteristics of a glaciated mountain valley may be further
illustrated by climbing to the Grimsel Pass, which is scratched and
striated (377, 385), and then descending the valley of the Aar to
Meyringen (377). Near the Grimsel Hospice are the characteristic rock
basin lakes (412), and upon the Aar Glacier to our left were carried
out the epoch-making researches of Louis Agassiz, the founder of the
glacial theory for explaining the drift. We encounter some thirteen
rock bars (377). Just before reaching Meyringen we pass the last of
these, the Gorge of the Aar, cut by the stream through limestone.
Interlaken (419) may be made the center for additional excursions up
the Lauterbrunnen Valley, with its prominent albs (376) and its ribbon
fall of the Staubbach (378). By the Jungfrau Mountain railway we may
now ascend partly in tunnels of the rock to the Ewigeismeer, and look
down upon the névé and bergschrunds of the Great Aletsch Glacier (370,
see Baltzer, Sammlung Geologische Führer, Vol. 10, Bernese Oberland,
1906). Returning to Interlaken by way of Grindelwald, one may study the
foot of a radiating glacier, the Untergrindelwald glacier, with its
tunnel and its milky and braided stream.
Crossing now the Alpine foreland to Villeneuve at the upper end of
Lake Geneva and upon a well-developed strath (426, 428), we may look
out upon the turbid waters extending far from the shore of the lake.
Journeying to Geneva by steamer we note the gradual clearing of the
water until at the outlet of the lake it is as clear as crystal. A
walking trip from Geneva takes us to the Bois de la Bâtie, where the
Arve with turbid waters meets this clear stream (427).
The railroad to Chamonix ascends another cascade stairway (376),
affords views of complexly folded sedimentary rocks (43), and at
Chamonix itself the mer de glace supplies opportunities for the study
of moraines (386, 393) and glacial movement (390-392). To experienced
Alpinists the summit of Mount Blanc offers a remarkably extended
outlook over the fretted upland of the Alps (pl. 18 A). From the
station of LeFayet below Chamonix, one may ascend to the Désert de la
Platé, where are Schratten in limestone due to solution (188).
Crossing by one of the passes to the valley of the Rhone at Martigny
we may reach Zermatt, to-day the climbing center of the Alps. From
the subordinate cirques surrounding this village descend the Gorner,
Findelen, St. Theodul, and other components of this radiating glacier.
A black tooth of rock, the Matterhorn, towers above the other peaks and
shows to greatest advantage this feature of glacial sculpture (374),
while the Gorge of the Gorner is a severed rock bar like that of the
Aar (377). Either on foot or over the mountain railway we may ascend to
the Gorner Grat, a subordinate comb ridge (373) which affords one of
the most magnificent and instructive views of radiating glaciers.
From Brig, farther up the Rhone Valley, an excursion is made to the
Eggishorn Hotel, a center for study on and about the Great Aletsch
Glacier (329, 371, 385, 388, 395, 410). The easy ascent of the
Eggishorn is rewarded by a view almost directly downward upon the
ice-dammed Márjelen Lake (329, 411).
From Brig one may make his entry into Italy, either over the
picturesque Simplon route afoot or by diligence, or else beneath it
through the railway tunnel. By an alternation of short steamboat and
rail trips the journey is continued in a direction transverse to the
longer axes of the border lakes Maggiore, Lugano, and Como, and later
southward to Milan. In leaving the village of Como we pass over heavy
morainic deposits on the apron borders of the expanded-foot glacier
(383, 385) which once occupied the valley above. On the journey from
Milan to Venice, over the fertile plains of Lombardy, the similar
accumulations about Lake Garda (414) are first encountered at the
little station of Lonato and left behind at Somma Campagna (Tornquist,
Sammlung Geologische Führer, Vol. 9, Northern Italy, 1902).
The city of Venice is built upon pile foundations in the lagoon behind
the barrier beach known as the Lido (242, 428-429). From here we may
reach the Karst country by way of Trieste, some of the more interesting
and typical features being found near Divača (187-189, 422, pl. 6 A).
In a different direction from Venice by way of Belluno we enter the
Dolomites with their patterned relief and battlemented towers (228,
445).
Additional centers for geological excursions on the route to our point
of departure from Italy are Rome and Naples. At the Italian capitol and
in its neighborhood we may study the volcanic Campagna with its beds
of tuff (105) and its crater lakes (405. See Sir A. Geikie, The Roman
Campagna, Landscape in History and other Essays, Macmillan, 1905, pp.
308-352; also Deecke, Sammlung Geologische Führer, Vol. 8, Campagna,
1901). From Rome it is an easy journey to the cataract of Tivoli with
its deposits of travertine (184). In the opposite direction from Rome
across the Campagna rise the Alban Hills, ruins of a composite cone
with several crater lakes on the sites of former vents. On the summit
of the encircling crater rim, like the Monte Somma of the Vesuvian
Mountain now a crescent only, is located the chief Italian station for
earthquake study.
From Naples we may reach in short excursions and study with some
care still active volcanic mountains. To the east is Mount Vesuvius
(94, 97, 122, 124, 127-137), which was in grand eruption in April,
1906. Westward from Naples are the Campi Phlegraeii, or burning
fields, with many craters. Of these Astroni offers a fine example of
a large-cratered cinder cone (105). In the same vicinity are Monte
Nuovo (96) and the Solfatara (97), the latter a type of volcano which
no longer erupts lava, but in its place emits carbon dioxide and other
gaseous emanations (Grotto del Cane). The starting point for excursions
in the Phlegræan fields is Pozzuoli with its Temple of Jupiter Serapis
(254-255), reached from Naples by an electric line which pierces the
wall of an immense crater (Posilippo) composed of fine yellow volcanic
ash known as Pozzuolan.
From Naples steamers make short excursions to Sorrento with its deep
ash deposits, and to Capri with its blue grotto (257-258). Herculaneum
(139) and Pompeii (122), buried during the eruption of 79 A.D., are on
the line of the Circum-Vesuvian Railway.
Steamships to New York from Naples call at Gibraltar, the land-tied
island _par excellence_ (241). Most steamships of the southern route
pass through or near the volcanic islands of the Azores, and certain
boats touch at Algiers, from which a line of railway gives access to
Biskra on the borders of the Desert of Sahara.
Throughout these pilgrimages the traveler should be on the alert to
note not only the agent responsible for the features which come under
his observation, but, especially where this is the common sculpturing
agent of running water, he should not fail to notice the stage of the
erosion cycle which is represented (Chapter XIII).
INDEX
Abrasion, beneath glaciers, 275.
Abyssinia, fissure eruptions in, 101.
Accordance, of tributary valleys, 162.
Adiabatic refrigeration, in relation to glaciers, 262.
Adolescence, in cycle of erosion, 169.
Advancing hemicycle of glaciation, 263-266.
Advective zone, of atmosphere, 270.
Aftershocks, of earthquakes, 83.
Agassiz, glacial lake, 325-328.
Agassiz, Louis, cited, 339, 400.
Age, of strata, 38, 52.
Aggradation, 162.
Aktian deposits, 36.
Alaskan coast, map of, 79.
Albs, 376.
Alden, W. C., cited, 316, 318, 319.
Algæ, growth of, in hot springs, 194.
“Alkali” in deserts, 201.
Alluvial bench, 214.
Alluvial cone, 213.
Alluvial-dam lakes, 423.
Alluvial fan, 213.
Alpine glaciers, 383, 386.
Alterations of minerals, 27.
Altitude, of different parts of lithosphere, 18.
American Falls, future extinction of, 357.
Amphiboles, 459.
Amphitheaters, formed on drift sites, 369.
Amundsen, R., cited, 23.
Analysis, of folds, 54.
Anderson, Tempest, cited, 146, 147.
Andersson, J. G., cited, 157, 295.
Andesite, 463.
Angular unconformity, 53.
Antarctica, 154, 281.
Antarctic protuberance, 17.
Antarctic shelf ice, 289, 290.
Anticlinal folds, 42.
Anticlines, 42;
tension in, 45.
Anticyclone, glacial, 284.
Ants, factor in rock decomposition, 156.
Apron, alluvial, 213.
Aprons, outwash, 280, 281.
Arbenz, P., cited, 195.
Arches, of folded strata, 42;
sea, 233, 234.
Architecture, of fractured earth superstructure, 55.
Arctic depression, 17.
Areal geological map, 62.
Arêtes, 373.
Arldt, Theodore, cited, 11, 19, 438.
Arnold, Ralph, cited, 157.
Arrangement of oceans and continents, 10.
Artesian wells, 190, 191, 196.
Ash, volcanic, 122.
Askja, eruption of, in 1875, 101.
Assmann, R., cited, 294.
Astronomical _vs._ geodetic observations, 12.
Atlantis, North, 16.
Atmosphere, compressibility of, 8.
Attack, of the weather, 149.
Atwood, W. W., cited, 7, 160, 298, 300, 313, 372.
Axial plane, of folds, 42.
Axis, of folds, 42.
Azurite, 453.
Bacteria, part taken in weathering, 156.
“Bad Lands”, control of relief in, 223, 224.
“Bad Land” topography, 214.
_Bajir_, 216.
Balance, between degradation and aggradation, 161.
Bandai-san, dissection of, 141.
Barchans, 211.
Barrancoes, 139.
Barrell, J., cited, 221, 447.
Barrier beaches, 240;
sections of, 242;
uplifted, 249, 250.
Barrier lakes, 420.
Barriers, 240;
mountain, in relation to glaciers, 262.
Bars, 240.
Basal conglomerate, 37, 53.
Basalt, 463;
faulted blocks of, 58;
of Hawaii, 105.
Base level, 159.
Basin-range lakes, 402, 403.
Basin Range structure, 440.
Basins, flat bottomed, separating dunes, 216;
of exudation, 272;
of sedimentation, earlier, 38.
Bastin, E. S., cited, 210.
Batholites, 143.
“Bath tubs”, 395.
Beach pebbles, 239.
Beach sand, 206, 238.
Beaches, remaining from ice-dam lakes, 410;
shingle, 239;
storm, 240;
uplifted, “feathering out” of, 344.
Bedded structure of rocks, 31.
Beede, J. W., cited, 195.
“Bee-hive” mountains, 380, 381.
_Belgica_ expedition, 289.
Belt of sea which divides land masses, 11.
Berghaus, H., cited, 424.
Bergschrund, 370.
Berson, A., cited, 294.
Berthaut, General, cited, 7.
“Bird-foot” delta, 167.
“Biscuit cutting” effect of glacial sculpture, 372.
Blackwelder, E., cited, 318.
Block mountains, 446.
Blocks, orographic, 58.
_Bocchi_, 125.
Bog, floating, 429.
Bogs, of peat, 429, 430.
Bonney, T. G., cited, 146.
Borax deposits, in deserts, 201.
Border drainage, about glaciers, 316, 320, 321.
Border lakes, 399, 414.
Bosses, 143.
“Bottoms”, from entrenched meanders, 173.
“Bowlder clay”, 310.
“Bowlder pavement”, 237.
Bowlders, faceted, 310;
glacial, 298;
“soled”, 276, 310;
thrown up during earthquakes, 69.
Bowlder trains, 306.
Bowman, Isaiah, cited, 179.
Box cañons, 214.
Braided streams, 280.
Branner, J. C., cited, 6, 91.
“Bread-crust” lava projectiles, 119.
Breakers, 232.
Breccia, fault, 60.
Bridges, nature of damage to, during earthquakes, 75, 76.
Brigham, A. P., cited, 424.
Brögger, W. C., cited, 66.
Bruce, W. S., cited, 290, 382, 399, 414.
Bryant, H. G., cited, 289.
Buckley, E. R., cited, 433, 434.
Built terraces, 235.
Bunsen, cited, 192.
Burns, G. P., cited, 434.
Burton, W. K., cited, 92.
Buttes, 216.
Bysmalite, 442, 447.
Calcareous ooze, 36.
Calcareous sinter, 184.
Calcareous tufa, 464.
Calcite, 455.
Caldera, 405, of composite volcanic cones, 126.
Camiguin volcano, birth of, 96, 97.
Campbell, M. R., cited, 178.
Cañons, 160;
box, 214.
Capri, blue grotto of, 257, 258.
Capture, river, 175, 176, 179.
Carbonization, 151.
Cascade Mountains, fissure eruptions of, 102.
Cascade stairway, 376.
Caspian Depression, 14.
Cauliflower cloud, 130.
Caverns, galleries directed by joints, 182;
of limestone, 182, 195;
refuge of predatory animals, 185.
Caves, sea, 234.
Cellular structure, of lava domes, 112.
Centers of dispersion, of North American Pleistocene glaciers, 298.
Centrosphere, 8.
Cerussite, 455.
Chaix, A., cited, 195.
Chaix, E., cited, 195.
Chalcopyrite, 453.
Challenger expedition, 38, 96, 97, 293.
Chamberlin, T. C., cited, 29, 156, 191, 196, 205, 221, 222, 293, 295,
318, 319, 337, 339.
Character profiles, coast, due to uplift or depression, 259;
composite, 229;
directly due to volcanic agencies, 145, 146;
from stream erosion in humid climates, 177;
of arid lands, 220;
of shore features, 243;
referable to continental glaciers, 318;
referable to mountain glaciers, 379.
“Checkerboard topography”, 226.
Chemical sediments, 34.
Chicago outlet, 331.
Chimneys, in “driftless area”, 300.
Chimneys, shore feature, 234.
China, loess of, 207.
Chlorite, 458.
Chlorite schist, 465.
Cicatrice, from dissection of volcanoes, 142.
Cinder cones, 105;
corrugations upon, 138;
diameter of crater in relation to violence of explosions, 123;
grander eruptions of, 117;
profiles of, 123;
secondary, 111.
Cinder eruptions, artificially simulated, 122.
Cirques, 371;
life history of, 371;
subordinate, 371.
Cities, destruction of, by drifting sand, 218.
Clastic rocks, 30.
Clay slate, 466.
Cleavage, mineral, 27, 450;
rock, 44.
Clefts, volcanic, in Iceland, 99.
Cliffs, notched, 233.
Climatic conditions, in relation to mountain sculpture, 443.
Clinometer, 48.
Cloudbursts, in deserts, 201, 212.
Cloud zones, 268, 269, 294.
Coals, 466.
Coast, Dalmatian, grottoes of, 258.
Coast, elevation of, during earthquakes, 80;
submergences of, during earthquakes, 80.
Coastal plains, 246;
belted, 247.
Coast lines, even, 246;
indicative of uplift or submergence, 245, 246;
ragged, 246.
Coast records, 245.
Coasts, Atlantic and Pacific contrasted, 438;
embayed, 251.
Coast terraces, 80, 250, 241;
uplift, effect of, on sediments, 38.
Coats Land, shelf ice of, 290.
Cobalt, in meteorites, 23.
Cobb, Collier, cited, 179.
Coigns, of earth’s tetrahedral figure, 15.
Coleman, A. P., cited, 318.
Colk lakes, 408, 409.
Colks, scape, 277.
Collet, L. W., cited, 39.
Colorado desert, 74.
Color, of minerals, 450.
Cols, 374;
origin of in cirque intersection, 372.
Comb ridges, 373.
Compass, geologist’s, 47, 48.
Competent layer, 42;
in relation to lava reservoirs, 144.
Composite cones, _caldera_ of, 126, 127.
Composite groups of joints, 57.
Composite volcanic cones, 105.
Composition of earth, 29.
Composition of the earth’s core, 21.
Compression of a district during earthquakes, 76.
Cones, alluvial, 213;
cinder, 105;
composite volcanic, 105.
Conformable series, 51.
Conglomerate, 34, 463;
basal, 37, 53.
Constructional topography, 309.
Construction of buildings, in earthquake regions, 89-91.
Continental glacier, behind rampart, 281;
in Victoria Land, 280-285;
of Antarctica, literature of, 295;
of Greenland, 271;
of Greenland, melting on margin of, 278;
of Greenland, literature, 295.
Continental glaciers, contrasted with mountain glaciers, 266-268;
defined, 266-267;
of “ice age”, 297;
of ice age, cross section of, 302;
nourishment of, 283, 286, 295;
profiles of, 267.
Continental platform, 19.
Continental shelves, 18, 19;
origin, 232.
Continents, arrangement of, 10;
development of, 14;
increase in area of, through wave action, 241;
past history of, 14.
Contortions of the strata, 40.
Contours, of topographic maps, 62.
Contraction of earth’s surface, during earthquakes, 74.
Contrary movements upon coasts, 254, 257.
Convective zone, of atmosphere, 270.
Conway, W. M., cited, 294.
Copernicus, cited, 10.
Copper glance, 455.
Coquina, 35.
Cornish, Vaughan, cited, 211, 222, 244.
Corrasion, 162.
Corrosion, of rocks, 156.
Coulée lakes, 406.
Coves, 233, 234.
Cracks, earthquake, 74.
Crater, evolution of form of, 128.
Crater lakes, 405, 406.
Craterlets, 84;
sections of, 85.
Craters, mechanics of explosions in, 115.
Crater, volcanic, 95.
Credner, G. R., cited, 179.
Crescentic levee lakes, 416, 417.
Crestline, of an anticline, 42.
Crevasse, marginal, on mountain glaciers, 370.
Crevasses, in connection with river cut-offs, 164;
on glaciers, 391.
Cross, Whitman, cited, 216, 441, 447.
Cross-bedded structure, 37.
“Crystal cellars”, 27.
Crystal form, of minerals, 449.
Crystals, behavior under special treatment, 24, 25;
essential nature of, 23;
forms of, 454, 457;
individuality of, 24;
mutilated, later growth of, 26;
symmetry of form of, 23.
Crustal shortening, 42.
Cuestas, 246, 247;
south of Lake Ontario, 361, 362.
Cut and built terrace, on steep shore of loose materials, 237.
Cut-offs, of meanders, 164.
Cut rock terraces, 235.
Cuvier, cited, 199.
Cvijić, J., cited, 195.
Cycle of glaciation, 263, 294.
Cycles, of glaciation, Pleistocene, 297;
of stream meanders, 163.
Dana, J. D., cited, 6, 104, 106, 109, 111, 146, 147.
Dana, E. S., cited, 29.
Daly, R. A., cited, 447.
Dante, cited, 9.
Darton, N. H., cited, 179.
Darwin, Charles, cited, 199, 322, 323, 339.
Daubrée, A., cited, 54.
David, T. W. E., cited, 23.
Davis, C. A., cited, 434.
Davis, W. M., cited, 7, 178, 179, 221, 247, 276, 317-319, 378, 382.
Deceptive unconformity, 53.
Decomposition, 149, 156;
mechanical results of, 150.
Débris cones, 395.
Deep sea deposits, 36, 38.
Deflation, 204.
Deforestation, in relation to agriculture, 156;
of Karst region, 188;
relation to erosion, 157.
Degeneration, 149.
De Geer, G., cited, 351, 366, 410.
Degradation, 161, 162.
Dekkan, fissure eruptions of, 101.