THE ROCKS
Broadly speaking a rock is an essential part of the crust of the earth,
and includes loose material, like sand, mud, or volcanic ashes, as well
as compact and solid masses, like sandstone and granite. Rocks are
aggregates of minerals, either several minerals grouped together, as are
mica, quartz and feldspar to make granite, or large quantities of a
single mineral, like quartz grains to make sandstone.
The rocks are most conveniently classified according to their mode of
origin, into three main groups, igneous, sedimentary, and metamorphic.
The igneous rocks are those which have solidified from a molten magma,
like lavas, granites, etc. The sedimentary rocks are those which
represent accumulations of fragments or grains, derived from various
sources, usually the weathering of other rocks, and deposited by such
agents as water, wind and organisms. Metamorphic rocks are those which
were originally either igneous or sedimentary, but have been altered by
the actions of heat, pressure and water, so that the primary character
has been changed, often to such an extent as to be obscured.
Rocks once formed in any of the above ways are being constantly altered
in character by the various processes of nature. Those exposed on the
surface are weathered to pieces, and the fragments are transported by
wind or water to accumulate elsewhere as sedimentary rocks. Those buried
deep beneath the surface are affected by the high temperature and
pressure of the depths of the earth and thus metamorphosed. For instance
a granite exposed on the surface is slowly weathered, some parts being
carried away in solution by the rain water, others less soluble
remaining as grains of quartz, mica or kaolin. These are transported by
water and sorted, the finer kaolin being carried to still and deep
water, the quartz and mica accumulating in some lowland as sand. This
sand will in time be cemented to a sandstone, later slowly buried
beneath the surface. If buried deep it will feel the effect of the
interior temperature, which increases as one goes down at the rate of
one degree F. for every 50 feet. If this should be in a region where
folding and mountain-making takes place, the material under the folds
would be melted (because of the relief from pressure which would permit
the high temperature to act freely) and become igneous rock, either
coming to the surface as lava, or remaining below the surface and making
a granite or similar rock; while the sedimentary material not melted but
near enough to the molten material to be affected, would be
metamorphosed, in this case to a quartzite. Much of the interest and
profit in studying rocks, will come from the understanding which they
will give as to the history of that particular part of the earth’s crust
where they are found.
Igneous Rocks
Igneous rocks are those which have formed from material that has been
melted, which involves temperatures around 1300° C.; or, if there is
water in the original material, temperatures as low as 800° C. will
suffice. Considering the increase of temperature to be a degree for
every 50 feet downward, this involves the rocks having been at depths of
5 to 10 miles below the surface. While at such depths the temperature
must be high enough to melt rocks, the great pressure of the overlying
rocks seems to keep them solid; for we know that the center of the earth
is solid, as is shown by a variety of observations, such as the rate at
which earthquake waves are transmitted through the earth, the lack of
tidal effects, etc. However, there is every reason to believe that if
the pressure is removed from the rocks which are five to ten miles below
the surface, there is heat enough at those depths to melt them. When the
crust of the earth is folded, as when mountain ranges are formed, the
areas under the arches or upward folds are relieved of pressure. Then
those rocks, which are under the arches and are relieved, become molten.
The molten magma may well up and fill the space beneath the arch where
it would cool again very slowly; or, if there is fissuring during the
folding, some of the molten material may be forced out through the
fissures and pour out over the surface as lava. Another area in which
pressures may be locally relieved is in the region of faulting, where
the crust of the earth is broken into blocks, between which there are
readjustments, some being tipped one way, some another, some uplifted.
Here again there would be areas of relieved pressure and molten magmas
would form, some of them solidifying in place, others rising to the
surface.
The molten material is termed the magma, and when it reaches the
surface, great quantities of water vapor and other gases escape: or
these gases may even escape from magmas which do not reach the surface,
rising through fissures. As these hot vapors pass through the fissures,
they are cooled, and may deposit part or all of their dissolved
compounds in the fissure, making veins. Lava is the magma minus the
vapors. Magmas vary greatly from place to place, indicating that they
are formed locally and do not come from any general interior reservoir,
as has sometimes been suggested.
When the molten magmas escape to the surface, they are termed extrusive,
and as they spread out in a layer this is termed a sheet. This rise and
overflow may be quiet, and from time to time one outpouring may follow
another making sheet after sheet. Or after one outpouring, the pressure
below may cease for a time and allow the lava to solidify and make a cap
or cover over the opening. Before more lava can rise, this cover must be
removed. This usually happens in an explosive manner, the lava below,
with the increasing pressure exerted by its expanding gases, finally
exerting enough pressure, so that the cover is broken, or shattered and
thrown in thousands of fragments into the air, as happened at Mt. Pelée
on the Island of Martinique in 1902. The fragments thrown into the air
are often termed volcanic ashes, though this is not a good word for
them, for they have not been burned.
In case the molten magmas under the relieved areas do not reach the
surface they are termed intrusive. Such magmas may remain in the space
under a mountain fold, or be forced in fissures part way to the surface.
When the magma is forced into more or less vertical cracks and there
solidifies, and these are exposed by erosion, they are termed dikes.
Sometimes the magmas have risen part way to the surface and then pushed
their way between two horizontal layers of rock and there hardened, in
which case they are termed sills, when uncovered. The Palisades along
the Hudson River are the exposed edge of a sill. Again the molten magmas
may well up and spread between two horizontal layers, but come faster
than they can spread horizontally, and then the magma takes the form of
a half sphere, and the overlying layers of rock are domed up over it.
Such a mass is termed a laccolith. In all these cases the mass of
igneous rock is only discovered when the overlying rocks have been
eroded off. The great mass of molten magma under the arches of mountain
ranges simply cools slowly into a granitic type of rock. These masses
are exposed when the thousands of feet of overlying rock are eroded off.
When these masses are exposed, if of but a few miles in extent, they are
called stocks, but, if of many miles in length and breadth, they are
batholiths, and are very characteristic of the heart of mountain ranges.
In all the above cases the exterior of the molten mass cools first, and
forms a shell around the rest. The shell determines the size of the
mass. As the cooling continues into the interior, it also solidifies,
and as all rocks shrink on cooling, cracks develop, separating the mass
into smaller pieces. There is usually no regularity about these cracks
and the mass is divided into blocks from six inches to three feet in
diameter. However, in some cases, especially in sills and laccoliths
where the cooling is slower, the shrinkage may be marked by a regular
system of cracks which bound the rock into more or less regular
hexagonal columns. The Palisades and the Devil’s Tower in Wyoming (See
Plate 52) show this structure. The Devil’s Tower is the remnant of a
laccolith, all except the central core of which has been eroded away.
All of the above terms have nothing to do with composition, but refer
entirely to the manner of occurrence.
While the igneous rocks are classified according to their composition,
the rate at which they cooled has much to do with their texture, and
certain names apply to the texture. For instance when the molten lava
cools very rapidly, there is no time for the formation of crystals, and
the resulting rock is glassy or non-crystalline. If the cooling is slow
as in large bodies, crystals have time to form and grow to considerable
size as in granites. Between these all grades may occur; and one
classification of igneous rocks expresses their rate of cooling, in such
terms as the following.
Glassy—lavas which have cooled so quickly that they are without distinct
crystallization, such as obsidian, pitchstone, etc.
Dense or felsitic—lavas which have cooled less rapidly, so that crystals
have formed, but in which the crystals are too small to be identified by
the unaided eye, such as felsite or basalt.
Porphyritic—magmas from which, in solidifying, one mineral has
crystallized out first and the crystals have grown to considerable size,
while the rest have remained small.
Granitoid—magmas which have solidified slowly, so that all the minerals
have crystallized completely, and the component crystals are large
enough to be recognized readily, as in granite.
Fragmental—a term applied to the fragments which have resulted from
explosive eruptions of igneous rocks. These fragments may be loose or
consolidated. Volcanic ashes are typical.
Porous—a term applied to the lava near the upper surface, which is
filled with gas cavities, such as pumice.
Amygdoloidal—is the term applied to porous lavas, when the cavities have
been filled by other minerals, such as calcite or some of the zeolites.
In determining a rock, first decide whether it is igneous, sedimentary
or metamorphic. The igneous character is recognized by its being either
glassy, or composed of masses of crystals irregularly arranged, there
being neither layering nor bedding.
CLASSIFICATION OF IGNEOUS ROCKS
Texture Excess of light colored minerals Excess of dark colored minerals
Glassy obsidian, perlite, pumice, pitchstone scorias, trachylyte, basalt-obsidian
Feldspar orthoclase Feldspar Plagioclase No feldspar
Mica and/or hornblende and/or augite Mica and/or hornblende with pyroxene augite and/or hornblende
and/or mica
+quartz -quartz +quartz -quartz +olivine -olivine +olivine -olivine
Dense rhyolite trachite dacite (felsite) andesite (felsite) basalt augitite or
hornblendite
Porphyritic rhyolite-porphyry trachite-porphyry dacite-porphyry andesite-porphyry basalt-porphyry augitite-porphyry
Granitoid granite syenite quartz-diorite diorite olivine-gabbro gabbro peridotite pyroxenite
Fragmental rhyolite, tuff trachite, tuff Dacite, tuff or andesite tuff or Basalt tuffs and breccias
or breccia or breccia breccia breccia
When it is located as igneous, turn to the key on page 177 and decide as
to which type of texture is present. If glassy, the color, luster and
type of construction will place it. If the rock is crystalline, first
decide whether feldspar is present, and if present, what type: then
determine the dark mineral, and lastly whether quartz or olivine is
present. In dense rocks the presence of quartz may be determined by
trying the hardness, for none of the other constituents of igneous rocks
have so great hardness. For example, if it is found that a rock is
composed of orthoclase hornblende and quartz, and the texture is
granitoid, it is granite: or if the rock is plagioclase feldspar and
pyroxene of any sort, it is gabbro, etc.
Granite
Pl. 53
The combination of orthoclase feldspar (or microcline), quartz, and
either mica, hornblende or augite is termed granite, if the texture is
coarse enough so the individual minerals can be recognized with the
unaided eye. The rock is light-colored because the feldspar and quartz
dominate. Accessory minerals may be present such as apatite, zircon,
beryl or magnetite. Varieties of granite are distinguished according to
the dark mineral present. When this is muscovite, it is a
_muscovite-granite_; when it is biotite, a _biotite-granite_; if it is
hornblende, a _hornblende-granite_; etc. The size of crystals in granite
varies widely. When they are as small as ¹/₁₂ of an inch in diameter, it
is termed fine grained; from ¹/₁₂ to ¼ of an inch, it is medium-grained;
when larger, it is coarse-grained. In some cases the crystals may be
over a foot in diameter which is known as _giant granite_.
Originally granite was a great mass of molten magma, which has cooled
very slowly, having been intruded or thrust up in great stocks or
batholiths beneath overlying rocks, which acted as a blanket to prevent
rapid cooling. These overlying rocks, in their turn, have been acted
upon by the heat and metamorphosed. Granite is particularly likely to
have been formed under mountain folds; so that, after the mountains have
been more or less completely eroded away, the great masses of granite
have come to the surface to mark the axes of the ranges; and even after
the mountains have been wholly worn away, the granite remains to mark
the sites on which they stood.
In the granite mass itself, there are often veins and dikes, which
probably resulted from the shrinkage of the cooling granite, and they
are filled with a different and usually coarser granite known as
pegmatite. This pegmatite formed from the residual magmatic material, so
that as some of the elements had already crystallized out, the granite
in these dikes is of different composition. The extreme coarseness of
these pegmatites seems to be due to the character of the mineralizing
agents left in the dikes. In some of these pegmatites the feldspar and
quartz are so intergrown, that when broken along the cleavage surface of
the feldspar, the quartz appears like cuneiform characters, and this
variety has been given the name _graphic granite_ (See Plate 53).
When granite is exposed to weathering, the feldspar is the first mineral
to be decomposed, altering eventually into carbonates, quartz and
kaolin. The dark minerals are only slightly less susceptible and they
break down into carbonates, iron oxides and kaolin. The original quartz
remains unchanged. Of these products the carbonates, some of the iron
oxide and a little of the quartz are carried away in solution. The
kaolin and some of the iron oxide is in fine particles and they are
carried by the water until it comes to the lakes or the sea. The quartz
is left in coarser grains, which are more slowly transported, and
deposited in coarser or finer sand and gravel beds.
Granites are widely used for building stone, because they can be worked
readily in all directions, and have great strength and beauty. The color
depends largely on the color of the feldspar, which may be white or
pink, in which case the granite will be gray to pink.
Granites occur throughout New England, the Piedmont Plateau, the Lake
Superior Region, the Black Hills, Rocky Mountains, Sierra Nevada, etc.
Syenite
Pl. 54
The combination of orthoclase and either mica, hornblende, or augite is
syenite, the texture being coarse enough so that the individual minerals
can be distinguished by the unaided eye. It differs from granite in the
absence of quartz. Syenite is a light-colored rock with the feldspar
predominating. Minerals like apatite, zircon, or magnetite may occur in
it, as accessory minerals. The foregoing would be an ideal syenite, but
usually there is some plagioclase feldspar also present. If this occurs
in such quantities as to nearly equal the orthoclase feldspar, the rock
is termed a _monzonite_; if it predominates, the rock becomes a diorite.
The presence of quartz would make this rock into a granite. Such a
compound rock has its type form, and when the proportions of the
component minerals are changed, it grades into other types.
Like the granite, syenite is an intrusive rock, which occurs in stocks
and batholiths along the axes of present or past mountain ranges. The
original magma welled up under the mountain folds, where it cooled
slowly, metamorphosing the adjacent rocks. Like granite it has only been
exposed after a long period of erosion has removed the overlying layers
of rock.
Syenites are not as abundant as granites, but they occur in the White
Mountains, near Little Rock, Ark., in Custer Co., Colo., etc.
Quartz-Diorite
The combination of plagioclase feldspar, quartz and either mica or
hornblende makes quartz-diorite, sometimes called _tonalite_. The above
would be the typical quartz-diorite, but there is usually some
orthoclase present, which if it equals the plagioclase feldspar in
amount makes this into a monzonite; or if it dominates, it makes the
rock a granite. Quartz-diorite is darker colored than the two preceding
rocks, the dark minerals being about as abundant as the light-colored
ones, such as feldspar and quartz. For this reason the weight is also
somewhat greater.
Like the others this is an intrusive rock, occurring in stocks and
batholiths, and indicative of great molten masses thrust up under
mountain folds, and only exposed after the overlying rocks have been
weathered away. It is by no means an abundant type of rock, but occurs
at Belchertown, Mass., Peekskill, N. Y., in the Yellowstone Park, etc.
Diorite
Plagioclase feldspar with hornblende or mica, or with both, is known as
diorite. It is distinguished from quartz-diorite by the absence of
quartz. There is generally some augite in it, but if this should be
equal to, or exceed the hornblende, the rock is then a gabbro. There may
also be a small amount of orthoclase present, without taking this rock
out of the diorite class, but if the orthoclase feldspar becomes
dominant, then the rock is a syenite. Thus there is gradation into other
groups in all directions. Apatite, magnetite, zircon, and titanite often
occur in small quantities as accessory minerals. Generally the
hornblende is in excess of the feldspar, so that the rock is a
dark-colored one.
Diorites occur in much the same manner as granites, being in stocks,
batholiths or dikes, and are often associated with granites and gabbros.
They are great intruded masses, associated with mountain making, and
like the preceding rocks, cooled far below the surface, and have been
exposed only after great thicknesses of overlying rocks have been
weathered away.
Peekskill, N. Y., the Sudbury nickel district in Canada, Mt. Davidson
above the Comstock Lode in Nevada, etc., are typical localities for
finding diorite.
Olivine-Gabbro
The combination of plagioclase feldspar with augite (or any of the
pyroxenes) and olivine makes olivine-gabbro. The feldspar is usually one
of those with considerable calcium in it, like labradorite; and as the
dark minerals predominate, the rock is dark-colored. It is an intrusive
rock, usually in dikes or stocks, where it solidified far below the
surface, and was only exposed after the overlying rocks were weathered
off. It is by no means an abundant type of rock, but is found in the
Lake Superior Region, and near Birch Lake, Minn.
Gabbro
Pl. 54
Plagioclase feldspar with any one of the pyroxenes, most commonly
augite, is gabbro. There is a wide range in the relative proportions of
the two minerals making gabbro. At one extreme are rocks made entirely,
or almost entirely, of plagioclase feldspar, which are known as
anorthosites, and occur in parts of the higher mountains of the
Adirondacks like Mt. Marcy, in several places in eastern Canada, etc.
Then there are the typical gabbros where the feldspar and augite are
more or less equally represented. At the other extreme come those
gabbros in which the pyroxene predominates, in the most marked cases the
feldspar being entirely lacking, and the rock being termed a pyroxenite.
When the pyroxene of a gabbro is either enstatite or hyposthene (usually
the latter) the gabbro is often called norite. Magnetite, biotite, and
hornblende may occur in small quantities as accessory minerals.
Gabbro is a common intrusive rock, occurring in stocks, batholiths, and
dikes, and often varies considerably in different parts of the mass.
Like granite the mass solidified far below the surface, under some
mountain fold, and has only been exposed as the result of weathering
away the layers of overlying rock. Gabbros appear much like diorites,
but are distinguished by the fact that the dark mineral is one of the
pyroxenes, instead of an amphibole or a mica. They are widely
distributed, being found in the White Mountains, near Peekskill, N. Y.,
Baltimore, Md., about Lake Superior, in Wyoming, the Rocky Mts., etc.
Peridotite
A rock made up of olivine and augite (or any of the pyroxenes) is
peridotite. As it contains no feldspar, and both augite and olivine are
dark-green to black in color, these rocks are always dark green to black
in color and of considerable weight. They are usually rather coarsely
crystalline. Peridotite is usually associated with gabbro, making dikes
which lead from the main gabbro mass. Less frequently it occurs
independently, making up an intrusive mass. Hornblende and mica may be
present in small quantities, as accessory minerals.
In general these are rather rare rocks, making dikes connected with
stocks or batholiths of gabbro. Peridotite is found near Baltimore, Md.,
in Custer Co., Colo., in Kentucky, etc.
Pyroxenite
This represents the extreme among coarsely crystalline igneous rocks, a
whole mass made up of one mineral, and that some one of the pyroxene
group. If the mineral can be exactly determined, the rock may be still
more definitely named. For instance if it is all augite, then the rock
would be called augitite. Like the preceding rocks, pyroxenite is an
intrusive rock, usually found in dikes, which are connected with gabbro,
and it represents the segregation of one mineral out of the gabbro, and
its solidification at one point. Hornblende, magnetite and pyrrhotite
may be present as accessory minerals. This is not a common rock, but it
illustrates the fact that all possible combinations do occur, if the
circumstances have warranted it. It is found near Baltimore, Md.,
Webster, N. C., and in Montana.
Rhyolite
This is a combination of orthoclase feldspar, quartz, and either
hornblende, mica or augite in which the crystals are of such small size
that they can not be identified with the naked eye. In composition it
corresponds to granite, but it is much finer in texture. It differs from
trachite by having quartz while the latter has none. This can usually be
determined by trying the hardness as none of the other minerals are as
hard as 7. It is much harder to distinguish it from dacite which differs
only in having plagioclase feldspar in place of the orthoclase, and only
the microscope will enable one to make this distinction. Where the
distinction cannot be made these light-colored lavas are often called
felsite.
Rhyolite is usually an extrusive lava, occurring in sheets, but
sometimes it is intrusive, occurring in sills, dikes, and laccoliths. In
all these cases the lava has solidified so rapidly, that the crystals
are tiny, and only the general effect of a crystalline structure is
distinguishable. Rhyolites may occur with porphyritic structure, in
which case the presence of the larger feldspar crystals will help to
distinguish whether they are orthoclase or not, making the determination
easier. The color of rhyolites is green, red or gray, always a decided
light shade.
Rhyolites are abundant in the western states, as in the Black Hills, the
Yellowstone Park, Colorado, Nevada, California, etc.
Trachite
The combination of orthoclase feldspar with mica, hornblende or augite
is termed trachite, if the texture is dense. It is usually an extrusive
lava of light color (green, red or gray), and corresponds in composition
to syenite. It can be distinguished from rhyolite by having no quartz,
and so nothing to show a hardness above 5.5; but it is difficult to
distinguish it from andesite, which differs only in having plagioclase
feldspar in place of orthoclase. It sometimes occurs with a porphyritic
structure, in which case the feldspar crystals are usually large enough
to be distinguished.
Trachites are not abundant in America, but some are found in the Black
Hills of South Dakota, in Custer Co., Colo., and in Montana.
Dacite
The union of plagioclase feldspar, quartz, and either hornblende or mica
is termed dacite, if the texture is dense. It is an extrusive lava,
occurring mostly in sheets and dikes. It corresponds in composition to
quartz-diorite. As the texture is dense it is difficult to distinguish
dacite from rhyolite, for both have quartz and differ only in the
character of the feldspar, so it is quite common to use the term felsite
which does not distinguish between the two, and only states that the
rock is dense, light-colored and extrusive. When, as often occurs, the
texture is porphyritic, and the feldspars are the large crystals, then
exact determination is fairly easy.
Dacites are rather common, occurring on McClelland Peak, Nev., in the
Eureka district, Nev., on Lassen’s Peak, Calif., Sepulchre Mt. in the
Yellowstone Park, etc.
Andesite
The union of plagioclase feldspar with mica, hornblende or augite, makes
andesite if the texture is dense. The lack of quartz, and so no mineral
which has a hardness of over 5.5, makes it possible to distinguish
andesite from dacite or rhyolite, but it is hard to distinguish this
rock from trachite, which differs only on having orthoclase feldspar in
place of plagioclase. When the texture is porphyritic and the feldspars
are the large crystals, then it is easy to make the distinction.
Andesite gets its name from being the characteristic lava of the Andes
Mountains, and is the commonest of all the extruded, light-colored
lavas, being the lava of hundreds of flows throughout the western United
States.
The union of plagioclase feldspar and biotite is the commonest type.
Plagioclase with hornblende or augite is less common, and, when they do
occur, they are usually distinguished as _hornblende-andesite_ or
_augite-andesite_. Magnetite, apatite and zircon may be present as
accessory minerals.
The lavas of Mt. Hood, Shasta, Rainier and others of the volcanic peaks
of the Cascade Range, those at Eureka and Comstock in Nevada, in the
Yellowstone National Park, and the porphyries of many peaks in Colorado,
like the Henry Mts., etc., which are exposed laccolithic intrusions, are
all andesites, as are many more.
Basalt
The combination of plagioclase feldspar with olivine and augite (or any
other pyroxene) makes a heavy, dark-colored, black to dark-brown rock
which, if its texture is dense or porphyritic, is termed basalt. This
usually has more or less magnetite in it as an accessory mineral, indeed
the magnetite may be so abundant as to be a component part of the rock.
This magnetite makes trouble for anyone trying to use a compass on or
about basalt rocks. These are extrusive or intrusive rocks and
correspond in composition to gabbro.
Basalts are among the commonest of igneous rocks, and are popularly
designated “_trap_,” much used as a road ballast on account of its
toughness, which is largely due to its dense texture. The coast of New
England is seamed with dikes of basalt, and through the Adirondack and
White Mountains there are a host of these dikes. The crests of such
mountains, as the Holyoke Range, the Tom Range, the Talcott Mts., East
and West Rocks at New Haven, etc., are all basalt sheets. The Palisades,
First Wachung and Second Wachung Mountains of New Jersey are sills of
basalt. The Lake Superior region is crisscrossed with basalt dikes. That
greatest of all lava fields the Columbia Plateau, covering over 200,000
square miles on the Snake and Columbia Rivers in Oregon, Washington and
Idaho, is all basalt. So it goes all down through Nevada, New Mexico and
California.
Porphyry
Pl. 55
This is a term which properly refers to texture alone, indicating a
lava, which has cooled in such a manner that one mineral has
crystallized out of the magma first and developed to a larger size,
while the mass of the material formed tiny crystals in which the larger
ones are embedded. The large crystals are technically known as
_phenocrysts_. The surrounding mass of tiny crystals is termed the
_matrix_. This porphyritic structure is especially characteristic of
lavas which have been extruded in large masses, and of intruded lavas in
such places as sills and laccoliths.
The term porphyry today has the above precise meaning. It is a much
abused word, and has had all sorts of meanings. In the past it was first
used to refer to lavas in general, then it came to be applied to lavas
which had been erupted before Tertiary times, that is to all ancient
lava sheets. This idea soon proved incorrect, lavas being of the same
composition whether ancient or recent. In the West the word is often
colloquially used today to designate almost every kind of igneous rock
occurring in sheets or dikes, if in any way connected with ore deposits.
When the composition of a rock with porphyritic textures can be
determined, the name due to the composition is coupled with that due to
texture, making such terms as _trachite-porphyry_, _basalt-porphyry_,
etc.
Tuff
Tuff, a term not to be confused with tufa on page 215, is the name used
to designate the finer fragmental ejecta of volcanic eruptions, which
are also often referred to as “volcanic ash,” but the word, ash, conveys
the false impression that the rock is a remnant of something burned, and
is therefore not a good term. When first ejected, tuff is loose
material, but it is usually soon cemented to make a more or less firm
mass of rock, for which the term, tuff, is still retained. In some
cases, while still loose, it is carried by streams to a distance and
deposited in more or less sorted and layered beds: and the finer tuff is
often carried by the winds and laid down, at a considerable distance
from its source, in so called “ash beds.” In both these cases,
sedimentary characteristics have been added to the tuff, and layering
which is characteristic of sedimentary deposits, is present. These
transported tuff beds are really sedimentary, but as there is little
change in the material, they are referred to here and not again. These
tuff beds are not at all uncommon in the sedimentary deposits of
Tertiary age in the Rocky Mountain region. The coarser material of
volcanic eruptions usually goes under the head of breccia.
Breccia
This term is used to describe the coarse fragmental ejecta of volcanic
eruptions. It is also used, in the section under sedimentary rocks, in a
broad sense to include all angular unworn fragmental material, whether
of igneous or sedimentary origin. For this reason, when dealing with
igneous rocks, it is usual to designate the fragments according to their
composition, making such terms as _trachite-breccia_,
_rhyolite-breccia_, etc.
While still loose (and also even when cemented into beds of rock), it is
customary to designate the smaller fragments, from the size of a grain
of wheat up to an inch or two in diameter, as _lapilli_; the larger
fragments, from two inches up to a foot or so in diameter, as _bombs_;
and the largest masses, often tons in weight, as _volcanic blocks_.
Obsidian
Pl. 55
Lavas, which have cooled so quickly that crystals have not had time to
form, have a glassy appearance, and are termed obsidian. If the color is
dark, due to the presence of large amounts of those elements which make
dark minerals, this lava is termed _basalt-obsidian_. Obsidian is
characterized by its glassy texture, a hardness around 6, and by
breaking with a conchoidal fracture, so called because the surface is
marked by a series of concentric ridges, something like the lines of
growth on a shell. Obsidians vary greatly in color, but are usually red
or green to black, and translucent on thin edges. While glassy, all the
obsidians contain embryonic crystals, which appear like dust particles
floating in the glassy matrix, or there may even be a few larger
crystals present, which are often arranged in flow lines. Most all large
masses of obsidian have streaks or layers of stony material in them
where crystallization has set in, in a limited way.
Near the upper surface, obsidians usually have gas cavities scattered
through them, and these may be small and few, or large and numerous.
Indeed the cavities may be so numerous as to dominate and give the rock
a frothy appearance. In this case, if the cavities are small and more or
less uniform, the rock is called _pumice_; if they are larger it is
_scoria_. If, as often happens when the lava is ancient and has been
buried beneath other rocks, the cavities have been filled with some
secondary mineral, then the lava is called an _amygdoloid_.
Obsidian is found in many localities, especially where there are recent
volcanoes, the most famous places being the obsidian cliffs in the
Yellowstone Park, those near Mono Lake in California, and many other
localities in the Rocky Mountains, the Sierra Nevadas, and the Cascade
Mountains.
Pitchstone
This is very like obsidian in appearance, but differs in that the glassy
material contains from five to ten per cent of water in its composition,
the most obvious effect of which is to make the luster resinous, instead
of vitreous, as is characteristic of obsidian. The colors are commonly
red, green or brown. Pitchstone is associated with recent volcanoes, and
some fine specimens have come from Silver Cliffs, Colo., and various
parts of New Mexico and Nevada.
Perlite
_pearlstone_
Perlite is a glassy lava, containing two to four per cent of water,
which, on cooling, has cracked into numerous rounded masses, with a
concentric structure, reminding one of the layers of an onion.
Scoria
While lava is cooling, there is a constant escape of gases, mostly
steam, and as these rise through the molten mass they make cavities,
near the upper surface, that portion on top often becoming frothy. If
this solidifies quickly so that the gas cavities are preserved it is
scoria. When the gas cavities are small and uniformly distributed, the
rock is called pumice, and often used as a scouring agent. When the
cavities are large and irregular the term scoria is generally used.
Molten lavas may form various structures, according to the conditions
under which they cool, dripping through cracks or from the roof of
caves, which often form where the molten lava escapes from a hardened
shell, and making stalactites, stalagmites, etc. The very thin lava of
the Hawaiian volcanoes may even be blown by the wind into fine threads,
known as “Pele’s hair.”
The presence of the gas cavities is so characteristic of the upper
surface of lavas which have been extruded; that, where one is dealing
with older lavas, now buried beneath other rocks, this fact helps to
determine whether the mass is a sheet, rather than a sill; for, in the
case of the sill, the lava was forced between layers of sedimentary
rocks, and the burden of the overlying rocks did not permit the escape
of steam and therefore the upper surface of sills does not have the
scoriaceous structure.
Amygdoloid
Pl. 56
When the upper surface of a lava is filled with steam holes, and this
lava has been buried beneath other rocks, the seeping waters slowly
bring such minerals as quartz, calcite and zeolites and fill the
cavities. Such a rock is known as an amygdoloid. It is often confused
with porphyry; but, if examined closely, it will be seen that the
outlines of the gas cavities are rounded, while the outlines of a
crystal, like a phenocryst, are always angular. This will be clear if
the amygdoloid on Plate 56 is compared with the porphyry on Plate 55.
The Sedimentary Rocks
To this class belong all those rocks which have been laid down by water
or wind, or are the results of organic depositions. They include loose
material like sand or day, and also the same materials, when cemented
into more or less solid rocks, like sandstone or shale. So long as the
material has not been altered from what it was when laid down, the rock
is termed sedimentary.
In general the material of which these rocks are composed comes from the
weathering and disintegration of other rocks. This does not apply to the
organic deposits, for each type of which there is a peculiar mode of
formation. To illustrate the typical formation of sedimentary rocks, we
may look at the fate of a granite when exposed. At once the surface is
attacked by changes of temperature, frost and rain. The various minerals
of the granite expand and contract with every change of temperature, but
each component mineral has a different coefficient of expansion under
heat, so that minute cracks are quickly formed between the minerals.
Water gets into these cracks and begins to dissolve the minerals.
Feldspar is the most easily attacked, part of it being dissolved and
carried away, a small part changing to quartz, and by far the largest
part changing to kaolin. The dark mineral is also attacked and partly
dissolved, and partly changed to kaolin and iron oxides. The quartz
resists solution almost completely. Of these products the kaolin and
iron oxides are carried far away and deposited in still water. The
quartz and perhaps some of the dark mineral are heavier and carried more
slowly, being deposited as sand. This happens to granite everywhere, but
in the regions where there is frost the action is greatly hastened; for
water gets into the cracks and expands every time it freezes and thus
widens the cracks rapidly, which greatly facilitates the entrance and
movement of water in the rock. In a similar way any original rock will
be disintegrated, and the residue, after the soluble part has been
carried away, becomes sand or clay or mud.
Particles of quartz, kaolin, and lime, separately, or mixed, loose or
more or less cemented, with accompanying impurities, make up the great
bulk of the sedimentary rocks. They are usually arranged in layers, of
varying thickness, as they were laid down by water or the wind. In the
same way layered accumulations which are either products of plants or
animals, or parts of the plants or animals, are considered sedimentary,
as for instance, coal, chalk, petroleum, etc.
A Classification of Sedimentary Rocks
Inorganic origin:
1. Coarse fragmentary material talus
resulting from weathering
2. The same fragmentary material breccia
cemented
3. Unsorted material resulting from soil
rock weathering
4. Coarse fragments rounded by the gravel
action of water and wind
5. The same material cemented conglomerate
6. Finer material deposited by water sand
or wind
7. The same material cemented sandstone
8. The finest material, mostly clay
kaolin, deposited by water
9. The finest material, deposited by loess
wind
10. The same material cemented shale
11. Fine particles of lime, pure or marl
impure
12. The same material cemented limestone
13. Unassorted material left by the till
glacial ice
14. The same material cemented tillite
Organic Origin:
15. Limes made from shells, etc. coquina, chalk, coral rock,
etc.
16. Silica from the shells of plants, diatomaceous earth, etc.
etc.
17. Carbon from plants peat, lignite, coal, etc.
18. Hydrocarbons from animals petroleum, asphalt, amber,
etc.
19. Phosphates from animals guano, phosphate rock, etc.
Talus
Where weathering is very active, especially on or below steep mountain
slopes, a mass of loose, angular fragments accumulates. This material is
termed talus, a term which refers only to the physical character of the
material, and not at all to its composition. If weathering continues
these fragments will be further broken up into one of the finer grained
rocks, which the water can carry away and deposit elsewhere. There is
little or no layering in talus. If the talus is not carried away but is
cemented where it was formed, the resulting mass is termed breccia, but
this is not very commonly the case.
Breccia
Pl. 58
The term breccia is used to cover all those rocks which are composed of
angular fragments, of any composition, and above sand in size, when they
are cemented into a solid mass, by any sort of cementing agent. Here the
term is used in its broad sense, as compared with the way it was used
under igneous rocks.
Breccias may result from the cementing of talus, but more often the
breaking up of the material into angular fragments was due to other
causes, such as crushing along a fault plane, or in the movements
involved in mountain making. In such cases the breccia is of limited
extent, but may occur repeatedly in the same neighborhood. Limestone,
which has been crushed and then recemented, often makes a rock which
takes a good polish and is used in several localities as an ornamental
stone in place of marble, in fact often goes in trade circles under the
name of “marble.” The breccia figured on Plate 58 is such a limestone.
Soil
Over most of the earth’s surface there is a covering of rock waste, the
product of weathering, some of which is unassorted, and some of it
sorted by water or wind. This is all termed soil. It is an ever-moving
cover resulting from the decomposition of the underlying rocks, to which
have been added in places layers of rock waste brought from afar by the
streams. Some soils are rock waste which had been carried clear to the
ocean and deposited on the floor of the sea, and is now above sea level,
because the floor of the sea has been elevated. Inasmuch as the
underlying rocks vary in composition, and as there are areas of
transported material, it is clear that the composition of soils must
vary from place to place, both as to composition and texture.
Soils range from the finest, composed mostly of clay, to coarse ones,
composed of sand, gravel or even boulders. Clay, the finest grained
soil, is composed of particles only about ¹/₁₀₀₀th of a millimeter in
diameter, of which it would take 720,000 billion particles to make a
gram’s weight. Ordinary soils however have about 2 to 5 million
particles to the gram.
The average specific gravity of soil with the usual amount of humus in
it is from 2.55 to 2.75. In this case however the specific gravity is of
less importance than is the volume weight. A cubic foot of water weighs
62½ pounds, that of soil from 75 to 80 pounds, the extremes being 30 lb.
for peaty soil and 110 lb. for calcareous sand. The terms “heavy” and
“light,” used in agriculture do not refer to the volume weight, for clay
which is actually relatively light (70-75 lb. per cubic foot) is classed
as a “heavy” soil; while sand, of much greater actual weight, is classed
as a “light” soil. These terms as used in agriculture refer to the ease
with which the soils are worked, and to their penetrability by plant
roots.
Soil is usually divided into an upper darker-colored layer, termed loam,
and into a lower, lighter-colored layer, termed subsoil. The presence of
humus, resulting from the decomposition of plant and animal remains is
the factor which darkens the color and distinguishes the loam; so that
loam is a complex of inorganic rock particles plus more or less humus,
colloid compounds, bacteria, living plants and animals. The subsoil is
mainly rock particles. The distinctions between these two layers break
down in arid soils, and often also in swampy regions.
It is this layer of soil on which the water of every rain and flood
works, picking part of it up and carrying it along, step by step, to the
sea. Though the amount moved on any one day is small, the sum of all the
soil transported is enormous, a large river carrying annual incredible
amounts. For instance the Mississippi annually deposits in the Gulf of
Mexico 476,900,000 metric tons (2204 lb. to the metric ton), of which
about a third is in solution. At this rate it takes about 7000 to 9000
years to remove a foot from over the whole drainage basin. This is
considerably slower than is the case of some other rivers. While on the
one hand soil is being continuously carried away from the surface, on
the other hand it is being constantly renewed from below, by the
weathering action of water, air and temperature.
Gravel
Gravel is a mass of loose fragments of rock, which have been rounded by
water and deposited with little or no sorting, so that larger and
smaller pebbles and sand all occur together. It is the deposit laid down
by comparatively fast water in inland lakes or along the storm-beaten
shores of the sea. Where a swift stream enters quiet water, as where it
empties into a lake, there it quickly drops its coarse material as
gravel, usually thus building a delta. Gravel also occurs in stream
beds, where for any reason the rate of flow is checked. During the
recent glacial period, the ice sheet brought down great masses of
unsorted material, which was deposited as till, or in moraines. Much of
this was then picked up by the running water and moved longer or shorter
distances, so that, all over the glaciated country of the northern and
eastern United States, there are unusually large numbers of gravel
deposits. Gravels are all water laid, and usually show more or less
clearly the bedded or stratified structure.
The size of the component pebbles of gravel ranges from great boulders
to fine sand, and the finer gravels grade into the coarser sands, the
line between gravel and sand being drawn at about the size of a pea, the
coarser being gravel, the finer sand.
Gravel is widely used as ballast for railroads and in making highways,
because of its tendence to pack well, while the hard pebbles resist
wear. It is also widely used in concrete work, bonding in well with the
cement, and making it go from three to five times as far.
Conglomerate
Pl. 58
Conglomerates are composed of rounded pebbles and sand of varying sizes,
cemented together into a solid rock. The pebbles may run up to boulders
in size, but they have all been more or less rounded by water, and
transported some distance. The pebbles may all be of the same
composition, or may represent a variety of rocks. When the pebbles are
all, or most all, of one sort, the resulting conglomerate is termed a
_quartz-conglomerate_, a _limestone-conglomerate_, a
_gneiss-conglomerate_, etc. So too the cementing material varies in
kind, silica, calcite and iron oxide being the commonest. The color will
depend on both the component pebbles and the cement, sometimes one
dominating, sometimes the other. There are some of the quartz- and
limestone-conglomerates which can be cut and polished to make very
handsome stone.
Conglomerates represent consolidated gravels, and always indicate an
aqueous origin, quite often the delta of an ancient stream, or the
invasion of the sea over the land; so they have become of importance to
geologists in interpreting past events.
Sand
Sand is a mass of small rock particles, from the size of a pea down to
¹/₅₀₀ of an inch in diameter. The material may be any sort of rock, or a
mixture of two or more kinds. Sand may be the result of the
disintegration of older rocks at the point where it is now found, in
which case the grains have the shapes they had in the original rock; but
more often the sand grains have been transported greater or lesser
distances, and in the process have been more or less rounded.
Those sands, which lie where they were formed are called _residual_, and
such sand is usually composed of a mixture of angular grains, some
harder and others softer, such as quartz, feldspar, mica and hornblende,
all mixed together. Where the sand has been transported, only the more
resistant minerals have remained, such as quartz, magnetite,
cassiderite, etc.; with which there are at times rarer minerals, such as
gold, platinum, garnets or topaz. Such sands are known as
_gold-bearing_, _topaz-bearing_, etc.
The sands from different localities differ greatly. The streams gather
the rock particles, and sort them according to the size, which the water
flowing at any given rate can carry. When the water is slowed down, it
drops all the particles above the size which the new rate of speed can
handle. The grains of sand from the bed of a stream are usually more or
less angular. The further they are carried, the more they are knocked
together and rounded; so that after being carried to the sea, and then
thrown up on the beaches, they have been well rounded, especially the
larger grains. As the air is less viscid than the water, sand which is
transported by the wind, is even more rounded; so that desert sands show
the most complete rounding, indeed are even polished; and this is true
even of the smaller grains. It is the wind-blown, or desert sands, which
flow so evenly in an hourglass. Between the angular residual sands and
the polished desert sands, there are of course all grades. Glacial sands
are angular or “sharp” almost to the degree characteristic of residual
sands; and lake-shore sands are between river sands and sea sands in the
degree of rounding.
Sands made of particles of lime, _calcareous sands_, are less resistant
to wear than are those of quartz. In regions where the water is “soft”
(free from lime), they do not last long, as they are dissolved; but in a
limestone region where the water is “hard” (saturated with lime), the
grains are not so quickly dissolved and may accumulate into beds of
great thickness, as in Florida. Along some shores of the ocean, there
occur “green sands,” which are ordinary quartz sands mixed with the dark
green mineral glauconite, which is a potassium iron silicate, forming on
the ocean bottom as a result of the action of decaying animal matter on
iron-bearing clays and potassium-bearing silicates, like feldspar. This
is particularly characteristic of some of the sands along the coast of
New Jersey.
In places, especially in the beds of rivers, there occur “quicksands.”
This is a deposit of fine sand, mixed with a considerable amount of
clay, and saturated with water; so that it will not support the weight
of a man or an animal. Much that goes under the name of quicksand is a
fluid mud, covered with a thin layer of sand.
Sand is used for a wide variety of commercial purposes, and under these
conditions gets various trade names; for instance “glass sand” is a
pure, colorless to white, quartz sand, which is used as one of the
components in making glass. It must be free from impurities, as these
color the glass, and much of the sand used for this purpose is quartz,
crushed to a fine sand-like condition. “Moulding sand” is a rather
fine-grained quartz sand, with a small but very definite admixture of
clay, and this is used to make the moulds for castings in foundries.
“Polishing sand” is one composed of angular fragments of quartz, usually
from stream beds or glacial deposits, or even crushed quartz, and is
used for cutting and polishing marble, for sandpaper, and for polishing
wood and softer stones. There are many other special uses, like
building, ballast, filters, furnaces, etc., in which quartz sand is
used, being screened if necessary to get the right sizes.
Sandstone
When sand of any sort is cemented so as to make a solid rock, it is
termed sandstone, which varies widely according to the size, color and
composition of the grains, and also with the sort and amount of the
cement. When the size of the grains is larger than that of a pea,
sandstone grades into conglomerate; when smaller than ¹/₅₀₀th of an
inch, especially if mixed with clay, it grades into shale. There are all
grades of firmness, due to the amount and kind of cement, ranging from
those which have little or no cement, but are compact as a result of the
pressure of the overlying rocks, to those in which the cement has filled
all the pore spaces. In general there is a considerable amount of space
between the grains of sand; so that a sandstone will absorb large
amounts of water, up to 25% of its bulk. In moist climates where it
freezes, this makes many sandstones unsuitable for use as building
stones, as they are likely to spale, or chip off, as is seen in the
“brown stone” so much used in New York City.
Sandstones are usually bedded rocks and are relatively easy to quarry,
and most of them are not so firmly cemented, but that they can be
readily worked or cut into shape by the stone cutter; and so, certain
sandstones are very popular for building stone or for trimming on
buildings, where they are not too much exposed to the weather.
Sandstone gets a variety of names according to the cement.
Siliceous sandstone is cemented with silica and usually very hard.
Calcareous sandstone is cemented with lime and usually rather soft.
Ferruginous sandstone is cemented with one of the iron oxides.
Argillaceous sandstone is held together with clay impurities, and is
usually both soft and of undesirable color.
According to their composition there is also a number of varieties.
Arkose is a sandstone composed of quartz and feldspar grains, usually
derived from the disintegration of granite and not transported far.
Graywacke is a sandstone composed of quartz, feldspar, and some other
mineral, like hornblende-augite, etc., also derived from the
disintegration of granites and not transported far.
Grit is a term applied to a coarse sandstone, composed of angular quartz
fragments, and used to a considerable extent for millstones.
Flagstone is a thin bedded sandstone, often with mica, which splits
easily and uniformly along the bedding planes; so that it can be
quarried in large slabs. It was widely used for sidewalks before the
advent of concrete.
Freestone is a thick-bedded sandstone, not over hard, so called, because
it can be worked freely and equally well in all directions.
Clay
Clay is a term used to describe a mass of fine particles, the most
prominent property of which is plasticity when wet. Clays range from
masses of pure kaolin to masses of kaolin and related minerals mixed
with as much as 60% of impurities, which may be sand, lime, iron oxides,
etc. The particles of a fine clay range around ¹/₁₀₀₀ of a millimeter in
diameter, while the impurities may be, and usually are, of larger size,
up to the size of sand grains.
All clays are of secondary origin, the result of weathering, especially
of feldspars, though clays may also result from the weathering of
serpentines, gabbros, etc. In some cases after the weathering of
feldspar or limestones, the clay may remain just where it was formed, as
a residual deposit; but, being so fine-grained, it is usually
transported by rain water or by the wind and deposited somewhere else as
a sedimentary bed. The quiet waters of a lake are favorable places for
such deposits, and many clay beds represent former lake bottoms. Impure
clays are often laid down on the flood plains of sluggish streams. In
fresh water the settling of the clay is a very slow process, requiring
days, or when very fine, weeks, before the water wholly clears. In salt
water, however, the clay sort of coagulates, the particles gathering
together in tiny balls, which settle rapidly, so that the water is soon
clear.
According to their mode of origin clays are classified as residual,
sedimentary, marine, swamp, lake, flood-plain, eolian, etc. But when
their uses are considered a very different classification is made, based
mostly on their composition, and we speak of China clays or kaolins,
fire or refractory clays, paving-brick clays, sewer-pipe, stone-ware,
brick, gumbo and slip clays.
The kaolin or china clays are residual clays, usually resulting from the
decomposition of pegmatite dikes. They must be white when burned, free
from iron oxides, and fairly plastic. A good deal of china clay is made
by crushing feldspar.
Ball clays are sedimentary clays which remain white when burned, are
usually very plastic, and free from iron oxides. They are mostly used in
the making of various sorts of china.
Fire clays may or may not have iron oxides in them, but they must be
free or nearly free from fluxing materials, such as lime, magnesia and
the alkalies (sodium and potassium compounds). They may be more or less
plastic, the essential quality being their ability to withstand high
temperatures without fusing. Silica (as sand) tends to diminish the
refractory quality; so that a clay otherwise suitable, if it has sand in
it, becomes at best a second grade fire clay. In coal mining sections it
is customary to term those beds of clay either above or below the coal,
“fire clay”; but this is an unfortunate designation, for though some of
them are true fire clays, the most of them are not.
Stone-ware clays are those with considerable sand and up to five per
cent of fluxing materials. They must be plastic enough to be readily
worked, and then burn to a dense body at comparatively low temperatures.
Sewer-pipe clays must be plastic, and carry a considerable amount of
fluxing material, as the surface of the pipe is expected to vitrify in
the burning.
Brick clays are low grade clays and vary greatly in composition. The
main requisites are that they mould easily and bake hard at relatively
low temperatures with as little warping and cracking as possible. As
most clays shrink both in the air drying and in the baking, sand is
added when the clay is being mixed. The color is mostly due to the
presence of iron impurities. If there are iron oxides and little or no
lime, the brick bakes to a red color, but if there is an excess of lime
over the iron oxides, it bakes to a cream or buff color, which on
vitrifying turns green.
Paving-brick clays range from surface clays, to semirefractory clays,
shale being often used. The essential component is enough fluxing
material, so that the bricks shall begin to vitrify, or fuse, at not too
high temperatures.
Slip clays are those with a high percentage of fluxing material; so
that, when baked at moderate temperatures, the surface fuses into a
glassy brown or green glaze.
Adobe is an impure calcareous clay, widely used in the western United
States for making sun-dried bricks.
Gumbo is a term applied to fine-grained plastic clays which shrink too
much in the burning to be useful in manufactures. They can be burned to
make an excellent ballast for railroads and highways. They are
especially abundant in the Middle Western States.
Loess
This is the name given to a fine grained homogeneous clay-like material,
which is a mixture of clay, fine angular fragments of sand, flakes of
mica and more or less calcareous matter. It is usually without
stratification, and cleaves vertically, so that, when eroded, it forms
steep cliffs. Loess covers great areas in the Mississippi Valley, in the
Rhine Valley, and in North Central China. By some it is thought to be an
accumulation of dust in those regions where the prevailing winds were of
diminished velocity and where the grass or other vegetation has served
to catch and hold the material; by others it is thought of as a river
and lake deposit; and by still others it is thought to be due to the
combination of the two modes, wind and flood. The writer inclines to the
first view expressed.
Shale
Pl. 59
When pure or impure clays, or loess, are consolidated, they are all
grouped under the name shale. It usually possesses a layered or
stratified structure, which makes it possible to split it into thin
layers. Of all the sedimentary rocks shale is the commonest, and it may
occur in all the places where clay could occur, but the most widely
distributed shale is that which made the sea bottom of former times and
is more or less calcareous, like the piece on Plate 59, in which bits of
shells are still visible. Shale has the same wide variation in
composition as has clay, the various types being designated according to
the impurity which is present, as:
_argillaceous shale_, made mostly of clay,
_arenaceous shale_, shale with more or less sand as an impurity,
_calcareous shale_, or one with more or less lime as an impurity,
_ferruginous shale_, or one with iron compounds as impurities,
_bituminous shale_, or one colored black by the presence of organic
matter, remains of either plants or animals.
While of no value as building material, shale may be ground or crushed,
and used as a substitute for any corresponding clay, and thus many
manufacturers use shale in making fire-clay products, bricks, tile, etc.
Marl
Where limestones or shells of any sort have been pulverized, and mixed
with more or less impurities, especially clay, the resulting
unconsolidated mass is known as marl. It is usually associated with
marine formations, and is the finer débris which has settled on the
ocean bottom well out from shore, that is out beyond the sandy and mud
deposits. Finding it therefore usually indicates a sea bottom recently
elevated. It is very characteristic of the southern coastal states, from
Maryland all along to Texas.
Limestone
Any mass of marl, or aggregate of calcareous shells, corals, etc., which
has become consolidated is known as limestone. It may, and usually does,
have a wide range of impurities, chief of which are clay, sand, iron
oxides, and bituminous matter, like plant or animal remains. Pure
limestone is white, but due to impurities it ranges through grays,
greens, browns, to black, and even red, but this last is rarer. It is
easily identified by the presence of calcium carbonate, which
effervesces in hydrochloric acid. It most often represents deposits in
fairly deep water on ocean bottoms of the past, but there is also a wide
range of limestones which were formed in fresh water.
Limestone is often burned at temperatures just above 900° C, at which
point carbon dioxide goes off as a gas, and leaves calcium oxide, or
lime. When this is mixed with water it makes calcium hydroxide, or
slaked lime, which is mixed with sand to give it body, and is used as
mortar. When exposed to the air, the slaked lime gives up water, and
takes back from the air carbon dioxide, and again becomes calcium
carbonate with its original hardness. Limestone is also used as one of
the elements in all cements. It is also considerably used as a building
stone, which, however, suffers in moist climates from the solution of
its lime by rains, but has stood up very well in dry climates.
The varieties of limestone are mostly distinguished according to their
mode of origin, some of them being as follows.
Bog Lime is a white calcareous powdery deposit on the bottom of ponds in
limestone regions, a deposit precipitated from solution by the action of
the plants inhabiting the ponds.
Coquina (Plate 59) is the rock formed by the rather loose consolidation
of shells and shell fragments. It is particularly characteristic of
tropical regions, and is very abundant near St. Augustine, Fla., in
which region it was, and still is, cut into blocks and used for building
stone. In that mild climate it has stood very well.
Chalk (Plate 60) is a soft fine-grained limestone, formed in the ocean
by the accumulation of myriads of the tiny shells of Foramenifera, which
are single celled animals, living either a floating life near the
surface of the sea, or a creeping life on the bottom. Chalk is composed
mostly of the shells of floating Foramenifera, which when the animals
died, settled to the bottom and there accumulated, mostly at depths of
600 feet or more. When the mass of unconsolidated shells is dredged up
from depths of 50 to 2000 fathoms, it is known as _Foramenifera ooze_.
Chalk beds are then indications of an uplifted sea bottom. When
consolidated, if pure or nearly so, it makes a white chalk, and the beds
may be of considerable thickness, as is the case of the famous cliffs
near Dover on either side of the English Channel. One of Huxley’s most
famous lectures is the one on chalk, found in his _Essays and Lay
Sermons_.
Coral Rock is made by the cementation of fragments of corals. The
binding material, as in most stones, is lime; and this sort of rock is
associated with coral reefs of either the past or the present. One of
the best illustrations of this being the “Dolomite Mountains” in Tyrol.
Coral rock, like coquina, has been cut into blocks and used as building
stone, as in Bermuda.
Encrinal Limestone (Plate 60) is a rock made by the cementation of
fragments of the skeleton of crinoids. These animals belong to the
group, echinoderms, and are now extinct except for a few so called
“sea-lilies.” They were animals with a central mouth surrounded by long,
jointed, flexible arms in multiples of five, and below this a small body
inclosed in calcareous plates, all at the top of a long jointed stem.
They lived in the sea and in the earlier geological times must have been
very abundant; for their remains are so common in places as to make
whole layers of limestone.
Hydraulic Limestone is a fine-grained, compact, yellowish limestone with
from 13 to 17% of sand, and some clay; which, when it is burned at a
temperature a little higher than that used in burning lime, makes a
product, that, while not as strong as Portland cement, still like it
sets under water.
Lithographic Limestone is a very fine-grained, compact limestone with
clay impurities, the finest of the grain making it usable for making the
stone plates used in lithographic printing. On slabs of this limestone
figures are drawn in reverse with a special crayon. Then the slab is
treated with acid, those parts which are not protected by the drawing
being etched away, while the points protected by the drawing remain in
low relief. From this slab figures can then be printed.
Travertine is a general name, applied to calcareous deposits from fresh
water lakes or streams, and has been precipitated either as a result of
cooling or evaporation. Some travertines are porous, while others are
dense; some are white, while others are colored, often beautifully, by
impurities in the water.
Porous deposits of travertine, when made on grass or other like
substances, are known as tufa or _calc sinter_. Such masses are common
around Caledonia, N. Y., Mammoth Hot Springs in the Yellowstone Park,
etc.
Onyx marble is a dense travertine, usually formed as a result of the
deposition of lime from the water of springs. It is often banded, due to
the presence of impurities in the water at one time, and their absence
at other times.
Till
Till is an unconsolidated mass of boulders, pebbles, sand and fine clay,
the unsorted material left behind by glaciers when they melted. The
boulders and pebbles, while they show some wear, are not rounded like
those that have been transported by streams, but have a more or less
angular shape; and some of them are polished or striated on one side,
where, while frozen in the ice, they were rubbed along the bottom.
One of the most recent geological events in America was the extension of
the ice sheet, now covering Greenland, down over north and northeastern
North America, until it extended as far south as northern New Jersey,
the Ohio River and the Missouri River, and as far west as the Rocky
Mountains, but not over the Great Basin, the Cascade Ranges or Alaska.
This great mass of ice, thousands of feet thick, moved from two centers,
one either side of Hudson Bay, scraping up the loose soil, and grinding
off the exposed surfaces of the underlying rock. All this material it
carried southward, until the melting along its lower margin equaled the
rate at which it advanced. When the melting was faster than the advance
the glacial sheet retreated. At the southern limit of the advance this
débris was dropped, either making long ridges (moraines) or while the
ice was retreating, thicker or thinner sheets. This deposited débris is
till.
The soil, and especially the subsoil, in all the regions formerly
covered by the ice sheet, is made up very largely of this till; which,
where it is undisturbed is often called “hardpan.” When till is mixed
with humus it becomes loam. This mixture of material, varying all the
way from the fine powdered products of the ice grinding to the great
boulder it picked up and carried south, is characteristic of this or any
other glaciated country. When this section of country was settled, the
boulders and stone were a hindrance to cultivation, and were picked up
and piled into stone walls, which are one of the first features to
strike the eye.
Tillite
When till is consolidated into solid rock, it is known as tillite. In
several cases it has been found buried far beneath the more recent
sedimentary rocks; testifying that there were other glacial periods
beside the last one which furnished the till.
The Coal Series
Disregarding minor constituents, the plants are largely made up of
cellulose, which is a combination of carbon, hydrogen, and oxygen,
(C₆H₁₀O₅). If this is heated in the air, where there is plenty of
oxygen, it disintegrates, or burns, making carbon dioxide and water; but
if the heating is done where the oxygen is excluded, as in a kiln, the
hydrogen and oxygen will be driven off and the carbon will remain behind
as charcoal. In Nature similar reactions go on, but more slowly.
Vegetable matter, exposed to the air, disintegrates into carbon dioxide
and water, and there is no solid residue. However, if the vegetable
matter is under water, which excludes the air more or less completely
including the oxygen in it, then disintegration still takes place, but
the products formed are water, (H₂O) marsh gas (CH₄), and some carbon
dioxide (CO₂), but a considerable part of the carbon remains behind and
accumulates.
Thus in bogs, swamps and ponds, where dead vegetation, especially that
growing in the water, piles up, the oxidation is incomplete; so that
there gradually accumulates on the bottom a layer of brown to black mud,
known as _peat_. More plant remains are constantly being added, and the
layer may increase to several feet in thickness. The decomposition is
incomplete and some oxygen and hydrogen remain, but the carbon is in a
constantly increasing ratio and in proportion far above that in
cellulose. In the cold northern climates sphagnum moss is the most
efficient peat producing plant, but in temperate and tropical climates
the moss is replaced by the leaves, twigs, trunks, etc., of trees,
bushes, and vines.
If these peat beds are buried beneath a layer or layers of sediment,
especially clay, the peat is sealed up and oxidation stops almost
entirely. With the pressure of the superincumbent beds, the peat becomes
more and more compact, and changes to a dark-brown or black color. It is
then known as _lignite_. If this lignite is buried still deeper, with
consequently more pressure and more time, it changes into the still
denser black _bituminous coal_. This is as far as it will go unless some
new agent is added to the forces already working.
The next step in the series of changes forming coal is associated with
mountain making. In case the layers of rock containing beds of coal are
folded, and that presupposes at least a moderate increase in heat, the
bituminous coal is altered to _anthracite_, which is still denser, and
so hard that it breaks with a conchoidal fracture. Alteration may be
carried a step still farther, in case the rocks between which lie beds
of coal are effected by such high temperatures as accompany
metamorphism. Then all the associated hydrogen, oxygen and moisture are
driven off, and only the carbon remains, which is then known as
_graphite_. All steps between the stages especially designated occur.
The following represent steps only in the series of changes.
Peat
Peat is a mass of unconsolidated vegetable matter, which has accumulated
under water, and in which the original plant remains are still, at least
in part, discernible. It contains a large amount of water, so that
before it can be used as a fuel, it is cut out in blocks, which are
piled up and left for a time to dry before using. It burns with a long
flame and considerable smoke. This country is so well supplied with
other fuels, that so far peat has been but little used.
Lignite
_brown coal_
Lignite is more compact than peat, and is found buried to some depth
under layers of clay or sandstone. It is dark brown to black in color,
and still retains pretty clear traces of the plants from which it was
derived. It also usually contains a considerable amount of moisture, and
when this is dried out, it tends to crumble badly, so that it is
undesirable to handle it much, or to ship it far, before using. It has a
fair fuel value and is fairly widely used; but it is very desirable that
some method be found, by which lignite could be treated to obtain its
by-products, and at the same time make it more compact, so it would not
crumble with the handling incident to using it in furnaces. There are
extensive lignite deposits in this country in North and South Dakota,
Montana, Wyoming, Colorado, New Mexico, Texas, Louisiana, and
Mississippi.
Bituminous Coal
_soft coal_
This type of coal is compact, black in color, and breaks readily, but
does not crumble as badly as lignite. It contains considerable water,
and still has some hydrogen and oxygen compounds in it. Bituminous coal
is the product of plant remains which have been preserved for long
periods, (millions of years), sealed from the air by the overlying beds
of rock. The pressure has made it compact, and nearly all traces of the
original plants have disappeared.
Bituminous coal is our most abundant fuel, occurring the world over in
seams from less than an inch in thickness to some over fifteen feet
thick. The United States is peculiarly fortunate in the abundant and
easily accessible deposits of this type of coal, in Pennsylvania, West
Virginia, Ohio, Kentucky, Tennessee, Indiana, Illinois, Michigan, Iowa,
Missouri, Kansas, Nebraska, Texas, Utah, and Colorado.
The volatile constituents, hydrogen and oxygen compounds, of bituminous
coal may be driven off by heating the coal in closed ovens, and the
residual mass is known as _coke_, almost pure carbon. This is
distillation, and the ovens in which this is done, without trying to
save the volatile products, are called bee-hive ovens, while the more
modern ovens which save the by-products are called by-products ovens. A
ton of bituminous coal treated in the typical by-products oven, will
yield on the average 1410 lb. of coke, 7.1 gallons of tar, 18.9 pounds
of ammonia sulphate, etc., 2.4 gallons of light oils, 10440 cubic feet
of illuminating gas, about half of this last being used to furnish the
heat for the distillation. The coal-tar dye industry is built on the tar
thus produced. Toluol, benzol, etc., come from the light oils; and half
the gas produced is available for household illumination, etc. Coke is
demanded, as it is a superior fuel for melting iron ores, iron and
steel, and is made regardless of whether the by-products are used. The
coke thus produced is hard, clean, and vesicular; but for some reason as
yet unknown, by no means all bituminous coal will produce a coke which
has this porous structure. These latter are known as “non-coking,” and
are of little use to the steel industry.
Cannel Coal
This is a compact variety of non-coking bituminous coal, with a dull
luster and a conchoidal fracture. It contains the largest proportion of
volatile hydrocarbon compounds of any variety of coal; so that when the
supply of petroleum and natural gas gives out, this will be one of the
important sources of obtaining substitutes. Cannel coals occur in Ohio,
Indiana, and eastern Kentucky. This cannel coal owes its peculiar fatty
nature to the material from which it is derived, it being supposed to
have resulted from the accumulation of the spores of lycopod trees, and
their conversion to jelly-like masses by bacteria in the fresh-water
marshes of those ancient days.
Anthracite
_hard coal_
Anthracite coal is hard, black, has a luster, and breaks with a
conchoidal fracture. It contains but a low percentage of volatile
matter, and so burns with a short flame, and less smoke, than is the
case with the other coals. It is always associated with folded rocks,
and appears to have been formed as a result of the combined pressure and
the higher temperatures, which accompanied mountain making. Still the
temperature was not high enough to metamorphose the adjacent rocks. Most
of our anthracite comes from northeastern Pennsylvania.
Carbonite
Carbonite is natural coke. It occurs in coal seams which have been cut
by dikes or intrusions of igneous rocks, the coal having been thus coked
by natural processes. It is not vesicular like artificial coke, for
which reason it is not useful as a fuel. Some carbonite is found in the
Cerillos coal field of New Mexico, in Colorado, and Virginia.
Jet
Jet is a dense variety of lignite, a fossil wood of black color, which
takes a high polish and cuts easily into various ornamental shapes. It
has been used for ornaments since early ancient times, beads of jet
being found in the early bronze period in England, the supply probably
coming from the Yorkshire coast, whence the principal supply comes even
to the present day. In Switzerland and Belgium it was used still
earlier, even as far back as the Palæolithic age. Jet seems then to have
had a talismanic value, and to have been worn to protect the owner.
About 700 A.D. crosses and rosaries began to be made of jet, the custom
starting at Whitby Abbey, the material being obtained nearby, so that it
came to be known as “Whitby jet,” and in the eighteenth century became
very popular. In recent times it has been used mostly as jewelry
suitable for mourning.
Amber
Pl. 61
Amber is a gum which oozed from coniferous trees and was petrified. It
is associated with lignite beds of middle Tertiary age. It is usually
pale-yellow in color, but at times has a reddish or brownish tinge, and
is more or less transparent. It occurs in rounded irregular lumps, up to
ten pounds in weight, though most pieces are smaller; and is mostly
picked up along certain coasts where it is washed ashore by the waves.
Since the earliest records amber has been cast up on the shores of the
Baltic, and it was used by peoples as early as in the stone age for
ornaments and amulets. It has been found among the remains of the cave
dwellers of Switzerland, in Assyrian and Egyptian ruins of prehistoric
age, and in Mycenæ in the prehistoric graves of the Greeks, the first
recorded reference to it being in Homer, and the Greek name for amber
being _elektron_ from which our word electricity comes. All these finds
were of Baltic amber which was doubtless gathered and traded by those
early men. Even down to the present many men make their living, riding
along the shore at low tide and hunting for the amber washed ashore by
the waves. As early as 1860 the German geologists concluded that the
source of the amber must be lignite beds outcropping beneath the sea
level, and started mining for the amber with fair success, so that today
two types of Baltic amber are distinguished, “sea stone” which is washed
ashore, and “mine stone” taken from the mines. Beside the Baltic
locality, it is found along the shores of the Adriatic, Sicily, France,
China, and occasionally of North America.
Some pieces of amber are found with insects inclosed and preserved
almost as perfectly as if collected yesterday. They were apparently
entangled in the gum while still viscid and completely embedded, before
fossilization.
The Petroleum Series
Certain sedimentary rocks contain larger or smaller quantities of
natural gas, petroleum, mineral tar and asphalt. These are compounds of
carbon and hydrogen, or hydrocarbons, and range from gases to solids,
each being a mixture of two or more hydrocarbon compounds. The crude
petroleum may have either a paraffin base or an asphalt base: in the
former case, when the gas, gasoline, kerosene, etc., have been removed
by distillation, the solid residue will be paraffin, as in most of the
Pennsylvania crude oils; while in the latter case, the solid residue
will be an asphalt, as in most of the California and Texas crude oils.
In the case of the paraffin series all the compounds belong to the
paraffin group, while the asphalt is due to the presence, in addition to
the paraffin group, of some of the benzine series of hydrocarbons.
Petroleum is found in sands and shales, which were originally deposited
on ancient sea bottoms, the shales generally being the real source of
the petroleum. The oil was once the fatty portion of animal bodies
(perhaps to some extent of plant bodies), and was separated during
decomposition as a result of bacterial activity. Oil thus produced is in
tiny droplets, which have a great affinity for clay. After being freed
by the bacteria, the oil droplets in muddy water attach themselves to
particles of clay, and as the clay settles the oil is carried down with
it, the two eventually making a bituminous shale. In clear water, or in
water which is in motion, the oil droplets rise to the surface and
eventually distill into the air.
The oil, or petroleum, may stay diffused through the shales, in which
case we have _oil-bearing shales_, with sometimes as much as 20% of oil.
Were there but ¹/₁₀₀₀ of a per cent of oil in a layer of shale 1500 feet
thick, this would amount to 750,000 barrels per square mile which is
equal to a rich production from wells. When the oil in shale amounts to
three per cent or more, it is commercially usable. There are large
stretches of petroleum-bearing rocks in New York, Pennsylvania, Ohio,
Indiana, and all the way out to the Pacific coast, some of them with oil
so abundant, that a blow of the hammer will cause them to smell of
petroleum.
In case these oil-bearing shales have been heavily overburdened and
compressed, the petroleum may have been more or less completely pressed
out of them. Then the droplets uniting have formed a liquid, which has
moved out from the shale, and gone wherever it could find open spaces.
Sandstones have frequently offered their pore space, and as it filled,
have been thus saturated with petroleum. If the sandstones were open to
the air, or if fissures extended from them to the surface, the oil has
escaped to the surface and evaporated into the air. But in those cases
where the sandstone (or other permeable rocks) was covered by an
impervious layer, like a dense shale or clay, the oil was confined below
the covering layer of rock. Crude oil is lighter than water; so that
when natural gas, petroleum and water were all present in the rocks, the
gas lies on top, the petroleum next, and the water underneath. With this
in mind it is easy to see, that in slightly folded or undulating layers
of rock, the gas and petroleum would be caught under upraised folds and
domes. This is the basis of prospecting for oil.
If petroleum-bearing layers are depressed far enough beneath the surface
to be affected by the high temperatures of the earth’s interior, or have
been near volcanic activity, of course the petroleum has been distilled
by natural processes, and at most only the residues, like paraffin or
asphalt, have remained. For this reason it is impossible to find
petroleum in igneous or metamorphic rocks.
Natural gas
Natural gas is the lightest portion of crude oil, and consists mostly of
marsh gas (“fire damp,” CH₄) together with other light hydrocarbons,
like ethane (C₂H₆), ethylene (C₂H₄), and some carbon dioxide and
monoxide. It is colorless, odorless, and burns with a luminous flame.
Mixed with air it is explosive. It is found in sedimentary rocks, mostly
sandstones, either with or without petroleum. Usually it is under
considerable pressure, and escapes with great force wherever a hole
permits. In time the gas all escapes through the hole or well, and then
the well “runs out.” If petroleum is present under the natural gas, the
hole may become an “oil well,” from which petroleum may be pumped, until
it in turn is exhausted. The end of an oil supply is usually indicated
by the appearance of water in the well. Natural gas is mostly associated
with oil districts, as in Pennsylvania, Ohio, Illinois, Texas,
California, etc.
Petroleum Crude Oil
Pl. 61
Petroleum is a mixture of paraffin compounds all the way from the gases,
through gasoline, kerosene, lubricating oils, and vasoline to paraffin.
In some of the crude oils there is also an admixture of compounds from
the benzine series, in which case, when all the volatile compounds have
been distilled off, an asphalt remains. The different components of
petroleum may be separated out by heating the crude oil in closed tanks,
and drawing off the various substances at the proper temperatures.
Petroleum occurs in sedimentary rocks of marine origin, usually rocks
which also contain the shells of some of the animals, the soft parts of
which made the oil. To have been preserved the millions of years since
the petroleum was first formed, the oil-bearing layers must have been
covered by some impervious layer of rock, beneath the domes and
anticlines of which the oil has lain ever since. When such a dome or
anticlinal fold is perforated by a well, the released oil flows to the
surface with a greater or less rush, according to the pressure. Wells
may keep flowing for 20 years, sometimes more, sometimes much less.
Those which flow with the greatest pressure usually are relatively short
lived, at times lasting only a year or two. When this easily obtained
oil is exhausted, there is an even greater supply to be obtained by the
distillation of the bituminous shales. Petroleum never occurs in igneous
or metamorphic rocks, but is found in either sandstones or shales, in
places favorable for accumulation, all across that great stretch of
ancient sea bottoms, extending from the Appalachian Mountains to the
Rocky Mountains, and in the Great Basin between the Rocky Mountains and
the Sierra Nevada Range, and also to the west of the Sierras.
Bitumen
Where petroleum has escaped through pores in the rocks, or by way of
fissures, and has come to the surface of the earth, the lighter
components, thus exposed to the air, have vaporized and escaped, leaving
behind a more or less solid residue, which is known as bitumen. If the
escape was through a fissure, the bitumen may have accumulated in the
fissure until it was filled, making vein bitumen. Or the escape may have
been so rapid that the petroleum formed a pool or lake from the surface
of which evaporation took place. In time such a pool will give off the
gases and volatile compounds, only a residue remaining to make a pitch
lake, like the one at Rancho Le Brea near Los Angeles, or an asphalt
lake like the one on the island of Trinidad. On account of their varying
hardness and composition, some of these bitumens have received special
names; as:
Albertite, a black bitumen with a brilliant luster on broken surfaces, a
hardness between 1 and 2, and a specific gravity a shade over 1.
Grahamite, a black bitumen, which is brittle, but has a dull luster, a
hardness of 2, and a specific gravity of 1.15.
Gilsonite or Uintaite, a black bitumen with a brilliant luster and a
conchoidal fracture, a hardness of 2 to 2½, and a specific gravity of
1.06.
Malta is a semi-liquid viscid natural bitumen, which has a considerable
distribution in California.
The above varieties of bitumen look a good deal like coal, but are
easily distinguished by their lightness (weight about half that of
coal), and the fact that with only moderate heat they melt, and become a
thick liquid like tar.
Guano
Guano is the accumulation of the excrement of birds (or of other animals
like bats) on areas so dry that, though soluble, it is not leached and
washed away. It may also contain some of the bones and mummified
carcasses of the birds which died on the spot. The greatest of these
deposits are on several small islands, just off the west coast of Peru,
and now “farmed” by the Peruvian government. In this country there are
no true guano beds, except a few accumulations of bat guano in certain
caves of Kentucky and Texas, but these are not large enough to become of
commercial importance.
Phosphate Rock
Phosphate rock is one composed chiefly of calcium phosphate along with
various impurities, such as clay and lime. It occurs in beds, irregular
masses, or as concretionary nodules in limestone or sand.
The bedded varieties are in the older sedimentary rocks, in which the
phosphate runs from a small percentage up to as high as 85%. Ultimately
the phosphate came from either animal excrement, or from bacterial
decomposition of animal carcasses and bones. In all the beds it seems to
be true that in the first instance the phosphate was laid down as a
disseminated deposit in marine beds, usually limestones. Later by the
action of water leaching through the rocks, the phosphate was dissolved,
and then redeposited elsewhere in a more concentrated form. This may be
either in the underlying sandstones, but is more often in limestones,
replacing the original lime.
In these secondary deposits, if the phosphate has been laid down in
cavities, the resulting phosphate will be in nodular masses. In the case
of the Florida and Carolina deposits, these nodules have been freed from
their matrix and washed along the river beds, remaining as pebbles in
the river sands. The bed deposits are mostly in Kentucky and Idaho. The
commercial use for such phosphate rocks is of course the making of
fertilizers.
Diatomaceous Earth
Pl. 62
Diatoms are tiny single-celled plants living in uncounted millions in
the fresh and salt water. Each diatom builds around itself two shells
which fit into each other like the cover and box of a pill-box, and each
shell is marvelously ornamented. The shells are composed of silica of
the opal type. In size the diatoms range from ¹/₅₀₀₀ of an inch in
diameter up to the size of a pin head, and they live in such numbers
that ordinary surface waters have hundreds of them to the quart, and
where they are flourishing up to 250,000 in a quart. When the plants
die, or in order to reproduce abandon the shells, these shells fall to
the bottom of the pond or the sea, and there accumulate, often making a
layer from a few inches thick up to hundreds of feet in extreme cases.
If unconsolidated, this mass of tiny shells is known as diatomaceous
earth; but if they are consolidated it is called tripolite, so named
because the first of them used commercially came from Tripoli.
As the shells are tiny and uniform in size and have a hardness of 6, the
diatomaceous earth is used to make a great variety of polishes, scouring
soaps, tooth paste, as a filler in certain kinds of paper, in making
waterglass, as an absorbent for nitroglycerine, and as packing in
insulating compounds, where asbestos would otherwise be used.
Deposits of freshwater diatoms are found all over the United States,
usually in thin layers of limited extent, especially in Massachusetts,
New York, Michigan, etc. The marine deposits of diatoms are on a much
larger scale, there being beds of diatoms in Anne Arundel, Calvert and
Charles Counties, Md., up to 25 or 30 feet in thickness. In Santa
Barbara County, Cal., there is one bed 2400 feet thick and another 4700
feet thick, beside many other smaller ones. The enormous former wealth
of life indicated by these great deposits may be suggested, when it is
remembered that it takes about 120,000,000 to make an ounce in weight.
They reproduce on an average about once in five days, so that from a
single diatom the offspring possible under favorable conditions would
amount to over 16,000,000 in four months or over 60 tons in a year. Of
such an order is the potential increase of animals or plants, no matter
how small, if the rate of reproduction is high.
Metamorphic Rocks
Either a sedimentary or an igneous rock, which has been altered by the
combined activities of heat, pressure and chemical action, becomes a
metamorphic rock. The process is essentially one, during which the
layers of rock come under the influence of such temperatures as are
associated with the formation of granite or lavas. Such material as is
actually melted becomes igneous rock, but adjacent to the masses
actually melted are other rocks which do not melt but, according to the
temperature, are more or less changed, and these are the metamorphic
rocks. At a distance from the molten masses the changes are minor, but
close to the molten magmas extensive changes take place. Though not
actually melted the rock near the heat center may be softened, usually
is, in which case pebbles and grains or even crystals become soft and
plastic, and, as a result of the great pressure, are flattened, giving
the rock, when it cools again, a striated appearance. At these high
temperatures the water in the rock and also some other substances
vaporize, and the hot steam and vapor are active agents in making a
great many chemical changes. In some cases material like clay is changed
into micas, or chlorite, etc.; in other cases the elements of a mineral
will be segregated and large crystals will appear scattered through the
metamorphic rock, such as garnets, staurolites, etc.
If one studies a layer of rock both near and far from the molten mass,
all grades of change will appear. For example, at a distance a
conglomerate maybe unaltered; somewhat nearer the molten mass, the heat
and steam may have softened (but not melted) the pebbles and then the
pressure has flattened them as though they were dough; and nearest the
molten mass, the outlines of the pebbles are lost, only a layered effect
remaining, and many of the materials have changed into new minerals,
like mica, garnets, etc., but still the layered effect is preserved.
One of the effects of heat and pressure is to flatten the component
particles of the rock, so that it tends to split in a direction at right
angles to the direction of the pressure, just as particles of flour are
softened and flattened under the pressure of the roller; and then when
the crust is baked it splits or cleaves at right angles to the direction
in which the pressure was exerted by the roller. This tendency to split
is not to be confused with either the layering, characteristic of
sedimentary rocks, nor the cleavage characteristic of minerals. It has
nothing to do with the way the particles were originally deposited, nor
with their cleavage; but is due to the pressure, and resembles the pie
crust splitting, being irregular and flaky. This is designated
_schistosity_ if irregular and _slaty cleavage_ if regular. Schistosity
refers to the flaky manner of splitting into thin scales as in mica
schists. Slaty cleavage is more regular, this being due to the fact that
the material of which slate is made is small particles of clay of
uniform size.
The metamorphic rocks are generally more or less folded, as they are
always associated with mountain making. These major folds are of large
size, from a hundred feet across to several miles from one side to the
other. Such folds may also occur in sedimentary rocks or even in igneous
rocks and simply express the great lines of yielding, or movement of the
crust of the earth. In addition to this there is minor folding or
contorting which is characteristic of metamorphic rocks only. When the
rocks were heated by their nearness to the molten igneous magmas, they
must expand, but being overburdened by thick layers of other rocks,
there is no opportunity for yielding vertically, so the layers crumple,
making minor folds from a fraction of an inch to a few feet across. Such
crumpling, which is so very conspicuous especially where there are bands
of quartzite in the rock, is entirely characteristic of metamorphic
rocks. It is seen on hosts of the rocks about New York City, all over
New England, and in any other metamorphic region. Plate 63 is a
photograph of such a crumpled rock which has been smoothed by the
glacial ice.
The metamorphic rocks are the most difficult of all the rocks to
determine and understand, because the amount of change through which
they have gone is greatest, but for this same reason they offer the most
interest, for the agents which caused the changes are of the most
dramatic type of any that occur in Nature. From one place to another a
single layer of metamorphic rock changes according to the greater or
less heat to which it was subjected, making a series of related rocks of
the same composition but with varied amount of alteration. For this
reason in naming metamorphic rocks, a type is named, and from that there
will be gradations in one or more directions, both according to
composition, and according to amount of heat involved. If it is possible
to follow a given layer of metamorphic rock from one place to another
this is of great interest; for by this means, many variations in the
type will be found, both those resulting from a different amount of
heat, and those due to the local changes in the composition of the
original rock.
One further consideration has to be kept in mind. When a rock is
metamorphosed the high temperatures either drive off all water, or the
water may be used up in the making of some of the complex minerals. When
such a metamorphic rock later comes near the surface and is exposed to
the presence of ground water, and that leaching down from the surface
into the rocks, several of the minerals formed at high temperatures will
take up this water and make new minerals such as serpentine, chlorite,
etc. They are always associated with metamorphic rocks, and have been
metamorphic rocks, but since then have become hydrated, forming minerals
not at all characteristic of high temperature.
The following shows the relation of the sedimentary and igneous rocks to
their metamorphic equivalents.
_Loose sediment_ _Consolidated sediment_ _Metamorphic
equivalent_
gravel conglomerate gneiss
sand (quartz) sandstone quartzite
mud (sand and clay) shale schist
clay shale slate or phyllite
marl limestone marble
peat bituminous coal anthracite to
graphite
coarse igneous rocks such as gneiss
granite, syenite, etc.
fine igneous rocks such as schist
trachite, rhyolite, etc.
In working out the past history of any given region, much of it is done
on the basis of this series of equivalents. The finding of limestone,
for instance, indicates that the given area was at one time under the
sea to a considerable depth, that is from 100 to 1000 feet, but not
ocean-bottom depths which run in tens of thousands of feet. Marble
indicates the same thing, and so one can go on through all these types
of rock.
Gneiss
Pl. 64
Gneiss is an old word used by the Saxon miners, and is often very
loosely used. Here it is used in its structural sense, and a gneiss may
be defined as: a banded metamorphic rock, derived either from a
sedimentary or an igneous rock, and is composed of feldspar, quartz, and
mica or hornblende, and is coarse enough, so that the constituent
minerals can be determined by the eye. It corresponds to a granite, or
some sedimentary rock like gravel or conglomerate.
Due to the action of pressure, all the gneisses are banded, and the
original constituent particles or crystals are distorted. The lines of
banding may be long or short, straight, curved or contorted. When the
banding is not conspicuous, the gneiss tends toward a granite. When the
banding is thin and the structure appears flaky, the gneiss tends toward
a schist. The color varies according to the constituent minerals, from
nearly white, through red, gray, brown, or green to nearly black. Plate
64 shows one gneiss which is in a less advanced stage, the pebbles being
simply flattened and the matrix partly altered to micaceous minerals,
and a second gneiss which is so far advanced that the original
constituents are all altered to other minerals and only the banded
structure remains. This latter type would have required but little more
heat to have completed the melting and changed this to a granite.
Gneisses are very compact and have little or no pore space in them. They
are hard and strong and resist weathering well, so that they are widely
used as building stone: but they are not as good as granite for this
purpose, as they split more readily in one direction and can not
therefore be worked so uniformly as can granite.
There are many varieties of gneiss, based either on their origin,
composition, or their structure, as follows:
Granite-gneiss is one derived by metamorphism from granite.
Syenite-gneiss is one derived by metamorphism from syenite.
Diorite-gneiss is one derived by metamorphism from diorite.
Gabbro-gneiss is one derived by metamorphism from gabbro.
Biotite-gneiss is one composed of quartz, feldspar and biotite.
Muscovite-gneiss is one composed of quartz, feldspar and muscovite.
Hornblende-gneiss is one composed of quartz, feldspar and hornblende.
Banded-gneiss is one in which the banded structure shows clearly.
Foliated-gneiss is one in which there is thin irregular layering.
Augen-gneiss is one which has concretionary lumps scattered through
it.
Gneisses have a wide distribution over all New England, most of Canada,
the Piedmont Plateau, the Lake Superior region, the Rocky Mountains, the
Sierra Nevada and the Cascade Ranges.
Quartzite
Quartzite is metamorphosed sand or sandstone, and frequently grades into
one or the other. It is a hard compact crystalline rock, which breaks
with a splintery or conchoidal fracture. It is distinguished from
sandstone by the almost complete lack of pore spaces, its greater
hardness and by its crystalline structure. In practice it may be
distinguished by the fact that a sandstone in breaking separates between
the grains of sand, while a quartzite breaks through the grains.
Some quartzites are almost pure quartz, but others contain impurities of
clay, lime or iron, which were in the original sandstone. These alter in
the metamorphism to such accessory minerals as feldspar, mica, cyanite,
magnetite, hematite, calcite, graphite, etc. The color of quartzite when
pure is white, but may be altered to red, yellow, or green by the
presence of these accessory minerals.
On account of the difficulty of working the quartzites, they are not
much used in building, though they are very durable. When crushed they
often make excellent road ballast, or filling for concrete work. The
pure varieties are sometimes ground and used in the manufacture of
glass.
According to the accessory mineral, the following varieties may be
distinguished; chloritic-quartzite, micaceous-quartzite,
feldspathic-quartzite, etc.
Quartzites are common in the New England, the Piedmont Plateau, and Lake
Superior metamorphic regions, and also in many western localities.
Schist
Pl. 65
Schist is a loosely used term, but is used here in its structural sense.
It includes those metamorphic rocks which are foliated or composed of
thin scaly layers, all more or less alike. The principle minerals are
recognizable with the naked eye. In general schists lack feldspar, but
there are some special cases in which it may be present. Quartz is an
abundant component of schists; and with it there will be one or more
minerals of the following groups: mica, chlorite, talc, amphibole or
pyroxene. Frequently there are also accessory minerals present, like
garnet, staurolite, tourmaline, pyrite, magnetite, etc.
All schists have the schistose structure, and split in one direction
with a more or less smooth, though often irregular, surface. At right
angles to this surface they break with greater or less difficulty and
with a frayed edge. As they get coarser, the schists may grade into
gneisses, losing their scaly structure: while on the other side, as the
constituent minerals become finer and so small as to be difficult of
recognition, schists may grade into slates.
The varieties of schist are based on the mineral associated with the
quartz; as mica-schist, chlorite-schist, hornblende-schist, talc-schist,
etc.
The color also is due to the constituent minerals other than quartz and
ranges widely, mica-schists being white to brown or nearly black,
chlorite-schists some shade of green, hornblende-schists from dark green
to black, talc-schists white, pale-green, yellowish or gray, etc.
Schists are found all over the same regions as gneisses and quartzites,
_i.e._, New England (especially good exposures of schist being seen
about New York City), the Lake Superior region, Rocky Mountains, etc.
Beside these regions where it occurs native, there are boulders of
schist all over the glaciated areas of eastern and northern United
States.
Slate
Slate is a metamorphic rock which will split into thin or thick sheets,
and is composed of grains so fine as to be indistinguishable to the
unaided eye. The cleavage is the result of pressure during metamorphism,
and has nothing to do with the bedding or stratification of the
sedimentary rock from which it was derived. The original bedding planes
may appear as streaks, often more or less plicated, and running at any
angle with the cleavage. If these bedding streaks are abundant or very
marked, they may make a slate unsuitable for commercial uses. The slaty
cleavage may be very perfect and smooth so that the rock splits into
fine sheets, in which case it is often used for roofing slate; but by
far the greater part of the slates have a cleavage which is not smooth
or perfect enough so that they can be so used. Slates are the
metamorphic equivalents of shales and muds, and represent the effect of
great pressure but with less heat than is associated with schists or
phyllite, and consequently with less alteration of the original mineral
grains.
The color ranges from gray through red, green and purple to black. The
grays and black are due to the presence of more or less carbonaceous
material in the original rock, the carbon compounds having changed to
graphite. The reds and purple are due to the presence of iron oxides,
and the green to the presence of chlorite.
While the particles of slate are so small as to be indistinguishable to
the unaided eye, the use of thin sections under the microscope shows
that slate is composed mostly of quartz and mica, with a wide range of
accessory minerals, like chlorite, feldspar, magnetite, hematite,
pyrite, calcite, graphite, etc.
According to their chief constituents slates may be distinguished as
argillaceous-slate or _argillite_, bituminous-slate, calcareous-slate,
siliceous-slate, etc.
Slate will be found here and there in the metamorphic areas of New
England, the Piedmont Plateau, the Lake Superior region, and in many
places in the west.
Phyllite
Pl. 66
Phyllite is a thinly cleavable, finely micaceous rock of uniform
composition, which is intermediate between slate and mica schist. In
this case the flakes of mica are large enough to be distinguishable to
the eye, but most of the rest of the material can only be identified
with the aid of a microscope. It is mostly quartz and sericite. Phyllite
represents a degree of metamorphism greater than for slate, but less
than for schist; and it may grade into either of these other rocks.
Garnets, pyrite, etc., may be present as accessory minerals. The color
ranges from nearly white to black, and it is likely to occur in the same
places as do slates.
Marble
Pl. 66
This is a broad term, and includes all those rocks composed essentially
of calcium carbonate (limestones) or its mixture with magnesium
carbonate (dolomite), which are crystalline, or of granular structure,
as a result of metamorphism. It takes less heat to metamorphose a
limestone, and for this reason the marbles have a more crystalline
structure than most metamorphic rocks; and they do not have the tendency
to split or cleave which is so characteristic of most metamorphic rocks.
It is only when there is a large amount of mica present that the typical
schistosity appears. Commercially the term marble is used to include
true marble and also those limestones which will take a high polish; but
in this book, and geologically speaking, no rock is a marble unless it
has crystalline structure.
Marbles range widely in color according to their impurities. Pure marble
is white. Carbonaceous material in the antecedent limestone is changed
to graphite in the metamorphic process, and makes the marble black, but
appears usually in streaks or spots, rather than in any uniform color.
An all black “marble” is usually a limestone. The presence of iron
colors the marble red or pink. Chlorite makes it green, etc.
Various accessory minerals are common in marbles, such as mica,
pyroxene, amphibole, grossularite among the garnets, magnetite, spinel,
pyrite, etc., through a long list.
Because it cuts readily in all directions and takes a high polish,
marble is widely used as a building stone. In the moist climate of the
United States it suffers in being soluble in rain water when used on the
outside of a building: but for interior decoration it furnishes some of
the finest effects.
The largest marble quarries are developed in Vermont, Massachusetts, New
York, Pennsylvania, Georgia, Alabama, Colorado, California, and
Washington.
Steatite
_Soapstone_
Steatite is a rock composed essentially of talc, which is associated
with more or less impurities, such as mica, tremolite, enstatite,
quartz, magnetite, etc. It is found in and with metamorphic rocks, and
is a rock which has been modified by hydration from a metamorphic
predecessor. It was probably first a tremolite or enstatite schist, in
which, after the metamorphic rock came into the zone where ground water
exists, the tremolite or enstatite was altered to talc, the impurities
remaining much as they were in the first place.
It is bluish-gray to green in color, often soft enough to cut with a
knife, and has a greasy feel. It is very resistant to heat and acids;
for which reasons it has proved very useful commercially in making
hearthstones, laundry tubs, and fire backs; and, when powdered, in
making certain lubricants. The Indians, in the days before Columbus,
took advantage of the ease with which it is cut, to make from it large
pots for holding liquids, which are today among the greatest treasures
in collections of Indian relics. They also carved pipe-bowls and various
ornaments and amulets from soapstone.
It is found in Vermont, Massachusetts, New York, New Jersey,
Pennsylvania, Maryland, Virginia, North Carolina, Georgia and
California.
Serpentine
Pl. 67
Pure serpentine is the hydrated silicate of magnesium, as described
among the minerals on page 138. Serpentine rock is serpentine with more
or less impurities, such as pyroxene, amphibole, olivine, magnetite,
chromite, calcite, magnesite, etc. It often also contains mica and such
garnets as pyrope, as accessory minerals. Serpentine, like steatite,
always occurs in and with metamorphic rocks, and was originally a
metamorphic rock, but has since been changed by the hydration of its
silicates, when it came into the zone in which ground water is present.
In the first instance it was some sort of shale, clay and dolomite,
which was metamorphosed to an amphibole or pyroxene schist. When this
was exposed to the action of ground water, the amphibole or pyroxene
minerals were changed to serpentine, resulting in a rock composed mostly
of serpentine, but retaining the impurities which were in the
metamorphic rock, and perhaps adding to them such amphiboles and
pyroxenes as were not altered during the hydration process. The above is
the commonest type of serpentine rock. It can and sometimes has been
formed in a similar way from an igneous predecessor, by the hydration of
its silicate minerals. In this latter case the serpentine would not be a
modified metamorphic rock, but a modified igneous one. It is a case
where such a rock as a diorite or a gabbro is exposed to ground water
and the pyroxene present altered to serpentine. A serpentine formed in
this way would be a very impure one.
Serpentine rock is used as an ornamental stone for interior decoration,
because it takes a high polish and has pleasing colors, various shades
of green. It is however decidedly soft and will stand very little
exposure to weather, and it is also filled with seams which make it
difficult to get out large slabs.
Serpentine rock occurs fairly commonly in the metamorphic belt of New
England and the Piedmont Plateau, and in some of the western states,
especially California, Oregon, and Washington.
Ophiolite
_Ophicalcite_
This name is given to marbles which are streaked and spotted with
serpentine. They are a mixture of green serpentine and a white or nearly
white calcite, magnesite or dolomite in variable proportions.
Ophicalcite occurs in and with metamorphic rocks, and represents an
impure limestone which has been metamorphised, the lime becoming marble,
and the impurities becoming such silicates as pyroxene, amphibole, or
olivine. This metamorphic rock has then come into the zone of
ground-water and the silicate minerals have been changed by hydration to
serpentine. Ophicalcite is then a metamorphic rock, in which secondary
chemical changes have since taken place. It may have a wide range of
accessory minerals present, such as magnetite, chromite, pyrope among
the garnets, olivine, etc. Verde antique is a trade name for one of the
ophiolites.
While not abundant, ophicalcite is in good demand as an ornamental stone
for interior work; for it takes a high polish, and is beautiful; but, on
the other hand, it will not stand exposure to the weather for the
calcite is soluble, and there are numerous seams and cracks in it making
it difficult to obtain large slabs.
It occurs in Quebec, Canada, in the Green Mountains of Vermont, and in
the Adirondack Mountains.