the silicious and calcareous oozes are formed through the agency of
organisms, they are not themselves strictly organic. There is, however,
a small but important group of rocks formed directly from organic
matter. In favorable situations the woody parts of plants, falling
into water, are so far preserved from decay that they accumulate in
beds, and by slow changes pass into _peat_, _lignite_, _bituminous
coal_, _anthracite_, and _graphite_. The first of these is composed
essentially of carbohydrates and hydrocarbons much as plants are, while
the last two are mainly carbon, and the intermediate members represent
stages of passage from the first to the last. They are all derived from
the strictly organic part of the plants, and spring essentially from
the atmosphere and hydrosphere. They are only indirectly associated
with the evolutions of the inorganic series.
INTERNAL ALTERATIONS OF ROCKS.
Besides the extreme alterations of rocks at the surface of the earth
by which they pass into solution and into residual mantle-rock, and
at length by transportation and re-sedimentation become stratified
rocks, as just described, those rocks which are not at the surface are
subject to changes that give rise to several varieties of _altered
rocks_. These changes are taking place constantly under ordinary
conditions, though usually very slowly. Under great pressure and heat
the changes are relatively rapid and intense, and lead to results
not reached under other conditions. These more profound changes are
termed _metamorphism_, and will be considered later. It is, however,
important to recognize the great fact that the outer part of the earth,
for perhaps 20,000 or 30,000 feet, is more or less fractured and
permeated by water containing in solution various substances dissolved
from the atmosphere, the soil, and the rocks through which it has
already passed, and that this permeating and circulating water is
now, and for long ages has been, working changes in the rocks, partly
by dissolving matter out of them, partly by depositing matter in them,
and partly by furnishing a medium through which new combinations of
their constituents may take place. This outer fractured portion of the
lithosphere has been called the _zone of fracture_.[201]
=Oxidation and deoxidation.=—At and above the surface of the
underground water, where the rocks are easily reached by atmospheric
waters carrying much free oxygen, and by the air itself, _oxidation_
prevails. Through oxidation the ferrous oxides are changed to ferric
oxides, a change which is usually manifested by a transition from a
gray, green, or blue color, to buff, brown, yellow, or red. The partial
progress of such oxidation is often shown in a fractured block or
bowlder whose exterior shows the latter colors, while the interior
shows the former. The sulphides, of which common pyrites (FeS₂) is the
most familiar, are oxidized into sulphates, and then sometimes pass
on into the higher oxides and other compounds. Thus copperas (FeSO₄)
arises from pyrites (FeS₂) by direct oxidation of both Fe and S. The
sulphuric acid of this compound, uniting with some base stronger than
the ferrous oxide, gives rise to further oxidation and results in
hematite (Fe₂O₃) and limonite (Fe₂O₃,3H₂O). In general, the mineral
constituents of the rocks in this upper zone take on their maximum
states of oxidation. This oxidation affects more or less profoundly the
character of the rock as a whole. Deeper in the earth oxidation is less
prevalent, and the action is sometimes reversed and deoxidation takes
place. So also wherever organic matter is undergoing decomposition
deoxidation is likely to occur.
=Solution and deposition.=—Solution preponderates in the upper part
of the zone of fracture, but deposition is prevalent in its deeper
parts. The calcium carbonate and silica dissolved near the surface are
often deposited below as calcite and quartz. The sulphates and other
sulphur compounds that are formed and dissolved near the surface are
apt to be changed into sulphides lower down by deoxidation. The soluble
oxides and other compounds formed near the surface are often likewise
precipitated below. This is particularly true where the descending
waters encounter decomposing organic matter, and where they mingle
with waters that have followed other routes and have become charged
with different solutes. On coming together, reaction between the
constituents takes place, resulting sometimes in new solutions and
sometimes in precipitation.
If these lower deposits of calcite, quartz, sulphides, etc., are made
in the pores of the rock, they change its texture and composition. If
they are made in fissures they constitute _veins_, and if a sufficient
percentage of the vein matter consists of valuable metallic compounds,
they constitute _ores_.
As the waters descend they suffer greater and greater pressure and some
increase of temperature, and these changes modify their power to hold
substances in solution. In general, the waters increase in solvent
power, but the effect is different for different mineral substances,
and hence as a rule the waters are taking up some substances and
laying down others as they proceed. After penetrating to greater or
less depths, the waters may come again to the surface, either because
they are pushed up by the higher head of the waters behind, or because
they become warmer and thus lighter, and are forced up by the heavier
cold waters above, or else they pass up by diffusion through the
descending waters. In any case, the deep, warm waters, usually rather
highly charged with material dissolved in their previous courses, are
apt to deposit some of their burden as they ascend to horizons of
lower pressures and temperatures. They are particularly liable to make
deposits where they commingle with other waters differently charged
with solutes. Thus internal changes in the body of the rocks are, and
for ages have been, taking place. In the upper part of the depositing
zone, calcite is the dominant mineral deposited, while in the lower,
quartz is more common; but much depends on local conditions and other
influences, and no rigid rule holds good.
=Hydration and dehydration.=—Water sometimes unites directly with some
of the constituents of a rock and produces hydrated minerals, i.e.,
minerals that have water as an element of their constitution, not
simply water absorbed into their pores. A large class of minerals known
as zeolites, because they swell up and undergo life-like contortions
when their basic water is driven off by heat, are examples of hydrous
products. A more familiar example is limonite (Fe₂O₃,3H₂O), of which
yellow ocher is a variety, which on heating sufficiently gives off its
water and becomes hematite (Fe₂O₃) or red ocher. The turning of yellow
clay to red brick on burning is a familiar example of dehydration.
The general tendency in the upper zones penetrated by water is toward
hydration. In the lower zones, where the pressure is great, Van Hise
holds that there is a tendency toward dehydration, if the rocks have
been previously hydrated. This may be the case if rocks have once
been near the surface and later deeply buried by the accumulation of
sediments on them. If the principle holds, rocks subjected to intense
lateral pressure may be dehydrated.
=Carbonation and decarbonation.=—The igneous rocks are largely
silicates. The carbonic acid of the surface-waters and of the air
acting upon them, converts them, in part, into carbonates. In this
way has arisen most of the original supply of calcium and magnesium
carbonates. Original carbonates formed in this way are precipitated and
redissolved again and again. The carbonates in river-waters are much
more largely solutions of previously solid carbonates than original
carbonates formed from the silicates. The potassium and sodium of the
silicates also form carbonates, but by preference they unite with the
sulphur and chlorine, and hence appear more largely as sulphates and
chlorides.
Carbonation is usually accompanied by oxidation and hydration. These
several processes break up the complex and relatively unstable
silicates into simpler and more stable silicates, carbonates, and
oxides. This is illustrated by the following formulas illustrative
of the changes undergone by augite and labradorite, two common
rock-forming minerals.
The composition of augite may be represented by the formula
{ CaO.(Mg,Fe)O.2SiO₂
{ (Mg,Fe)O.(Al,Fe)₂O₃.SiO₂.
Assuming Mg and Fe to be equal in amount in the first half of the above
formula, and Mg and Fe to be equal in the first part of the second
half, and Al and Fe to be equal in the last part of the second half,
doubling the whole and allowing it to be acted on by CO₂ and H₂O, we
have
2CaO.2MgO.2FeO.Al₂O₃.Fe₂O₃.6SiO₂ + 6CO₂ + 2H₂O
= 2CaCO₃ + 2MgCO₃ + 2H₂O.Al₂O₃.2SiO₂ + 2FeCO₃ + Fe₂O₃ + 4SiO₂.
The hydrous silicate of the last part of the equation is kaolin.
The composition of labradorite is represented by the formula
{ CaO.Al₂O₃.2SiO₂
{ Na₂O.Al₂O₃.6SiO₂.
Assuming the two molecules represented by this formula to be equally
abundant, and allowing the whole to be acted on by H₂O and CO₂, we have
CaO.Na₂O.2Al₂O₃.8SiO₂ + 4H₂O + 2CO₂
= CaCO₃ + Na₂CO₃ + 2(2H₂O.Al₂O₃.2SiO₂) + 4SiO₂.
When waters charged with carbonates descend into the earth they are
likely to precipitate a portion of their burden, forming calcite and
other crystalline carbonates, and hence these are among the most
common minerals found in veins and rock cavities. Carbonates are
also deposited when carbonate-charged waters come to the surface and
evaporate or lose a part of their carbon dioxide.
Decarbonation also takes place, but it is, at least at the surface, a
much less common process, and its conditions are less well understood.
Sufficiently high heat will drive off the carbon dioxide, as in the
artificial process of burning lime, but this is rarely observed in
nature. Even lava intrusions do not usually reduce limestone to
caustic lime at any appreciable distance from the contact. It is
believed, however, that in the deeper zones, where high pressure and
heat prevail, carbonates are changed into silicates, thus in a way
reversing the process that prevails at the surface, and setting free
again a portion of the carbon dioxide that had become locked up in the
formation of the carbonates. To this action some of the carbon dioxide
of deep-seated thermal springs is assigned.
The carbonation of the silicates takes place at the expense of
the carbon dioxide of the atmosphere and hydrosphere, and hence
in proportion as the igneous rocks are changed into carbonates,
the atmosphere and hydrosphere are depleted of carbon dioxide, new
supplies being neglected. As plants are dependent on carbon dioxide
for their principal food, and as animals are dependent on plants for
their food, directly or indirectly, the process of carbonation has
a profound bearing on the life-history of the earth, and will often
invite attention in the historical chapters. It is sufficient here to
note that carbonation is one of the chief processes in the alteration
of igneous rocks and furnishes, directly and indirectly, a larger
percentage of the mineral substances dissolved in the waters that flow
from the land, than any other single process.
=Molecular rearrangements.=—Besides these and similar changes that
involve additions and subtractions through the agency of percolating
water, the molecules of some of the rock constituents rearrange
themselves, or the elements enter into new chemical relations; thus,
pyroxene may pass into hornblende by a change of the crystalline
arrangement of the molecules. The change may sometimes be caught in
progress, the outer part of the crystal being hornblende (which when
thus formed is called _uralite_), while the heart of the crystal
remains pyroxene. So aragonite may pass into calcite.
By changes of the foregoing kinds, many crystalline rocks are much
altered. Some become _chloritic_ from the development of the soft,
green hydrated mineral, chlorite, derived from the pyroxene, amphibole,
biotite, and perhaps other silicates of the original rock. Others
become _talcose_ from the development of talc, a very soft, unctuous,
hydrous magnesian silicate developed from the magnesian minerals
of the original rock. _Soapstone_ or _steatite_ is a rock composed
essentially of such secondary material. _Serpentine_ is a rock made
up of a similar secondary mineral (serpentine) apparently derived
from chrysolite (olivine) and other magnesian minerals. _Epidote_, a
complex lime-iron-alumina silicate, often recognizable by its peculiar
pistachio-green color, is derived from other silicates, and is rather
common in many varieties of crystalline rocks. _Melaphyre_ is a name
applied rather loosely and variously to certain altered basic rocks of
the basalt family. _Diabase_ is essentially an altered dolerite. Nearly
all the very ancient basaltic rocks show notable degrees of alteration,
even though they appear to have escaped unusual dynamic conditions
since their original formation, and hence their alteration seems to
have resulted chiefly from the operation of unobtrusive agencies, chief
among which is the circulation of water.
_The Salient Features of Rock Descent._
The foregoing processes by which primitive or igneous rocks are
disintegrated and their constituents converted into fragmental material
may be said to constitute _the descent of rocks_ in its fuller
sense. Viewed chemically, the great features of the process are (1)
the breaking down of the complex silicates, and (2) the gathering
of the resultant simpler silicates (mainly aluminum silicates) into
the silt and clay beds, (3) the assembling of a large part of the
free acidic element (the quartz) into the sand and gravel beds, and
(4) the concentration of a large part of the earthy basic element
(the calcium, magnesium, and iron oxides) into the calcareous,
magnesian, and iron deposits, while (5) a large part of the alkaline
basic remainder (the sodium, and potassium oxides) is dissolved and
held in the sea-water. Physically, the great features are (1) the
disaggregation of the antecedent rock, and (2) the separation from
one another of products which are physically unlike, that is, the
coarser from the finer, and the heavier from the lighter, and (3) the
aggregation of these diverse materials in more or less distinct beds.
It is to be noted that while the rearrangement of the sediments is
made on the basis of their physical characters, it results in chemical
differentiation as well, for the products of rock decay, which are
physically diverse, are often chemically diverse as well. The physical
assortment and the stratification are to be looked upon as a step in
the direction of a simpler grouping of the material. On the whole, the
process is descensional in character.
THE REASCENSIONAL PROCESS.
Running hand in hand with this descensional process, there has
always been a reascensional process by which the coherence, the
crystallization, and in some measure the complex composition of the
rocks are restored. This is partially due to external mechanical
agencies, but chiefly to internal chemical and molecular forces.
Two general phases of this reconstructional work are recognized. The
first, simplest and most universal, is that by which the incoherent
materials produced by the descensional processes, i.e., the muds,
sands, and clastic materials generally, are hardened into firm,
coherent shales, sandstones, and limestones, and incidentally more or
less changed in composition and molecular arrangement. The second is
that by which more profound changes of induration and of composition
are wrought, bringing the rock back to a state resembling its original
crystalline character. This is known as _metamorphism_. Often, however,
it is but an extension and intensification of the more common processes
of the first class. Metamorphism is essentially reconstruction.
=Induration under ordinary pressures and temperatures.=—All kinds
of loose fragmental material, whether soils, earths, clays, sands,
gravels, volcanic ashes, cinders, or other forms of clastic or
pyroclastic material, may become hardened into firm rock either by
_pressure_, or by _cementation_, or by both. Pressure and cementation
commonly act together and aid each other. The ordinary pressures
arise from the weight of the overlying material, and these of course
increase with depth. Extraordinary pressures arise from the shrinkage
of the earth and perhaps from other sources. The fragments of the
clastic material, on being pressed together for long periods, weld
more or less at the points of contact. If they are irregular, angular,
or elongate, they come to interlock more or less like the fragments
of macadam, and this coöperates with the welding. The process is
greatly aided by water-bearing solutions of lime, silica, etc. which
are deposited at the points where the fragments press upon each other.
It is here that the capillary spaces are most minute and deposition
is most liable to take place. Sometimes a film of mineral matter is
laid down over the surfaces of the fragments and serves to bind them
together. This process goes on wherever the ground-waters are in a
depositing condition, just as the opposite process of disintegration
takes place wherever the waters are in a solvent state. At and near the
surface of the land, the waters are usually in the latter condition and
disintegration is in progress, as already noted, but this is not always
so. At times and places, the water from within the rock-mass may come
to the surface and evaporate, and in so doing leave all its dissolved
material on the surface, or within the outer pores of the mass, as
cementing material. The exterior thus becomes firmly bound together,
“case-hardened,” as it is termed. This may be seen in the drying of
a lump of mud, the exterior of which often becomes quite firm. It is
seen in quarry-rock, especially sandstone, which is sometimes soft and
easily worked when taken wet from the earth, but which hardens as the
water—the “sap” of the quarrymen—dries out and deposits its solutes in
the capillary spaces of the grains of the surface. It is obvious that
it is the very last of the “sap” which contains the most concentrated
solutes, and that this last remnant is held in the minute capillary
spaces where the grains touch each other, and hence the last stage of
drying leaves the cement at the points where it is most effective. In
natural exposures of sandstone, the pores of the outer shell sometimes
become almost completely filled in this way with silicious deposits,
and the sandstone is changed into a quartzite.
In the sea, and in the deep water underground, the common habit of
the water is to deposit more than to dissolve, though it is doing
more or less of both. As a rule, therefore, loose material in these
situations becomes bound more or less firmly into rock, and hence what
were originally loose sand beds become _sandstones_; what were soft
muds become _shales_ or _limestone_, according to composition; what was
gravel becomes _conglomerate_; what was chipstone becomes _breccia_;
what were volcanic ashes, cinders, and lapilli become _tuffs_; and
what were masses of volcanic blocks and coarse fragments become
_agglomerates_.
[Illustration:
+Fig.+ 354.—Quartz crystal enlarged by secondary growth. The shaded
outline represents the outline of the sand grain; the solid lines,
the outline after secondary growth. Magnified 67 diameters. (Van
Hise.)
+Fig.+ 355.—Sandstone and quartzite texture. The shaded outlines
represent the surfaces of the sand grains before growth, the
intervening white portions, the added quartz, and the black portions,
unfilled spaces. Open spaces characterize sandstone. When the spaces
are filled with quartz, the rock becomes quartzite. Magnified 35
diameters. (Van Hise.)]
The cementing process works at times in specially interesting ways. In
quartz sandstones, the grains are worn fragments of quartz crystals,
formed originally in quartz-bearing rock. The crystalline force in
these remnants controls the arrangement of the new molecules of silica
deposited about them. The result is that the new deposits tend to
build up the original forms of the crystals from which the sand grains
were derived (Fig. 354). Sometimes a film of iron oxide has formed
about the grain of sand before the addition of the new silica. This, or
some difference of color, may clearly distinguish the original grain
from subsequent additions. Sometimes the adjacent grains of sandstone
are rebuilt in this way until the interstices are completely filled.
When this has been accomplished, the sandstone becomes a quartzite
(Fig. 355). Most quartzites indeed appear to have been formed in this
way, but mainly under special conditions that promote the deposition
of silica. Grains of other minerals, such as feldspar, are subject to
similar secondary enlargement (Fig. 356).
[Illustration: +Fig.+ 356.—Feldspar crystals enlarged by secondary
growth. Magnified 50 diameters. _AA_ = original grains; _BB_ =
enlargements; _D_ = unfilled spaces. (Van Hise.)]
Sometimes the new material is deposited in the form of concentric
shells about the particles of sediment, building them up into little
spheres. Rock formed of such spherules is known as _oolite_, from the
resemblance of the grains to the roe of fish (Fig. 357). Sometimes the
nuclei of the concretions are grains of quartz sand, and the added
concentric layers are of calcium carbonate. In this case the structure
is quite obvious; but perhaps more frequently the nuclei are minute
and difficult to identify, and the concentric shells make up the main
mass of the grains. Certain formations, as the oolitic limestone of
Indiana and elsewhere, and the Upper and Lower Oolites of England,
are characterized by this structure. In most cases these accretions
probably grew in depositing waters that gently rolled the grains while
layers were being added. They thus do not fall under the head of
cementation after the beds were formed; but concentric additions to the
grains appear sometimes to have taken place after they were formed into
beds.
=Cavity filling.=—When cavities of some size occur in rocks and the
percolating waters are in a depositing state, the interiors of the
cavities are sometimes lined with concentric layers of deposit. Here,
instead of building _out_ from a nucleus, the waters build _in_ from
the walls of the cavity. The _agate structure_ (Fig. 358) is a case of
this kind, in which the successive layers are commonly silica in the
form of chalcedony and differ from each other in color and texture.
Often before the cavity is entirely filled, the deposit changes from
chalcedony, to crystals of quartz, which grow with their bases on the
walls and their pyramidal points toward the center of the cavity.
_Geodes_ are examples of a similar process in which the cavity is
but partially filled with crystals which have their bases set on the
walls of the cavity and their points directed inwards (Fig. 359). The
crystals of geodes are most commonly quartz or calcite, but they may
be any other mineral that the waters are capable of depositing. Very
large cavities lined in this way are known to miners as _vuggs_, and
these grade on into caves lined with crystals and with _stalactite_
and _stalagmite_. These are the largest expression of the solidifying
process by means of internal deposition.
[Illustration: +Fig.+ 357.—Oolitic texture. About natural size. (Photo.
by Church.)]
[Illustration: +Fig.+ 358.—Agate structure. The cavity was first coated
with mineral matter deposited from solution. The contracted cavity
was then nearly filled with the same sort of material deposited in
layers, apparently over the bottom, until the cavity was nearly
obliterated.]
=Fissure-filling; veins.=—Cracks, crevices, and fissures filled by
deposition in a similar way give rise to _veins_ (Fig. 360). Here
the filling grows from the walls toward the center, and hence often
has a banded appearance. By this filling of cracks and crevices,
the circulating water heals the breaks in the rocks. Frequently a
crushed zone is thus restored to a solid state. When the fissures
are deep and wide and traverse different formations, conditions are
afforded for very complex deposits, and for the concentration of
rare and valuable material originally dispersed through a great mass
of rock. Ore deposition in such veins is usually treated as a theme
by itself, but it is really but a declared expression of the work
which the percolating waters are doing throughout all the rocks which
they penetrate. Most of the fine crystals that grace mineralogical
collections were formed in cavities and fissures by deposition from
circulating mineralized waters.
[Illustration: +Fig.+ 359.—A geode. About half natural size. (Photo. by
Church.)]
=Solution as well as deposition.=—A further phase of the process needs
attention. The percolating waters are constantly taking up matter as
well as throwing it down, and so, while they are cementing fragments
together and healing fractures, they are also removing material, and
a rock may be growing porous and cavernous at the same time that its
fragments are being united. Cavities may be formed at one stage and
filled at another; matter may be taken up at one point and put down at
another, and so an internal reconstruction is in slow progress.
[Illustration: +Fig.+ 360.—Veins of calcite in limestone. Calciferous
formation near Highgate Springs, Vt. (Walcott, U. S. Geol. Surv.)]
=Concretions.=—A notable phase of this internal reconstruction is the
assembling together of like kinds of matter. For instance, silica that
was probably deposited in the form of the silicious shells and spicules
of plants and animals, and was disseminated through the sediments as
originally formed, is aggregated into nodules of chert or flint (Fig.
361); similarly, concretions of ferrous carbonate or calcium carbonate
grow in sands, silts, or muds; clusters of crystals of pyrite (FeS₂),
of sphalerite (ZnS), and galenite (PbS) are formed in clayey layers,
pressing the clay back as they grow; and in many other cases, kind
comes to kind. Some concretions probably form during the accumulation
of the beds in which they lie.
=Replacements and pseudomorphs.=—So also there are replacements,
sometimes resulting in imitative or false forms. Frequently the calcium
carbonate of corals, molluscan shells, etc., is replaced by silica, and
this substitution is brought about so gradually, particle by particle,
that the minutest details of structure are sometimes fully preserved.
This is often of great service in their study, since the limestone
in which they are imbedded may often be dissolved away, while the
silicified fossil is unaffected. So woody matter is sometimes replaced
by silica, forming silicified wood. Similarly, the molecules of one
crystal are sometimes replaced by different material, as the molecules
of calcite by zinc carbonate, giving a pseudomorph of zinc carbonate
after calcite.
[Illustration: +Fig.+ 361.—Nodule of chert. About half natural size.
(Photo. by Church.)]
=Incipient crystallization.=—A more general change is incipient
crystallization. Some common limestones and dolomites are now largely
made up of small crystals, though the mass was originally a calcareous
mud or ooze. Incipient crystals are formed in shales and other
sediments. This process, like the preceding, is a kind of incipient
metamorphism or reconstruction, but it is a pervasive process, taking
place under ordinary conditions of heat and pressure, and through the
agency of circulating ground-waters.
By these and similar processes the fragmental deposits are solidified
into firm rock and undergo internal changes which more or less
reorganize the matter of which they are composed. The process is a very
slow one usually. Some of the sands and muds of very early geologic
ages are yet imperfectly solidified; e.g., much of the St. Peter’s
sandstone, a very ancient formation, is yet so incoherent as to break
down into sand in being dug out, and is used for mortar sand much more
than for building stone. Some of the Hudson River shales of scarcely
less age are more nearly clay than hard rock. But these are examples of
excessive slowness and slightness of change. In general, all but the
most recent deposits show notable progress in reconstruction.
[Illustration: +Fig.+ 362.—Figure showing the elongation of pebbles
resulting from pressure. Carboniferous formation, Bancroft Place,
Newport, R. I. (Walcott, U. S. Geol. Surv.)]
_Reconstruction under Exceptional Conditions._
Two special conditions greatly influence changes in rocks, viz.,
pressure and heat. Their action gives rise to three general cases,
but these blend indefinitely: (1) exceptional pressure without great
heat, (2) great heat without exceptional pressure, and (3) great heat
and great pressure conjoined. Exceptional pressure may arise from the
weight of overlying rocks, or from lateral thrust due to the shrinkage
of the globe, and occasionally from other causes. Exceptional heat may
arise from pressure, from the intrusion of hot lavas, and occasionally
from other sources. In the case of intruded lavas there may or may not
be exceptional pressure. Thrust usually gives heat as well as pressure,
but if lateral thrust acts on rocks near the surface, they may be
mashed into new forms without becoming very exceptionally heated,
though some rise of temperature is inevitable.
[Illustration: +Fig.+ 363.—Pre-Cambrian fossiliferous slate. Deep Creek
Canyon, 16 miles southeast of Townsend, Mont. (Walcott, U. S. Geol.
Surv].)
(1) =Slaty structure.=—When rocks made up of clastic particles are
compressed in a given direction and are relatively free to expand at
right angles to the direction of pressure, the particles that are
already elongated tend to take positions with their longer axes at
right angles to the direction of pressure, and all particles, whether
elongate or not, are more or less flattened in a plane transverse
to the direction of pressure. This may be readily seen where the
particles are large (Fig. 362). As a result of the orientation and
flattening of their particles, rocks so affected split more readily
between the elongate and flattened particles than across them. In
other words, the rocks cleave along planes normal to the direction of
compression, and break with difficulty and with rough fracture across
the planes of cleavage. The condition thus induced is known as slaty
structure (Fig. 363), and is best illustrated by roofing-slate, which
was originally a mud, later a shale, and finally assumed the slaty
condition under strong compression. Sometimes the original bedding
may still be seen running across the induced cleavage planes (Fig.
364). As the original mud beds were horizontal or nearly so, and as
the thrust is usually horizontal or nearly so, the induced cleavage
commonly crosses the bedding planes at a high angle (Fig. 364); but
after the beds are tilted or bent, the lines of pressure take new
directions relative to the bedding planes, and the angles between the
original bedding and the slaty cleavages usually become smaller, and
may even disappear in exceptional cases. Limestones, sandstones, and
conglomerates are not so easily compressed as mudstones, and they
usually take on only an imperfect cleavage normal to the direction of
pressure. Often they merely show some little compacting, while the
shaly strata between them are converted into slate. Obviously the
direction of slaty cleavage may be used to determine the direction of
the compressing force, and is thus serviceable in dynamic studies.
[Illustration: +Fig.+ 364.—Slaty structure and its relation to bedding
planes. Two miles south of Walland, Tenn. (Keith, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 365.—Foliated rock. (Ells, Can. Geol. Surv.)]
=Foliation, schistosity.=—A more intense application of pressure in
a given direction is capable of breaking down and deforming the most
resistant rock. This must necessarily be attended with the evolution
of much heat, and thermal effects are mingled with pressure effects,
but the thermal effects may be neglected for the moment. The first
stage of the mechanical effect of the compression may be to crush the
rock more or less. It thus becomes granular or fragmental, and is
really a peculiar species of clastic rock (_autoclastic_). At a further
stage, the fragmented material may be pressed into layers or leaves,
much as in the development of slaty cleavage, but as a result of the
nature of the material, the cleavage is less perfect. This is often
attended by more or less shearing of the material upon itself, and thus
a rude fissility and foliation is developed. The result, including the
attendant metamorphism about to be described, is a _foliated_ or
_schistose structure_ (Figs. 365 and 366). Even the most massive rocks
may be reduced to the foliated form by this process; thus, a granite
may be mashed into a _gneiss_—which is a granite in composition, but
has a foliated structure—or a basalt may be converted into a _schist_,
a common term for foliated crystalline rocks. Porphyritic rock rendered
schistose by pressure is shown in Fig. 366. When massive rocks like
granite or basalt are thus crushed down into the foliated form, the
process is in a sense degradational. It is a kind of _katamorphism_
or downward change. It is often difficult to differentiate the
schists thus derived by degrading massive rocks, from those developed
by ascensional processes from clastic formations (_anamorphism_).
The action of heat is important in the evolution of schists of both
classes, but the effects of heat may best be taken up where it acts
measurably alone.
[Illustration: +Fig.+ 366.—Porphyry rendered schistose by pressure.
Near Green Park, Caldwell Co., N. C. (Keith, U. S. Geol. Surv.)]
[Illustration: +Fig.+ 367.—Schistose structure developed by pressure
shown in the left half of the figure, while it is wanting in the
right half. The vertical line is a bedding plane. The layer to
the left was of sufficiently different composition or subject to
sufficiently different movement to develop schistosity, while that
to the right was broken (brecciated) instead. The rock at the left
would be called quartz schist, while that at the right is quartzite.
Huronian formation near Ableman, Wis. (Atwood.)]
=Metamorphism by heat.=—When a mass of lava is poured out upon the
surface, it bakes the mantle-rock which it overruns, in greater or less
degree, depending on the mass and temperature. The nature of the effect
is much the same as in the process of brick-making, a dehydration of
the material, a hardening of the loose matter by the partial welding of
the particles, and sometimes the partial fusion of the surface and the
development of new compounds, usually glassy, but sometimes partially
crystalline. In both the natural and the artificial process, the time
element is short, the pressure trivial, and the water action limited.
If the heat were to become sufficiently intense, the result would be
fusion, i.e., a lava which would solidify into a glass. In such a case,
the rock cycle would be carried back to the initial molten state and a
new cycle instituted, but this does not usually take place when lava
merely overflows the surface.
If lavas, instead of rising to the surface, wedge in between layers of
rock and form _sills_, or interstratified sheets, the surface above
as well as that below is baked, and as the excess of heat of the
lava can only escape through the neighboring rock, the effects for
a given mass of lava are more considerable, and as the time element
and the water action (and sometimes the pressure) are usually greater
than in the case of extruded lavas, the effects tend rather toward
chemical and crystalline change than to simple baking. This tendency
increases with increase in the mass of the lava and in its temperature.
Sometimes enormous masses of very hot lava are thrust in between or
among the strata that lie beneath the surface, and bring to bear upon
them intense heat for a long period. So also, when a vent or fissure
is the passageway for lavas that continue to come to the surface for
long periods, as in the case of persistent volcanoes, the rocks which
form the walls of the vent or fissure are heated for a long time, and
this gives rise to metamorphism through heat, without very unusual
pressure, but usually with the free aid of water. In these cases the
chief effect is chemical recombination and crystallization. In the
limestones and sandstones it is simple; in the shales more complex. In
pure limestones and dolomites little chemical change takes place, but
the molecules are rearranged into larger and more perfect crystals, and
_marble_ is the result. The coarseness of the crystals is, in a general
way, a measure of the length of time during which the heat acts, and
of its intensity, but much depends on the freedom of the attendant
water circulation. Crystals an inch or two across are sometimes formed
in the contact zone, where the attendant water action is important.
If impurities, as silica, alumina, iron, etc., are present, various
minerals, such as _tremolite_ and _actinolite_, may be formed in the
marble. In pure quartzose sandstones, the effect is to cause the
building up of the quartz grains until the interspaces are essentially
filled and the whole becomes a massive _quartzite_. Here, as in the
marbles, impurities form adventitious crystals, a very common one being
_hematite_, formed from the segregation of the ferric oxide of the
sandstone.
In the shales, the material to be acted upon is more complex, for,
while the main mass is an aluminum silicate, there is usually much
free quartz, not a little potash and iron, and more or less of lime,
magnesia, soda, and other ingredients, for the muds from which the
shales arose contained not only the fully decomposed matter of the
original crystalline rocks, but the fine matter worn from them by
wind and water without decomposition. When this mixed matter is acted
upon by high heat and moisture, it tends to return to its original
crystalline state, so far as its changed constitution permits. The
potash chiefly unites with alumina and silica, and forms potash
feldspar (orthoclase chiefly) and potash mica (muscovite). The iron
often unites with magnesia, alumina, and silica to form biotite or one
of the ferromagnesian minerals, chiefly an amphibole. The lime usually
aids in the formation of other silicates of either the feldspar or
the ferromagnesian group, while the surplus silica crystallizes into
quartz. There is usually a predisposition to form mica in preference
to other silicates if the proper constituents are present, and the
result is that _mica schists_ and _gneisses_, in which mica abounds,
are common products of the metamorphism of shales by contact with
bodies of lava. Mica schists and micaceous gneisses are also formed
in other ways, and other schists, dependent on the composition of
the shales, are formed about intrusions of igneous rock. In all such
cases pressure probably attends the heat and is a factor in the
development of the schists. When the change induced by the heat is less
considerable, the shale is baked, with incipient recrystallization, and
often takes the form of _argillite_, a compact, massive sort of shale.
Beds of hydrous iron oxide (limonite) or of iron carbonate (siderite)
are usually converted by heat into hematite or magnetite. Beds of peat,
lignite, and bituminous coal are converted into anthracite by the
driving off of the volatile hydrocarbons. If the process goes to the
extreme, graphite is the result.
=Metamorphism by heat and lateral pressure.=—As already indicated,
the more common intense pressures experienced by rocks at and near
the surface are those that come from lateral thrusts arising from the
shrinkage of the earth. These affect one dimension of the rock-mass,
while they permit it to expand in one or both of the other dimensions.
This produces a strain in all the constituent particles of the rock,
and under such strain they pass more readily into solution than when
free from strain, and more readily rearrange their molecules internally
into positions of less strain. The crystals grow most freely along the
planes of least stress, i.e., at right angles to the pressure.[202] As
a consequence, where unidimensional pressure and high heat resulting
from the compression unite their influence, the metamorphic changes
are not only facilitated, but the rearrangement is controlled by the
pressure and results in a parallel arrangement of the constituent
crystals, giving a foliated or schistose character to the new rock.
The changes themselves are much the same as those produced by heat
and water without exceptional pressure, though some distinctions may
be noted. It is to be observed, however, that two kinds of work are
embraced here: the metamorphism of clastic rocks into crystalline
schists, which may be regarded as an upbuilding process, anamorphism,
and the mashing down of massive crystalline rocks into schists, which
may be regarded as a degradational process, katamorphism. In both
cases, however, there is solution and rearrangement of the molecules.
The katamorphism of basalts and other basic rocks gives basic schists;
that of granitic and similar rocks gives gneisses. The anamorphism of
basic pyroclastic tuffs and wackes gives basic schists, while that
of acid pyroclastics and most shales gives gneisses, mica schists,
or similar acidic schists. It is obvious that ordinary shales cannot
usually become basic schists, because in producing the original
muds, the bases were generally removed; but when shales are highly
calcareous and magnesian, as when they grade toward the limestones and
dolomites, they may become basic schists by metamorphism, e.g., certain
hornblendic schists. It is even more obvious that the limestone and
sandstone formations must largely retain their distinct composition. It
is thus seen that, in general, a sedimentary series anamorphosed must
differ from a crystalline series katamorphosed, though both give rise
to foliated or schistose rocks.
=Deep-seated metamorphism.=—When the exceptional pressure arises from
the weight of rocks felt at great depth, it is practically equal in
all directions and the crystallization probably develops normally and
is not forced into the parallel or foliated form. Rocks metamorphosed
under these conditions probably tend to take the massive form rather
than the schistose form, but this conclusion is theoretical rather than
observational, for little or nothing is known of the history of such
rocks.
=Completion of the rock cycle.=—The crystallizing processes of
metamorphism are fundamentally similar to the processes by which rocks
crystallize out of magmas, only in the first case the work is done
chiefly by the aid of an aqueous solution, while in the second it is
done through a mutual solution of the constituents in themselves,
where water was but an incident. If the heat factor in metamorphism
be sufficiently increased, aqueous solution may actually grade into
magmatic solution through various degrees of softening and melting, and
the cycle of changes be closed in upon itself.
VARIOUS CLASSIFICATIONS AND NOMENCLATURES.
From the foregoing sketch of the processes of rock-making it may easily
be inferred that the varieties of rocks may be almost unlimited, and
that they may be defined, named, and classified on many different
bases; for example.
(1) If _the mode of origin_ is chiefly in mind, rocks may be classed
as _igneous_ (lavas, tuffs, etc.); _metamorphic_ (schists, gneisses,
anthracite, magnetite, etc.); _aqueous_ (water-laid sediments,
stalactites, travertine, etc.); _eolian_ (dunes, loess in part);
_glacial_ (till, moraines); _clastic_ (mantle-rock, sandstone,
conglomerate, etc.); _organic_ (peat, lignite, coal, etc., and
indirectly, limestone, chalk, infusorial earth, etc.); and so on.
(2) If the _textural or structural characters_ are in mind, rocks are
designated vesicular (pumice, scoria, etc.); rhyolitic (flow-structure
rocks); glassy (obsidian, tachylite); porphyritic (distinct crystals
in obscure matrix); granitic (well-grained); compact, porous, earthy,
arenaceous (sandy), schistose, etc.
(3) If the _chemical composition_ is chiefly regarded, they may be
classed as silicious, calcareous, carbonaceous, ferruginous, etc.; or,
if the _chemical nature_ is considered, they are grouped as acidic,
basic, or neutral.
(4) If the _crystalline character_ is made the basis, they
are designated phanerocrystalline (distinctly crystallized),
microcrystalline (minutely crystallized), cryptocrystalline (hiddenly
crystallized), and amorphous (non-crystalline).
(5) If attention is fastened on _certain ingredients_, rocks
are characterized as quartzose, micaceous, chloritic, talcose,
pyritiferous, garnetiferous, etc.
(6) When rocks are regarded as _mineral aggregates_, if (_a_) the
_aggregates are simple_, they are named from the dominant minerals,
as dolomite, hornblendite, garnetite, anorthite, etc.; and if (_b_)
the _aggregates are complex_ they take special names, as syenite
(orthoclase and hornblende), gabbro (plagioclase feldspar and
pyroxene), etc.
(7) When the point of view is _structure of the mass_, they are classed
as massive, stratified, shaly, laminated, slaty, foliated, schistose,
etc.
(8) When _physical state_ or _genesis_ is considered, they are grouped
as clastic, fragmental, or detrital (conglomeratic, brecciated,
arenaceous, argillaceous, etc.); or pyroclastic (tufaceous,
scoriaceous, agglomeratic); or massive, in a sense slightly different
from that above (7).
As sometimes one of these characteristics and sometimes another is most
important in a given rock, or in a given study, no one classification
is satisfactory in all cases, yet each has its advantages in particular
cases.
_New System of Classification and Nomenclature._
The present systems of classifying and naming rocks have grown up
gradually out of earlier and cruder methods, many of which were
inherited from popular usage. Most of the names and definitions came
into use before microscopical and other modern means of study were
adopted. These systems, therefore, retain many inherited crudities
and inconsistencies, and lack adaptation to present needs. They are
too complex and difficult for field use and for general discussions,
while not sufficiently exact and systematic for the more rigorous
petrological discussions. A more adaptive and consistent practice
has been earnestly sought by petrologists, and a new system of
classification of igneous rocks has been offered by a group of leading
American petrologists, an outline of which is here given.[203] To
some extent this may be extended to the metamorphic crystalline
rocks with necessary modifications and additions. The classification
and nomenclature of the secondary rocks must probably always remain
variable and plastic to express the various points of view which it
is desirable to take. During the transition to this or some other new
system, which seems inevitable, the appended alphabetical reference
lists of the most common minerals and rocks, with brief definitions in
accordance with current usage, will be found serviceable. The proposed
system includes two parts, a _field system_ and a _quantitative
system_, the one applicable to rocks on casual inspection, and the
other, only after detailed study.
_The proposed field system._
The proposed field names are based largely on _texture_ and _color_.
The mineral constituents are used for subdivisions when they can be
determined; otherwise they are neglected.
Classifying chiefly on the basis of texture and crystalline state,
there are three groups: _Phanerites_, in which all the leading mineral
constituents can be seen megascopically; _aphanites_, in which all, or
at least an appreciable part, of the constituent minerals cannot be
distinguished megascopically; and _glasses_, in which the material is
wholly or largely vitreous.
I. The =Phanerites= may be further classified by their chief mineral
constituents as follows:
1. _Granites_ (f.n.),[204] consisting largely of _quartz_ and
_feldspar_ of any kind, with or without mica, hornblende, pyroxene,
or other minerals. This differs from the present common use in not
regarding mica as an essential constituent, and in not distinguishing
between alkali feldspars and calcic feldspars, thus broadening the
class.
2. _Syenites_ (f.n.), consisting predominantly of _feldspar_ of any
kind, with subordinate amounts of hornblende, mica, or pyroxene, but
with little or no quartz. This differs from the common use in giving
hornblende a subordinate place, and in embracing rocks with calcic
feldspars, thus broadening the class.
3. _Diorites_ (f.n.), consisting predominantly of _hornblende_ and
subordinately of _feldspar_ of any kind, with which there may be mica,
pyroxene, or other minerals. This is nearly the present use except that
any kind of feldspar may form the subordinate element.
4. _Gabbros_ (f.n.), consisting predominantly of _pyroxene_ and
subordinately of _feldspar_ of any kind, with or without other
minerals. This nearly coincides with one of the various present uses of
the term except that the range of the feldspar is increased.
5. _Dolerites_[205] (f.n.), consisting predominantly of _any
ferromagnesian mineral_ not distinguishable as hornblende or pyroxene,
with subordinate elements of _feldspar_ of any kind, and with or
without other accessory minerals. A name to be used when the dominant
mineral is clearly ferromagnesian, but cannot be satisfactorily
identified as either hornblende or pyroxene, although it may probably
be one of these. In other words, the dolerites (deceptive) embrace the
whole diorite-gabbro group when too obscure for separation.
6. _Peridotites_, consisting predominantly of _olivine_ and
_ferromagnesian minerals_, _without_ feldspar, or with very little.
7. _Pyroxenite_, consisting essentially of pyroxene without feldspar or
olivine.
8. _Hornblendite_, consisting essentially of hornblende without
feldspar or olivine.
II. The =Aphanites= may be _non-porphyritic_ or _porphyritic_.
(_a_) Non-porphyritic aphanites when light-colored may be classed as
_felsites_; when dark-colored, as _basalts_.
(_b_) The porphyritic aphanites or _porphyries_, when light-colored,
are _leucophyres_; when dark-colored, are _melaphyres_ (f.n.). They may
be classified further, according to the kind of phenocryst imbedded in
the aphanitic ground-mass, as
_Quartz-porphyries_, or quartzophyres;
_Feldspar-porphyries_, or feldspaphyres (not felsophyres);
_Hornblende-porphyries_, or hornblendophyres; and so on.
These may be subclassed by color, as
_Quartz-leucophyres_, light-colored quartz-porphyries;
_Quartz-melaphyres_, dark-colored quartz-porphyries;
_Feldspar-leucophyres_;
_Feldspar-melaphyres_; and so on.
III. The glasses are classified, according to color and luster, into
_obsidians_ or _pitchstones_ when dark and lustrous; _perlites_, when a
spheroidal fracture gives them a pearly appearance; and _pumice_ when
greatly inflated by included gases.
In general discussions, it is regarded as serviceable to use the term
_granitoids_ in a broad generic sense, to include all crystalline
rocks of the general granitoid type, including the granites, syenites,
gneisses, etc. In a similar broad way, the term _gabbroids_ may be
used to include the dark crystalline rocks in which the ferromagnesian
minerals predominate, as the diorites, gabbros, dolerites, peridotites,
etc. In this convenient and comprehensive way, two contrasted groups of
igneous rocks may be designated. As the granitoids are usually acidic
and the gabbroids usually basic, the grouping represents a broad fact
of importance.
THE PROPOSED QUANTITATIVE SYSTEM.
The distinguishing characteristic of the more rigorous system designed
to meet the needs of scientific petrology is its quantitative chemical
character. All igneous rocks are classified _primarily_ according to
their chemical composition and only secondarily according to their
mineral constituents, texture, and other characters. The rigorous
application of the system requires chemical analyses of the rocks, but
as these are not available in many cases, the authors of the system
have devised a method of optical mineral analysis by which the nearly
exact proportions of all the constituent minerals can be determined,
and by knowledge of their chemical nature the results may be converted,
by computation, into chemical terms. This can only be done for
holocrystalline rocks whose crystals are large enough to be measured
under the microscope, but aphanitic rocks may often be approximately
classified by comparison with similar rocks already accurately
determined. To facilitate this method of chemical analysis by measuring
the minerals, the chemical composition of certain common rock-making
minerals is expressed in proportional parts and tabulated, and is used
somewhat as molecular weight is in ordinary chemical analysis. Certain
of these are selected as _standard_ minerals, the selection being such
that the standard minerals embrace all the essential elements that
enter into the composition of rocks. All other minerals are converted
into their chemical equivalents in terms of these standard minerals by
the use of the tables. All the mineral constituents being thus reduced
to standard minerals, the classification is built up systematically on
these standard (or standardized) minerals.
A new system of names is required, and these have been very skillfully
formed by selecting significant letters from the names of the leading
minerals or from words signifying their preponderance, so that short
terms which carry their meaning in their forms, are secured, and
this has been done so that these are usually euphonious, however
strange they may seem to our preoccupied senses. For example, minerals
composed chiefly of _s_ilica and _al_umina are called _salic_;
those of _fe_rro_m_agnesian minerals, _femic_; those of _al_uminous
_fe_rromagnesian minerals, _alferric_, etc. When in a combination of
salic and femic minerals, the salic are extremely abundant, the rock is
_per_salic; if notably _do_minant, _do_salic; if the salic and femic
minerals are nearly equal, _salfemic_; if the femic are _do_minant,
_dofemic_; if extremely abundant, _per_femic, and so on, the system
being mnemonic. This method of deriving names is applicable only to a
portion of the necessary divisions. For the rest, a series of roots
derived from geographic names, with a system of terminations, has been
employed.
All standard minerals are divided into two groups of primary
importance: one of minerals characterized by alumina, as the
feldspars,—orthoclase, albite, anorthite,—leucite, nephelite, sodalite,
noselite, and corundum, to which are added the closely associated
minerals, quartz and zircon. This is called the _salic_ group. The
second group contains minerals characterized by iron and magnesia with
no alumina, as hypersthene (enstatite), acmite, olivine, magnetite,
hematite, and ilmenite, to which are added the closely associated
minerals, titanite, perofskite, rutile, apatite, and all other
rock-making minerals except those containing alumina together with
iron and magnesia. The second group is called _femic_.
Aluminous ferromagnesian minerals, such as hornblende, augite, mica,
etc.; are called _alferric_, and are not classed as standard minerals,
because their complexity of composition makes it better to treat them
as though made up of the simpler minerals of the standard list.
The composition of all igneous rocks can be expressed in terms of the
relative proportions of the two groups of the standard minerals, salic
and femic. By subdividing these groups successively on a mineral and
chemical basis, a series of classificatory divisions of greater and
greater precision has been formed. In each stage of the series, two
factors only are compared, and a simple set of ratios has been selected
to limit the divisions. Assuming the possibility of a continuous range
of variable mixtures of the two factors (_A_ and _B_) from an extreme
composed wholly of one (_A_), and an extreme composed wholly of the
other (_B_), five ideal cases have been chosen as types or centerpoints
about which variation in mixture may take place. These are:
_A_ = 1 _A_ = 3 _A_ = 1 _A_ = 1 _A_ = 0
——— —, ——— —, ——— —, ——— —, ——— —.
_B_ = 0 _B_ = 1 _B_ = 1 _B_ = 3 _B_ = 1
Division lines half-way between these points occur as the following
ratios:
_A_ 7 _A_ 7 > 5 _A_ 5 3 _A_ 3 1 _A_ 1
(1) ——— > —, (2) ——— < — > —, (3) ——— < — > —, (4) ——— < — > —, (5) ——— < —.
_B_ > 1 _B_ < 1 > 3 _B_ 3 5 _B_ 5 7 _B_ 7
These ratios are used throughout the system. In (1) _A_ is _extreme_;
in (5) _B_ is _extreme_; in (2) _A_ _dominates_ over _B_; in (4) _B_
_dominates_ over _A_; in (3) _A_ and _B_ are _equal_ or _nearly equal_.
All igneous rocks are grouped in five (5) primary divisions called
_Classes_ on a basis of the proportions of the salic and femic
minerals, thus:
Class I. Sal 7
——— > —, extremely rich in salic minerals, called _persalane_.
Fem 7
II. Sal 7 5
——— < — > —, with dominant salic minerals, called _dosalane_.
Fem 1 3
III. Sal 5 > 3
——— < — > —, salic and femic minerals, equal or nearly equal,
Fem 3 5
called _salfemane_.
IV. Sal 3 1
——— < — > —, with dominant femic minerals, called _dofemane_.
Fem 5 7
V. Sal 1
——— < —, extremely rich in femic minerals, called _perfemane_.
Fem 7
Each of these classes is divided into two _subclasses_ according to
the proportions of two subgroups of the preponderant group of standard
minerals. Of salic minerals one subgroup includes quartz, feldspars,
and the feldspathoids; the other includes corundum and zircon. Of femic
minerals one subgroup includes the silicates with magnetite, ilmenite,
hematite, and rutile; the other contains apatite and the remaining
minerals of this group. Most known igneous rocks fall into the first
subclass of each class.
The classes are further divided into _orders_ according to the
proportions of certain minerals in the preponderant subgroups. Thus
Classes I, II, and III are each divided into nine orders on a basis of
the proportions of quartz and the feldspars, and of the feldspars and
the feldspathoids, quartz and feldspathoids not occurring together. The
orders may be described in the same terms for each of the first three
classes as follows:
Order I. Q 7
— > —, extremely rich in quartz, _perquaric_.
F 1
II. Q 7 5
— < — > —, quartz dominant over feldspar, _doquaric_.
F 1 3
III. Q 5 3
— < — > —, quartz and feldspar equal or nearly equal, _quarfelic_.
F 3 5
IV. Q 3 1
— < — > —, feldspar dominant over quartz, _quardofelic_.
F 5 7
V. Q or L 1
—————— < —, extremely rich in feldspar, _perfelic_.
F 7
VI. L 3 1
— < — > —, feldspar dominant over feldspathoids (lenads), _lendofelic_.
F 5 7
VII. L 5 3
— < — > —, feldspar and lenads equal or nearly equal, _lenfelic_.
F 3 5
VIII. L 7 5
— < — > —, lenads dominant over feldspars, _dolenic_.
F 1 3
IX. L 7
— > —, extremely rich in lenads, _perlenic_.
F 1
In classes IV and V the preponderant minerals are femic, and in
subclass 1 they are silicates, titanates, and ferrates, with hematite
and rutile. These are subdivided as follows:
Silicates—pyroxenes and olivine with akermanite in one subgroup; the
other minerals, magnetite, hematite, ilmenite, titanite, perofskite,
rutile, in the second subgroup. This first group is called _polic_,
mnemonic of pyroxene and olivine; the second group is called _mitic_,
mnemonic of magnetite, ilmenite, titanite.
There are five orders in each of these classes, as follows:
Order I. PO 7
—— > —, extremely rich in pyroxene or olivine, _perpolic_.
M 1
II. PO 7 5
—— < — > —, dominant pyroxene or olivine, _dopolic_.
M 1 3
III. PO 5 3
—— < — > —, pyroxene or olivine, equal or nearly equal to the mitic
M 3 5
minerals, _polmitic_.
IV. PO 3 1
—— < — > —, dominant mitic minerals, _domitic_.
M 5 7
V. PO 1
—— < —, extremely rich in mitic minerals, _permitic_.
M 7
In the first three orders a distinction between pyroxene and olivine is
recognized by sections, five in number:
Section 1. P 7
— > —, extremely rich in pyroxene, _perpyric_.
O 1
2. P 7 5
— < — > —, dominant pyroxene, _dopyric_.
O 1 3
3. P 5 3
— < — > —, pyroxene and olivine, equal or nearly equal, _pyrolic_.
O 3 5
4. P 3 1
— < — > —, dominant olivine, _domolic_.
O 5 7
5. P 1
— < —, extremely rich in olivine, _perolic_.
O 7
In the last two orders a distinction between the preponderant mitic
minerals is recognized by suborders, five in number. The minerals
containing Fe₂O₃ are compared with those containing TiO₂. The former,
magnetite and hematite, are called _hemic_, mnemonic of hematite; the
latter subgroup, titanite, ilmenite, perofskite, rutile, are called
tilic, mnemonic of titanite and ilmenite. Of orders 4 and 5, there are
Suborder 1. H 7
— > —, hemic minerals extreme, _perhemic_.
T 1
2. H 7 5
— < — > —, dominant hemic minerals, _dohemic_.
T 1 3
3. H 5 3
— < — > —, hemic and tilic minerals equal or nearly equal,
T 3 5
_tilhemic_.
4. H 3 1
— < — > —, dominant tilic minerals, _dotilic_.
T 5 7
5. H 1
— < —, tilic minerals extreme, _pertilic_.
T 7
Further subdivision, producing rangs and subrangs, is made on the
character of the chemical bases in the standard minerals used in
forming orders and is expressed in terms of the molecular proportions
of certain oxides. For the salic minerals, forming orders in the first
three classes, the bases are alkalies—K₂O and Na₂O—and lime, CaO. For
the femic minerals, forming orders in the last two classes, the bases
are MgO, FeO, CaO and alkalies, K₂O, Na₂O. In classes I, II, and III
rangs are formed by comparing salic alkalies, K₂O′ + Na₂O′, with salic
lime, CaO′; and subrangs are formed by comparing K₂O′ with Na₂O′.
Rang 1. K₂O′ + Na₂O′ 7
——————————— > —, alkalies extreme, _peralkalic_.
CaO′ 1
2. 7 5
„ < — > —, alkalies dominant, _domalkalic_.
1 3
3. 5 3
„ < — > —, alkalies and lime equal or nearly so,
3 5
_alkalicalcic_.
4. 3 1
„ < — > —, lime dominant, _docalcic_.
5 7
5. 1
„ < —, lime extreme, _percalcic_.
7
Subrang 1. K₂O′ 7
———— > —, potash extreme, _perpotassic_.
Na₂O′ 1
2. 7 5
„ < — > —, potash dominant, _dopotassic_.
1 3
3. 5 3
„ < — > —, potash and soda equal, _sodipotassic_.
3 5
4. 3 1
„ < — > —, soda dominant, _dosodic_.
5 7
5. 1
„ < —, soda extreme, _persodic_.
7
In classes IV and V rangs are formed by comparing femic MgO + FeO +
CaO″ with femic alkalies K₂O″ + Na₂O″.
Minerals containing magnesia, iron, and lime are called _mirlic_.
Rang 1. MgO + FeO + CaO″ 7
———————————————— > —, extremely mirlic, _permirlic_.
K₂O″ + Na₂O″ 1
2. 7 5
„ < — > —, dominantly mirlic, _domirlic_.
1 3
3. 5 3
„ < — > —, equally mirlic and alkalic, _alkalimirlic_.
3 5
4. 3 1
„ < — > —, dominantly alkalic, _domalkalic_.
5 7
5. 1
„ < —, extremely alkalic, _peralkalic_.
7
Sections of rangs distinguish between MgO + FeO and CaO″. Minerals with
MgO + FeO are called _miric_.
Section 1. MgO + FeO 7
————————— > —, extremely miric, _permiric_.
CaO″ 1
2. 7 5
„ < — > —, dominantly miric, _domiric_.
1 3
3. 5 3
„ < — > —, equally miric and calcic, _calcimiric_.
3 5
4. 3 1
„ < — > —, dominantly calcic, _docalcic_.
5 7
5. 1
„ < —, extremely calcic, _percalcic_.
7
Subrangs distinguish between MgO and FeO, thus:
Subrang 1. MgO 7
——— > —, extremely magnesic, _permagnesic_.
FeO 1
2. 7 5
„ < — > —, dominantly magnesic, _domagnesic_.
1 3
3. 5 3
„ < — > —, equally magnesic and ferrous, _magnesiferrous_.
3 5
4. 3 1
„ < — > —, dominantly ferrous, _doferrous_.
5 7
5. 1
„ < —, extremely ferrous, _perferrous_.
7
Finally a recognition of the character of the subordinate standard
minerals leads to further subdivisions known as _grads_ and _subgrads_.
They only occur in classes II, III, and IV, because these are the only
ones in which the subordinate minerals are in notable amounts. _Grads_
are formed in a manner similar to that employed to produce orders. Thus
grads in classes II and III correspond to orders in class IV and the
reverse. Subgrads are the same in form as rangs when the difference in
the treatment of salic and femic minerals is borne in mind. The names
given to these divisions, which in fact recognize only the character of
the magma, are derived from geographical localities and embrace many of
those already in use, except that the names of orders are taken from
countries or nations. Specific terminations indicate the place in the
series of divisions:
_ane_ for class, _one_ for subclass.
_are_ for order, _ore_ for suborder.
_ase_ for rang, _ose_ for subrang.
_ate_ for grad, _ote_ for subgrad.
This may be illustrated as follows:
Class I. _persalane_, all rocks extremely salic.
Order 4. _britannare_, feldspar dominant over quartz, quardofelic. Many
rocks of granitic composition whether crystalline or glassy.
Rang 1. _liparase_, peralkalic, rocks in which the potential feldspars
are extremely alkalic, orthoclase, or albite.
Subrang 2. _Omeose_, dopotassic, rocks in which the extremely alkali
feldspars are dominantly potassic, orthoclase, with subordinate albite.
Examples of omeose are: granite from Omeo, Victoria, Australia, and
rhyolite from Silver Cliff, Colorado.
The presence of distinctive minerals not indicated in the standard
mineral composition of _norm_ is expressed by qualifying the magmatic
name by the name of the distinctive mineral; as, a hornblende-monzonose.
The precise texture of the rock is expressed by qualifying the magmatic
name by a textural adjective; as, a grano-monzonose, a vitro-monzonose,
a phyro-monzonose, etc.
REFERENCE LIST OF THE MORE COMMON MINERALS.
=Actinolite=—a magnesium-calcium-iron amphibole (q.v.); commonly bright
green to grayish green; crystals usually slender or fibrous.
=Agate=—a banded or variegated chalcedony (quartz, q.v.).
=Alabaster=—a fine-grained variety of gypsum (q.v.), either white or
delicately colored.
=Albite=—a soda feldspar (q.v.), an aluminum-sodium silicate; H. 5–6;
cleavage perfect in two planes; luster vitreous or pearly white;
occasionally bluish gray, reddish, greenish; sometimes opalescent.
=Amethyst=—a variety of quartz of purple or bluish-violet color, due
probably to manganese.
=Amphibole=—the type of an important group of rock-forming minerals
known as the amphibole or hornblende group; a ferromagnesian silicate,
monoclinic, H. 5–6; luster vitreous to pearly; fibrous varieties
often silky; black, ranging through various shades of green to
light colors; embraces the magnesium-calcium varieties, tremolite
and nephrite; the magnesium-calcium-iron variety actinolite; the
aluminous-magnesium-iron-calcium variety _hornblende_, and others.
=Analcite=—analcine, one of the zeolites; a hydrous aluminum-sodium
silicate; luster vitreous, colorless, white; occasionally grayish,
greenish, yellowish, reddish, transparent to opaque.
=Andesine=—a plagioclase feldspar (q.v.); a sodium-calcium-aluminum
silicate, intermediate in composition between albite and anorthite; H.
5–6; white, gray, grayish, yellowish, flesh red; luster subvitreous,
inclining to pearly.
=Andalusite=—an aluminum silicate; luster vitreous; whitish, rose red,
flesh red, variety pearly gray, reddish brown, olive-green; H. 7.5,
infusible; impurities sometimes so arranged in the interior as to
exhibit a colored, crossed, or tesselated appearance in cross-section
(chiastolite).
=Anhydrite=—a calcium sulphate; H. 3–3.5; luster pearly to vitreous;
white, sometimes bluish or reddish; differs from gypsum in absence of
water and in its greater hardness.
=Anorthite=—a plagioclase feldspar (q.v.); a calcium-aluminum silicate;
varies much by impurities and admixtures; H. 6–6.5; pearly or vitreous
luster; white, grayish, reddish.
=Anthracite=—hard coal; hydrocarbon with impurities; supposed to be
derived from bituminous coal by metamorphism.
=Antimony=—a native metal, tin-white, brittle; rather rare in native
form.
=Apatite=—essentially calcium phosphate with chlorine or fluorine;
hexagonal; H. 5; luster vitreous or subresinous; colors usually
greenish to bluish, characterized by a hexagonal form.
=Aragonite=—a calcium carbonate; differs from calcite in _cleavage_,
and in being orthorhombic; H. 3.5–4; luster vitreous or resinous;
white, also gray, yellow, green, and violet.
=Asphaltum=—asphalt; mineral pitch, bitumen; a natural mixture of
different hydrocarbons; odor bituminous; melts at 90 to 100 degrees
C.; burns with a bright flame; graduates into mineral tars and through
these into petroleum; probably the residue of the latter.
=Augite=—one of the pyroxenes (q.v.); an
aluminum-calcium-magnesium-iron silicate; H. 5–6; monoclinic, crystals
usually thick and stout; sometimes lamellar; also granular; black,
greenish black, deep green; an important rock-forming mineral.
=Beauxite=—essentially hydrated alumina; occurs in concretionary grains
of clay-like form, whitish to brown; valuable as a source of aluminum.
=Beryl=—a beryllium-aluminum silicate; hexagonal; prismatic; H. 8;
luster vitreous or resinous; marl-green, pale passing into whitish;
closely resembles apatite, but distinguished by superior hardness and
in composition.
=Barite=—barites, heavy-spar, barium sulphate; orthorhombic, H. 3–3.5;
luster vitreous to resinous, sometimes pearly; white, inclining to
yellow, gray, blue, red, or brown; very heavy, sp. gr. 4.3–4.7.
=Biotite=—black mica, a potash-aluminum-magnesium-iron silicate;
monoclinic; easy basal cleavage into thin laminæ; sometimes occurs as
a massive aggregation of cleavable scales; H. 2.5–3; luster splendent
on cleavage surface; black to dark green; cleavage surfaces smooth and
shining; a very common constituent of crystalline rocks.
=Bitumen=—the same as asphaltum (q.v.).
=Bismuth=—a metal of whitish color and rather brittle nature; occurring
occasionally native, usually as an ore.
=Bronzite=—a variety of enstatite (q.v.); grayish green to olive-green
and brown with luster on cleavage surface often adamantine, pearly or
bronze-like and submetallic.
=Calcite=—calcspar; calcium carbonate; rhombohedral, perfect
rhombohedral cleavage; often taking the forms known as dogtooth spar,
nail-head spar; frequently stalactitic and stalagmitic; H. 2.5–3.5;
luster vitreous; white, occasionally pale shades of gray, red, green,
blue, violet, yellow, brown; strong double refraction; embraces variety
called Iceland spar; a very common mineral; the essential basis of
limestone.
=Cassiterite=—tin stone; an oxide of tin; tetragonal; luster
adamantine, usually splendent; brown or black, sometimes red, gray,
white, or yellow; an important source of tin.
=Catlinite=—essentially a hardened red clay, rather a rock than a
mineral; much prized by Indians for pipes.
=Chalcedony=—a cryptocrystalline variety of quartz having a wax-like
luster, either transparent or translucent; white, grayish, pale brown
to dark brown, black, sometimes delicate blue, occasionally other
shades; frequently occurs as the lining or filling of cavities, taking
on a botryoidal or mamillary form.
=Chiastolite=—andalusite (q.v.).
=Chlorite=—the type of an important group of secondary minerals usually
characterized by a green color, softness and smoothness or unctuousness
of feeling; they are usually aluminum-magnesium-iron silicates, with
chemically combined water; derived from several other species, as
pyroxene, amphibole, biotite, garnet, etc.; embraces a number of
species, among which are clinochlore, penninite, prochlorite, and
delessite.
=Chromite=—chromic iron; essentially an iron chromate; isometric;
luster submetallic; iron black to brownish black; opaque; sometimes
magnetic; resembles magnetite.
=Chrysolite=—olivine; essentially a magnesium-iron silicate;
orthorhombic; H. 6–7; luster vitreous; green, commonly olive-green,
sometimes yellow, brownish, grayish green; highly infusible; a common
constituent of certain basic igneous rocks; the name olivine is more
commonly used by geologists.
=Chrysotile=—a delicately fibrous variety of serpentine (q.v.).
=Corundum=—alumina; an oxide of aluminum; H. 9; rhombohedral; large
crystals usually rough; luster vitreous; color blue, red, yellow, gray,
and nearly white; purer forms of fine colors are _sapphires_; the red
variety is _ruby_, the yellow, oriental _topaz_, the green, _emerald_,
and the purple, _amethyst_; dark colors, with iron oxide, _emery_.
=Delessite=—a ferruginous chlorite, usually olive-green or blackish
green; occurring commonly in the cavities of amygdaloids.
=Diallage=—a variety of pyroxene (q.v.); H. 4; characterized by thin
foliæ; usually grayish green to grass-green, or deep green; luster on
cleavage surface pearly, sometimes metalloid or brassy; an essential
mineral in the gabbros, as sometimes defined.
=Elæolite=—a variety of nephelite (q.v.); occurring in large coarse
crystals or massive, with greasy luster, from which the name is
derived; a characteristic constituent of elæolite syenite.
=Enstatite=—one of the pyroxenes; essentially a magnesium silicate;
orthorhombic; H. 5.5; luster a little pearly on cleavage surface;
metalloidal in the bronze variety (bronzite); grayish white, yellowish
white, greenish white to olive-green and brown; very infusible; a
common mineral in certain basic crystalline rocks.
=Epidote=—a complex aluminum-calcium-iron silicate of varying
composition; monoclinic; H. 6–7; luster vitreous, pearly, or resinous;
color usually pistachio-green, or yellowish green to brownish green;
can usually be detected by its peculiar pistachio hue, which is seldom
found in other minerals; common in many crystalline rocks, usually as a
secondary product.
=Feldspar=—a group of minerals of the first importance in rock
formation, embracing orthoclase, microcline, albite, oligoclase,
andesine, labradorite, anorthite, and numerous variations; aluminum
silicates, with either potassium, sodium, or calcium or two or more
of these; crystallizes in both the monoclinic and triclinic systems;
possesses very distinct cleavage in two directions; H. 6–6.5; range in
color from white through pale yellow, red, or green, and occasionally
dark; triclinic feldspars frequently called plagioclase (see individual
feldspars).
=Fluorite=—fluorspar; calcium fluoride; isometric, usually cubic;
H. 4; luster vitreous, sometimes splendent; white, yellow, green,
rose, crimson red, violet, sky-blue, and brown; yellow, greenish, and
violet most common; occurs usually in veins or cavities in beautiful
crystalline form.
=Galenite=—galena; lead sulphide; isometric, usually cubic; perfect
cubic cleavage; luster metallic; lead-gray; a common ore of lead;
occurs in veins and layers, also as linings of cavities.
=Garnet=—a complex silicate of varying composition, embracing aluminum,
calcium, magnesium, chromium, iron, and manganese, but usually only
two or three of these are present in abundance, and the varieties
are characterized by the leading constituent; isometric, usually
in dodecahedrons or trapezohedrons; H. 6.5–7.5; luster vitreous to
resinous; commonly red or brown, sometimes yellow, white to blue, green
or black; common in mica schist, gneiss, hornblende schist; also in
granite, syenite, and metamorphosed limestone.
=Geyserite=—a concretionary deposit of silica in the opal condition;
formed about geysers; white or grayish.
=Glauconite=—green-sand, a hydrous potassium-iron silicate usually
impure, amorphous, or earthy; dull olive-green or blackish, yellowish,
or grayish green; opaque, commonly occurs as grains or small
aggregations.
=Graphite=—plumbago, black lead; a form of carbon, usually impure;
rhombohedral, but rarely appearing as a crystal; more often as thin
laminæ of greasy feel; yields a black adhesive powder; hence its
common use for lead pencils; occurs in granite, gneiss, mica schist,
crystalline limestone; sometimes results from alteration of coal by
heat; occasionally occurs in basaltic rocks and meteorites.
=Gypsum=—a hydrous calcium sulphate; monoclinic; perfect cleavage into
smooth polished plates; occurs in a variety of forms, including fibrous
and granular; H. 1.5–2; luster pearly and shiny; white, sometimes gray,
flesh-red, yellowish, and blue; impure varieties dark; crystallized
varieties include selenite, satinspar, alabaster, etc.; easily
recognized by its softness and want of effervescence with acids; occurs
in beds; calcined and ground constitutes plaster of Paris.
=Haüynite=—a complex sodium-aluminum silicate and calcium sulphate;
crystals dodecahedrons; luster vitreous or somewhat greasy; bright
blue, sky-blue, or greenish blue, or green; occurs in certain igneous
rocks, commonly associated with nephelite and leucite.
=Hematite=—ferric oxide, Fe₂O₃, iron-sesquioxide; rhombohedral, more
commonly columnar, granular, botryoidal, or stalactitic; luster
metallic, sometimes earthy; iron-black, dark steel-gray, red when
earthy; gives red streak or powder; a leading iron ore, 70 percent.
metallic iron when pure; the chief source of the red color of soils and
rocks generally.
=Hornblende=—an amphibole; name sometimes used as a synonym for
amphibole; sometimes to designate a variety under amphibole (q.v.).
=Hyalite=—a variety of silica in the opal condition; clear and
colorless like glass, consisting of globular concretions or crusts.
=Hypersthene=—one of the pyroxenes; a ferromagnesian silicate;
orthorhombic; H. 5–6; luster somewhat pearly on cleavage; surface often
iridescent; dark brownish green, grayish, or greenish black and brown;
a frequent constituent of crystalline rocks.
=Iceland spar=—a form of transparent calcite (q.v.).
=Ilmenite=—menaccanite; a titanium iron oxide; rhombohedral; resembles
hematite; luster submetallic; iron-black; powder black or brownish red;
occurs frequently in crystalline rocks associated with magnetite.
=Iron pyrites=—pyrite (q.v.).
=Kaolin=—kaolinite; essentially a hydrous aluminum silicate; usually
in clay-like or earthy form; white or grayish white; often tinged with
impurities; commonly arises from decomposition of aluminous silicates,
especially the feldspars; basis of pottery and china.
=Labradorite=—a plagioclase feldspar; essentially an
aluminum-calcium-sodium silicate; composition intermediate between that
of albite and anorthite; triclinic; H. 6; luster pearly or vitreous,
gray, brown, or greenish; sometimes colorless or white; frequently
shows play of colors; important constituent of various crystalline
rocks, especially of the basic class; usually associated with a
pyroxene or amphibole.
=Lepidolite=—lithia mica; essentially like muscovite (q.v.) except that
potash is replaced by lithia.
=Leucite=—essentially an aluminum-potassium silicate, allied to the
feldspars; H. 5–6; luster vitreous, white, ash-gray, or smoke-gray;
occurs in certain volcanic rocks, particularly lavas of Vesuvius.
=Limonite=—brown hematite, ocher;—a hydrous iron oxide; commonly
earthy; also concretionary, stalactitic, botryoidal, and mamillary,
with fibrous structure; H. 5–5.5; luster silky, sometimes submetallic,
but commonly dull and earthy; brown, ocherous yellow; streak and powder
yellowish brown; constitutes ocher, bog-ore, ironstone, etc.; is the
chief source of the yellow color of soils and rocks; arises from the
alteration of other iron ores.
=Magnesite=—magnesium carbonate; rhombohedral; white, yellowish,
grayish white to brown; fibrous, earthy, or massive; found in altered
magnesium rocks.
=Magnetite=—magnetic iron ore; iron oxide, Fe₃O₄; octahedral or
dodecahedral; strongly magnetic; H. 5.5–6.5; abounds in igneous and
metamorphic rocks.
=Marcasite=—white iron pyrites; iron sulphide; same composition as
pyrite, which it closely resembles; H. 6–6.5; luster metallic, pale
gray, bronze, or yellow; prone to decomposition; disseminated through
various rocks, particularly plastic clays containing organic matter.
=Martite=—iron sesquioxide; originally magnetite, which by oxidation
has assumed the composition of hematite.
=Mica=—the type of an important group of rock-forming minerals well
known for their perfect cleavage into thin elastic laminæ; among the
leading varieties are the common potassium mica (muscovite), the sodium
mica (paragonite), the lithium mica (lepidolite), the magnesium-iron
mica (biotite), the magnesium mica (phlogopite), and the iron-potash
mica (lepidomelane).
=Menaccanite=—ilmenite; titanium iron ore (q.v.).
=Microcline=—a triclinic feldspar, closely resembling orthoclase in
appearance and having the same composition.
=Muscovite=—common or potash mica; essentially an aluminum-potassium
silicate; H. 2–2.5; monoclinic; remarkable for its basal cleavage;
splits easily into exceedingly thin, flexible, elastic laminæ; luster
vitreous, more or less pearly or silky; colorless or variously tinged
brown, green, or violet; a common mineral in crystalline rocks,
particularly in the granites or gneisses.
=Nephelite=—nepheline; essentially an aluminum-sodium silicate with
potash; allied to the soda-feldspars; hexagonal; usually in thick
prisms; H. 5.5–6; luster vitreous to greasy, white or yellowish,
varying to greenish, bluish, and red; occurs in volcanic rocks; the
variety elæolite characterizes the elæolite syenite.
=Nosite=—nosean; a complex sodium-aluminum silicate and sulphate, like
haüynite, but with little calcium; common in phonolites.
=Oligoclase=—a plagioclase feldspar; essentially an
aluminum-calcium-sodium silicate which may be regarded as a mixture of
albite and a small amount of anorthite; triclinic; luster vitreous,
pearly, or waxy; whitish grading into greenish and reddish; H. 6–7;
common in crystalline rocks.
=Orthoclase=—a potash feldspar; essentially a potassium-aluminum
silicate; varying by the replacement of the potassium by sodium and
less frequently by other substitutions; monoclinic; occurring in
distinct crystals and also in cryptocrystalline forms; cleavage planes
perfect with pearly luster on cleavage surface; white, gray, and
flesh-red, occasionally varying to greenish white and bright green;
H. 6–6.5; difficultly fusible; sanidine a glassy variety; felsite
a cryptocrystalline form; a very common mineral, especially in the
granites and gneisses.
=Olivine=—chrysolite (q.v.).
=Omphacite=—a variety of pyroxene of grass-green color and silky to
fibrous luster; allied to diallage.
=Opal=—silica with a varying amount of water; differs from quartz in
a lack of crystallization and in lower degree of hardness; amorphous,
massive; sometimes reniform, stalactitic, or tuberous; also earthy; H.
5.5–6.5; luster vitreous, inclining to resinous; white, yellow, red,
brown, green, gray, blue, generally pale; colors arise from admixtures;
sometimes play of colors as in precious opal.
=Ozocerite=—a native paraffine, mineral wax.
=Petroleum=—naphtha; a native mineral oil; a hydrocarbon, commonly
believed to arise from organic matter, both animal and vegetable, but
held by some to be due to deep-seated chemical and thermal action.
=Pictotite=—a variety of spinel, containing chromium.
=Pisolite=—a concretionary variety of calcite.
=Picrolite=—a variety of serpentine.
=Piedmontite=—a manganese epidote.
=Plagioclase=—a general term embracing the triclinic feldspars whose
two cleavages are oblique to each other; embracing albite, oligoclase,
andesine, labradorite, and anorthite (q.v.).
=Plumbago=—graphite (q.v.).
=Psilomelane=—essentially a hydrous manganese oxide occurring
in massive, botryoidal, reniform, and stalactitic forms; luster
submetallic; iron-black, passing into dark steel-gray; H. 5–6; the
common ore of manganese.
=Pseudomorph=—a false form, i.e., having the form of one mineral and
the composition of another; usually arises from the replacement of a
mineral, particle by particle, by a solution of another substance,
leaving the original form unchanged.
=Pyrite=—iron pyrites, fool’s gold, iron sulphide; isometric; commonly
in cubes; H. 6–6.5; luster metallic, splendent, or glistening; pale
brass-yellow; occurs widely disseminated throughout a large class of
rocks; usually harder and lighter in color than copper pyrites, and
deeper in color than marcasite, which has the same composition.
=Pyroxene=—the type of a large and important group of rock-forming
ferromagnesian minerals; varies in composition and embraces a large
number of varieties; usually a magnesium-iron-calcium silicate;
crystals usually thick and stout, but varying greatly; sometimes
lamellar and fibrous; H. 5–6; luster vitreous inclining to resinous;
green of various shades verging towards light colors, occasionally
more often to browns and blacks; among the minerals belonging to the
pyroxene group are augite, bronzite, diallage, diopside, enstatite,
hypersthene, and others.
=Quartz=—crystallized silica; rhombohedral; crystals commonly six-sided
prisms capped by six-sided pyramids; without cleavage; H. 7; scratches
glass; usually transparent, glassy, colorless when pure, shaded by
impurities to yellow, red, brown, green, blue, and black; varieties,
amethyst, purple, or violet; false topaz, yellow, rose-quartz, smoky,
milky, cat’s eye, opalescent; aventurine, spangled with scales of mica;
chalcedony is a cryptocrystalline variety; carnelian, a red chalcedony;
chrysoprase, an apple-green chalcedony; prase, a leek-green variety;
agate, a variegated or banded chalcedony; moss-agate, a chalcedony
containing moss-like or dendritic crystallizations of iron or manganese
oxide; onyx, a chalcedony in layers; sardonyx, like onyx in structure,
but includes layers of sard (carnelian); jasper, an opaque-colored
quartz, usually red or brown; flint, an opaque impure chalcedony;
chert, an ill-defined term applied to an impure flinty rock; hornstone,
a translucent, brittle, flinty rock.
=Rutile=—titanium oxide; tetragonal, crystals commonly in prisms; H.
6–6.5; luster metallic, adamantine; reddish brown, passing to red;
sometimes yellowish, bluish, violet, and black; occurs in crystalline
rocks and is a common secondary product in the form of microlites.
=Sanidine=—a glassy variety of orthoclase feldspar.
=Satinspar=—a variety of selenite or gypsum.
=Selenite=—a distinctly crystallized transparent form of gypsum.
=Serpentine=—a hydrous magnesium silicate; usually in pseudomorph
forms; also fibrous, granular, cryptocrystalline, and amorphous; H.
2.5–4; luster subresinous to greasy, pearly or earthy, resinous or
wax-like; feel, smooth and somewhat greasy; leek-green to blackish
green and siskin green verging into brownish and other colors;
apparently derived most commonly from chrysolite or olivine and also
from other magnesian minerals; sometimes constitutes the bulk of rock
masses.
=Siderite=—iron carbonate; rhombohedral; H. 3.5–4.5; luster vitreous,
more or less pearly, ash-gray, yellowish or greenish, also brownish;
occurs as extensive iron deposits and in crystalline rocks.
=Smaragdite=—a form of amphibole or hornblende (q.v.).
=Spherosiderite=—a globular form of siderite.
=Spinel=—a magnesium-aluminum oxide; crystals, octahedrons; red of
various shades, passing into other colors; spinel-ruby is a variety.
=Staurolite=—a complex hydrous iron-magnesium-aluminum silicate;
orthorhombic; disposed to cruciform shapes; occurs in schists and
gneisses.
=Steatite=—soapstone, a variety of talc (q.v.); a hydrous magnesium
silicate.
=Sulphur=—a well-known element occurring native in volcanic regions;
also formed by the decomposition of sulphides, particularly pyrites.
=Talc=—a hydrous magnesium silicate; usually in foliæ; granular or
fibrous forms; also compact; easy cleavage into thin flexible laminaæ,
but not elastic; feel greasy; luster pearly on cleavage surface;
apple-green to silvery white; H. 1–2; a secondary product from the
alteration of magnesian minerals; distinguished by its soft, soapy
feel, soapstone being one variety; whitish form is known as French
chalk.
=Titanite=—calcium-titano-silicate; monoclinic; luster adamantine to
resinous; brown, gray, yellow, green, and black; H. 5–5.5; occurs in
various crystalline rocks.
=Topaz=—an aluminum silicate, with part of the oxygen replaced by
fluorine; orthorhombic; H. 8; luster vitreous; colorless, straw-yellow
verging to various pale shades, grayish, greenish, bluish, and reddish;
distinguished by its hardness and infusibility; occurs in crystalline
rock.
=Tremolite=—a calcium-magnesium amphibole; a common constituent of
certain crystalline rocks.
=Viridite=—a general term used for green products of rock alteration,
usually hydrous silicates of iron and magnesia; mainly chlorite.
=Wad=—bog manganese; a variety of psilomelane (q.v.).
=Zeolite=—a group of minerals derived from the alteration of various
aluminous silicates.
=Zircon=—zirconium silicate; H. 7.5; luster adamantine; pale yellowish,
grayish, yellowish green, brownish yellow, and reddish brown;
infusible; occurs characteristically in square prismatic forms; found
in crystalline rocks and granular limestone.
REFERENCE LIST OF THE MORE COMMON ROCKS.[206]
=Adobe=—a fine silty or loamy deposit formed by gentle wash from
slopes and subsequent lodgment on flats; especially applied to silty
accumulations in the basins and on the plains of the western dry region.
=Agglomerate=—an aggregate of irregular, angular, or subangular blocks
of varying sizes, usually of volcanic origin, distinguished from
conglomerate in which the constituents are rounded.
=Alluvium=—sediment deposited by streams.
=Amygdaloid=—a vesicular igneous rock whose cavities have become filled
with minerals; the fillings are called _amygdules_, because sometimes
almond-like in form.
=Andesite=—an aphanitic igneous rock consisting essentially of the
plagioclase feldspar _andesine_ (sometimes oligoclase) and pyroxene (or
some related ferromagnesian mineral); sometimes cellular, porphyritic,
or even glassy; usually rich in feldspar microlites.
=Anorthosite=—a rock consisting mainly of the feldspar labradorite.
=Aphanite=—a rock whose constituents are so minute as to be
indistinguishable to the naked eye; rather a condition of various rocks
than of any specific rock.
=Aqueous rocks=—a general term applied to rocks deposited through the
agency of water.
=Arenaceous rocks=—either those which are mainly sand or those in
which sand is a notable accessory.
=Argillite=—a clayey rock; usually applied to hard varieties only.
=Arkose=—a sand or sandstone formed of disaggregated granite or similar
rock in which a notable part of the grains are feldspar or other
silicate; sand when undefined, is understood to be quartzose.
=Augitite=—a rock mainly made up of augite.
=Basalt=—a dark, compact basic igneous rock consisting of a mass
of minute crystals sometimes with more or less glassy base, often
containing also visible crystals; composed of plagioclase and pyroxene,
with olivine, magnetite, or titaniferous iron as common accessories;
a basic lava in which the crystallization has taken place rapidly;
usually rich in crystallites or microlites; graduates into dolerite and
basic andesite.
=Bituminous coal=—common soft coal, intermediate between lignite and
anthracite; contains much bituminous matter, i.e., hydrocarbons.
=Bowlders=—rounded masses of rock, particularly those that have been
shaped by glaciers.
=Breccia=—a rock composed of angular fragments, contrasted with
pudding-stone or conglomerate, in which the fragments are rounded.
=Buhrstone=—a compact, flint-like silicious rock full of small
cavities, so named from use as millstones.
=Calc-sinter= (calcareous tufa)—a loose cellular deposit of calcium
carbonate made by springs; travertine is the better term, as tufa
should be left for volcanic elastics.
=Cannel coal=—a very fine-grained homogeneous bituminous coal, giving
off much gas and burning with a candle-like flame.
=Chalk=—a fine-grained soft rock composed essentially of calcium
carbonate derived from minute marine organisms.
=Chlorite schist=—a schistose rock in which chlorite is a predominant
mineral; usually greenish, whence the name.
=Clastic rock=—formed from the débris of broken-down rocks; the same as
fragmental or detrital rock.
=Clay=—a term commonly applied to any soft, unctuous, adhesive deposit,
but in strict use confined to material composed of aluminum silicate;
many so-called clays are chiefly silicious silts or loams.
=Clay ironstone=—a clayey rock heavily charged with iron oxide, usually
limonite; commonly in concretionary form.
=Clinkstone=—a name applied to phonolite because of its metallic
clinking sound when struck; composed of orthoclase, with nephelite and
one or more of the ferromagnesian minerals as accessories.
=Chert=—an impure flint, usually of light color, occurring abundantly
in concretionary form as nodules in certain limestones.
=Coal=—a carbonaceous deposit formed from the remains of plants by
partial decomposition.
=Concretions=—aggregates of rounded outlines formed about a nucleus;
the material is various: clay, iron ore, calcite, silica, etc.
=Conglomerate= (pudding-stone)—a rock formed from rounded pebbles,
consolidated gravel.
=Coquina=—a rock formed almost wholly of small and broken shells;
especially applied to a shell limestone of Florida.
=Dacite= (quartz-andesite)—an andesite (q.v.) with quartz.
=Diabase=—a dolerite (q.v.) which has undergone alteration; consists
essentially of plagioclase feldspar and augite, with magnetite or
titaniferous iron as a common accessory; one of the greenstones.
=Diatom ooze=—a soft silicious deposit found on the bottom of the deep
sea, made largely or partly of the shells of diatoms; similar deposits
are formed from the shells of radiolaria.
=Diorite=—an igneous rock usually of dark-greenish color, consisting
of plagioclase feldspar and hornblende; often speckled from the
commingling of light feldspar and dark hornblende.
=Dolerite=—a fine-grained igneous rock composed of plagioclase feldspar
(labradorite or anorthite) and augite (or related ferromagnesian
mineral, as enstatite, olivine, or biotite), with magnetic or
titaniferous iron as common accessories; crystals usually of medium
size, assuming the ophitic structure; embraces many of the greenstones;
graduates into basalt on the one hand and gabbro on the other.
=Dolomite=—a magnesian limestone.
=Drift=—in common American usage, a mixture of clay, sand, gravel, and
bowlders formed by glacial agencies.
=Eolian rocks=—deposits formed by wind, embracing especially dunes and
one variety of loess.
=Felsite= (felstone)—a light-colored aphanitic rock composed of
feldspar often with quartz, in which the crystallization is very
imperfect or obscure, giving a close-grained texture with conchoidal
fracture and flinty aspect; certain varieties are called petrosilex and
hälleflinta.
=Flint=—a compact dark chalcedonic or lithoid form of quartz.
=Freestone=—a sandstone of uniform grain without special tendency to
split in any direction.
=Fulgurites=—glassy tubes, produced through fusion by lightning in
penetrating sand, earth, or rock.
=Gabbro= (euphotide)—a crystalline rock composed of the plagioclase
feldspar, labradorite (or anorthite), and diallage (or a related
ferromagnesian mineral), with magnetite or titaniferous iron as a
common accessory.
=Gangue=—a term applied to the crystalline material in which ores are
imbedded.
=Gannister=—essentially a quartz silt or pulverized quartz used for
lining iron furnaces.
=Garnetite=—a rock composed largely of garnets.
=Geest=—residual earth or clay left by the decomposition of rocks,
especially limestones.
=Geyserite=—the silicious sinter deposited about hot springs.
=Globulites=—minute spherical bodies embraced in volcanic glass.
=Gneiss=—a foliated granite, consisting typically of quartz, feldspar,
and mica; the feldspar typically orthoclase.
=Granite=—a granular crystalline aggregate of quartz, feldspar, and
mica; the feldspar typically orthoclase; popularly and properly used
for any distinctly granular crystalline rock.
=Granitell=—a name used to designate a quartz-feldspar rock.
=Granitite=—a biotite granite with quartz.
=Granulite=—a fine-grained granite with little or no mica.
=Greensand=—a sand or sandstone containing a notable percentage of
grains of glauconite.
=Greenstone=—a comprehensive term used to designate igneous and
metamorphic crystalline rocks of greenish hue and of intricate and
often minute crystallization; they are mostly dolerites, diabases, and
diorites; a convenient term for field use where the constituents cannot
be determined, and for general use when the variety is unimportant.
=Greisen=—an aggregate of quartz and mica, i.e., a granite without
feldspar.
=Greywacke=—a sand rock in which the grains are basic silicates instead
of quartz.
=Hälleflinta=—a compact flint-like felsitic rock.
=Hornblendite=—a rock essentially composed of hornblende.
=Hornstone=—a very compact, silicious rock of horn-like texture, allied
to flint; term also applied to flinty forms of felsite.
=Hypogene rocks=—those formed deep within the earth under the influence
of heat and pressure.
=Ironstone=—a rock composed largely of iron, usually applied to clayey
rocks having a large iron content.
=Infusorial earth= (tripolite)—an earthy or silt deposit consisting
chiefly of the silicious shells of diatoms.
=Itacolumite=—a flexible sandstone whose pliability is due to an open
arrangement of sand grains which are held together by scales of mica.
=Jasper=—a reddish variety of chalcedonic quartz.
=Keratophyre=—a felsite with a large percentage of soda.
=Kersantite=—a mica dolerite consisting chiefly of plagioclase, augite,
and biotite.
=Lapilli=—small fragments of lava ejected from volcanoes; volcanic
cinders.
=Laterite=—a red, porous, ferruginous residual earth of India and other
tropical countries.
=Lava=—a molten rock, especially applied to flows upon the surface,
whether from vents or from fissures; also applied to the solidified
product.
=Lignite= (brown coal)—a soft, brown, impure coal.
=Limburgite=—a compact basic igneous rock of the basaltic class,
composed essentially of augite and olivine, with magnetite iron and
apatite as common accessories.
=Limestone=—a rock composed primarily of calcium carbonate, though
magnesium sometimes replaces a part of the calcium. (See dolomite and
marble.)
=Liparite= (rhyolite)—an acidic igneous rock of aphanitic or glassy
texture, characterized by flowage lines and various microscopic
crystals; rhyolite is the more common American name.
=Loess=—a very fine porous silicious silt containing some calcareous
material which often collects in nodules (_Löss Kindchen_) or in
vertical tubules; characterized by a peculiar competency to stand in
vertical walls; held by some to be eolian, by others to be fluvial or
lacustrine, and by still others to be partly eolian and partly aqueous.
=Marble=—typically a granular crystalline limestone or dolomite
produced by metamorphic action; but the term is variously applied to
calcareous and even to other rocks that are colored ornamentally and
susceptible of polish.
=Marl=—an earth formed largely of calcium carbonate, usually derived
from the disintegration of shells; or the calcareous accretions
of plants, notably the stoneworts; term also sometimes applied to
glauconitic and other fertilizing earths.
=Melaphyre=—a term of varying usage; most commonly applied perhaps to
an altered basalt (q.v.), especially an olivine-bearing variety.
=Meta-diabase=—a term sometimes used for a metamorphic diabase; in like
manner _meta_ is prefixed to dolerite, syenite, etc.; not in general
use.
=Meta-igneous rock=—a metamorphosed igneous rock.
=Metamorphic rock=—a rock which has been altered, particularly one
which has been rendered crystalline, or recrystallized by heat and
pressure.
=Meta-sedimentary rock=—a metamorphosed sedimentary rock.
=Microgranite=—a very fine-grained granite.
=Microlites=—incipient crystals found in glassy lavas; usually
needle-shaped, or rod-like; occurring singly and in aggregates.
=Millstone=—see buhrstone.
=Minette= (mica-syenite)—a rock consisting essentially of orthoclase
and mica, or a syenite in which mica replaces hornblende or
predominates over it.
=Monzonite=—a granitic rock composed of orthoclase and plagioclase
in nearly equal proportions, with ferromagnesian minerals; a rock
intermediate between syenite and diorite.
=Mudstone=—solidified mud or silt, shale.
=Nephelinite=—a rock composed essentially of nepheline and augite, with
magnetite and other accessories.
=Nevadite=—a variety of rhyolite of granitoid aspect due to an
abundance of porphyritic crystals.
=Nodules=—concretionary aggregations of rounded form.
=Norite=—a fine-grained rock consisting of plagioclase and hypersthene.
=Novaculite= (honestone, oilstone)—a very fine-grained, hard sandstone
or silt-stone, used for whetstones.
=Obsidian=—a typical form of volcanic glass usually of the acidic class.
=Onyx=—a variety of chalcedonic quartz having colored bands alternating
with white; the “Mexican onyx” is a crystalline calcium carbonate,
variegated with delicate colors due to iron and manganese.
=Oolite=—a limestone or dolomite composed of small concretions
resembling the roe of fish.
=Ooze=—an exceedingly soft watery deposit of the deep sea;
characterized usually by microscopic shells from which it is mainly
derived, as diatom ooze, globigerina ooze, etc.
=Orthophyre= (orthoclase porphyry)—a rock consisting of crystals of
orthoclase in an aphanitic base.
=Peastone= (pisolite)—a very coarse variety of oolite.
=Peat=—the dark brown or black residuum arising from the partial
decomposition of mosses and vegetable tissue in marshes and wet places.
=Pegmatite=—a term of ill-defined usage applied to rocks whose grain
varies from coarser to finer, and often takes on peculiar aspects due
to the simultaneous crystallization and mutual intergrowths of the
crystals; graphic granite is a distinct type of pegmatite in which
quartz and orthoclase crystals grew together along parallel axes so
that cross-sections give figures resembling certain Semitic letters
(Fig. 345).
=Peridotite=—a very basic igneous rock composed chiefly of olivine with
augite or related ferromagnesian minerals, with magnetite and chromite
as accessories.
=Pelites=—a general term embracing clay rocks.
=Perlite= (pearlstone)—a form of glassy lava made up in part of small
spheroids formed of concentric layers which have a lustrous appearance
like pearls.
=Petrosilex=—an old name for felsite or hälleflinta.
=Phonolite= (nephelite-trachyte, clinkstone)—a compact resonant igneous
rock formed of sanidine and nephelite with accessories.
=Phyllite= (argillite)—a variety of indurated, partly metamorphosed,
clay silt in which finely disseminated micaceous scales are abundant
and lustrous; intermediate between typical clay slate and mica-schist.
=Pitchstone=—a dark vitreous, acid, igneous rock of less perfect glassy
texture than obsidian and more resinous and pitch-like.
=Plutonic rocks=—igneous rocks formed deep within the earth under the
influence of high heat and pressure; hypogene rocks; distinguished from
eruptive rocks formed at the surface.
=Porphyrite=—a term sometimes used for an altered form of andesite,
usually porphyritic in structure.
=Porphyry=—a rock consisting of distinct crystals embedded in an
aphanitic ground-mass.
=Propylite=—an altered form of andesite and similar igneous rocks.
=Protogine=—a hydrated micaceous or chloritic variety of granite or
gneiss.
=Pumice=—a glassy form of lava rendered very vesicular through
inflation by steam.
=Pyroclastic rocks=—fragmental or clastic rocks produced through
igneous agencies, embracing volcanic ashes, tuffs, agglomerates, etc.
=Pyroxenite=—an igneous rock consisting essentially of pyroxene.
=Quartzite=—a rock consisting essentially of quartz, usually formed
from quartzose sandstone by cementation or metamorphic action.
=Regolith=—a name recently suggested by Merrill to embrace the
earthy mantle that covers indurated rocks, chiefly residuary earths;
mantle-rock.
=Rhyolite=—an aphanitic or glassy igneous rock showing flowage lines,
usually applied only to the acidic varieties.
=Sandstone=—indurated sand usually composed of grains of quartz, but
not necessarily so; sometimes formed of calcareous grains or of grains
of the various silicates.
=Schist=—a crystalline rock having a foliated or parallel structure,
splitting easily into slabs or flakes, less uniform than slate; they
are mainly composed of the silicate minerals.
=Scoriæ=—light, cellular fragments of volcanic rock, coarser than
pumice; cinders.
=Septaria=—concretions the interior of which have parted, and the
gaping cracks become filled with calcite or other mineral deposited
from solution (Figs. 375–77).
=Serpentine=—a rock consisting largely of serpentine; derived in most
cases by alteration from magnesian silicate rocks.
=Shale=—a more or less laminated rock, consisting of indurated muds,
silts, or clays.
=Slate=—an argillaceous rock which is finely laminated and fissile,
either due to very uniform sedimentation or (more properly) to
compression at right angles to the cleavage planes; e.g., common
roofing-slate (Fig. 362).
=Soapstone= (steatite)—a soft unctuous rock, composed mainly of talc.
=Stalactites=—pendant icicle-like forms of calcium carbonate deposited
from dripping water.
=Stalagmite=—the complement of stalactites formed by calcareous waters
dripping upon the floors of caverns.
=Steatite=—see soapstone.
=Syenite=—a granitoid rock composed of orthoclase and hornblende,
or other ferromagnesian mineral; the name was formerly applied to a
granitoid aggregate of quartz, feldspar, and hornblende.