Rocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools online

cence in acetic acid (vinegar) and but very slowly in cold
hydrochloric; if it contains admixed calcite this reacts
very readily. The best test is a chemical one for magnesia



308



ROCKS AND ROCK MINERALS



in a solution obtained by boiling the powdered rock in
dilute hydrochloric acid.

The following analyses show the chemical composition
of some examples of this rock.





Si0 2


A1 2 3


Fe 2 O 3


FeO


MgO


CaO


CO 2


H 2 O


XyO


Total


I....


3.2


0.2


0.2


0.1


20.8


29.6


45.5


0.3




99.9


II...


3.1


_


0.1


0.9


20.0


29.7


45.3


0.3


0.2


99.6


III..


1.1


0.4








19.9


31.5


45.6


1.3


0.1


99.9


IV..


5.0


1.0


0.5





16.8


32.2


43.8








99.3



I, Knox Dolomite, Morrisville, Alabama; II, Dolomite, Sunday
Lake, Gogebic district, Michigan; III, Dolomite, Tornado Mine,
Black Hills, South Dakota; IV, Dolomite (magnesian limestone),
Newcastle, England.



The origin of dolomite is a matter which has been much discussed
and many theories have been propounded by geologists and
chemists in explanation of it. When all the facts are taken into con-
sideration, it is clear that dolomite is not an original rock, but has
been formed from pure limestones by the substitution of a part of
the lime by magnesia, from waters containing magnesium salts in
solution. Dolomite is a denser and more stable compound than
calcite; if the latter were subjected to the action of soluble magne-
sium salts there would be a constant tendency for dolomite to form
and part of the lime to be liberated, as illustrated in the following
reaction,

2 CaCO 3 + MgCl 2 = CaMg (CO 3 ) 2 + CaCl 2 .

It is evident that if the magnesia solution was strongly concen-
trated the exchange, in a given mass of limestone, would be effected
more quickly. If the solution were heated it would also act more
quickly; if it acted under pressure the result would also be hastened,
and, finally, as time is an important element, the longer the limestones
have been subjected to the solutions the more completely we may
expect them to be changed to dolomite. If we consider in addition,
that not only sea- water contains magnesium salts, but also the circu-
lating ground waters and thermal waters ascending from the depths,
in greater or lesser amount, it is clear that in harmony with the



DESCRIPTION OF STRATIFIED ROCKS 309

above principles, the change, which we may call dolomitization, must
take place in a variety of ways and under various conditions, not
only in the sea, but also on the land; that all limestones are not
converted completely into dolomite before they emerge from the sea
must be due to certain reasons; that the solution is too dilute, that
it is not hot enough, that there has not been sufficient time, that the
deposits are too compact to permit sufficient penetration and cir-
culation of sea-water, etc. But if lime deposits are subjected in
an enclosed basin to constantly concentrating sea-water they may
become more rapidly converted. This might happen, for instance,
if a coral atoll were somewhat elevated and its lagoon wholly or
nearly shut off from access to the sea. The formation of dolomite
in such enclosed basins of sea-water would also explain its frequent
association with gypsum and anhydrite. The application of the
principles stated above would also lead us to conclude, that the
older and more deeply buried a limestone was, the more apt it would
be to become a dolomite; that in disturbed and folded mountain
regions, limestones of the same age and formation would be more
likely to be dolomitic than those of undisturbed areas, because the
rocks are there more fractured and filled with thermal solutions, and
in practice the facts are found to confirm these views. The connec-
tion with thermal waters also explains the frequent association with
lead and zinc ores.

The mineral dolomite is denser than calcite and in the change
above mentioned a considerable reduction of volume, amounting
to 12 per cent, must occur in the limestone. This would apparently
help to explain why dolomites are so frequently very porous or
cavernous rocks, though if deeply buried, all such cavities would be
closed by the pressure.

Limestones and dolomites are rocks of such general
distribution in all parts of the world where stratified
rocks are found, that their occurrence needs no special
mention.

Oolite. Oolitic Limestone. This is a well-characterized
variety of limestone, which consists of minute to small
spherical concretions, presenting very much the aspect of
a fish roe, whence the name from the Greek, meaning egg-
stone. The round grains vary in size from very minute
up to those as large as a pea. In the larger ones it may often
be observed that they have a concentric shelly structure
and thus consist of successive coats. An illustration of a



310 ROCKS AND ROCK MINERALS

coarse oolite or pisolitic limestone from Bohemia is shown
on Plate 28. There is usually more or less limy cement
binding the grains together.

Examination of oolites generally shows that some object, such as
a bit of shell, a grain of sand or something similar, has served as a
nucleus around which the coatings of lime carbonate have accumu-
lated. On the shores of Great Salt Lake at the present time oolitic
sands are forming from the waters which are charged with lime and
other salts in solution. As the particles are rolled on the beach, or
agitated in the water, all parts become equally coated and the
spherical form is assumed. By a similar process oolitic grains are
forming in springs charged with lime salts, as at Carlsbad in Bohemia.
The concretionary structure is best seen under the microscope;
it is rarely sufficiently coarse to be observed with the eye alone, but
may be sometimes made out with a lens. Oolitic limestones con-
stitute large and important formations, often of great thickness and
of different geological ages. They are especially important in the
Jurassic strata of England and elsewhere in Europe. It is a
structure also assumed by some American limestones.

Chalk. Typical chalk is a soft, white, friable rock, whose
use for marking and blackboard crayons is well known.
While generally pure white it may sometimes be tinted
gray, flesh color, or buff. It consists of a fine calcareous
powder, which by examination under the microscope has
been found to consist of the tiny shells of foraminifera,
mingled with minute fragments of the shells and hard
parts of various organisms, as well as the siliceous spicules
of sponges, shells of diatoms and radiolarians, together
with occasional microscopic fragments of various min-
erals. It is the siliceous material of the sponge spicules,
diatom shells, etc., that has concentrated into the
nodules and concretions of flint, so commonly found in
certain beds of chalk, and whose analogue is seen in
the layers and masses of chert in limestones. Chalks,
in spite of their fine grain, are very porous rocks, absorb-
ing as much water as 20 per cent of their weight in some
cases.

Chemically, chalks are quite pure carbonate of lime, as



I



A. OOLITE, VARIETY PISOLITE, BOHEMIA.






B. COgUIXA, FLORIDA.



DESCRIPTION OF STRATIFIED ROCKS



311



shown in the following analyses of three specimens given
by different authorities.

It has been customary to consider chalk a formation produced on
the bottom of the deep sea, from its resemblance to the calcareous
oozes or muds found underlying the depths of modern oceans. It
has evidently not always been formed in this way, as shown by the
fossils indicative of shallow water which some chalks contain, as well
as the perfect skeletons of birds, pterosaurs and other vertebrate ani-
mals. The facts in some cases would point rather to its having been
formed in clear, warm and shallow seas, free from the products of
land waste.

Closely related to chalk, but differing in the fact that they do not
predominantly consist of foraminiferal shells, are light, chalky, earthy
limestones formed in a variety of ways, such as from coral sands and
muds; from those materials accumulated by the wind on coral
islands ; from ground-up shells in clear, shallow seas, etc. A whitish,
fragile rock formed on the coasts of Florida, which consists of shells
and their fragments of all sizes somewhat lightly compressed and
cemented together, is known as Coquina, from the Spanish word for
shell. (See Plate 28.)





CaCO 3


MgCO 3


SiO 2


(FeAl) 2 3


H 2 O


Total


I..


94.2


1.4


3.5


1.4


0.5


101.0


II....


96.4


1.4


1.6


0.4


0.2


100.0


III...


98.4


0.1


1.1


0.4


~~


100.0



I, White chalk, White Cliffs, Little River, Arkansas; II, Lower
Cretaceous chalk, Burnet Co., Texas; III., White chalk, Shore-
ham, Sussex Co., England.



Chalk is found extensively in Europe in England,
Germany, France, etc., where its occurrence is the result
of a distinct geologic epoch, named on this account
the Cretaceous. It also occurs widely distributed in
the Cretaceous formations of the southern trans-Missis-
sippi States, in Nebraska, Arkansas, and especially in
Texas.



312 ROCKS AND ROCK MINERALS

Travertine, Calcareous Tufa. In the preceding chaptei
it was shown how material of the land surface is taken into
solution and carried into the sea. This is especially
important with regard to lime, which goes to the sea as a
sulphate and carbonate, the latter being much the more
momentous. This lime carbonate comes, not only from
pre-existent carbonate rocks, but also from the lime sili-
cate minerals of the igneous and metamorphic ones, which
under atmospheric agencies are converted into carbonates.
The lime carbonate on its way to the sea may be tem-
porarily deposited, giving rise to rock-masses of some
magnitude and importance.

Carbonate of lime has little solubility in pure water, but if the latter
contains carbon dioxide, the lime carbonate is converted into a soluble
bicarbonate and the amount of the latter formed and taken into
solution depends on the amount of carbon dioxide present. Thus
in regions where limestones or other carbonate rocks abound, the
natural waters, under atmospheric pressures, attack such rocks and
take the lime carbonate into solution in a relatively slow manner,
but in spring waters, and especially thermal ones coming from depths,
the pressure may be great, the amount of contained carbon dioxide
large and the quantity of dissolved carbonate of lime proportionately
so. Such waters on coming to the surface lose the greater part of
the dissolved carbonic acid in the form of gas, and the lime in solu-
tion is consequently deposited rapidly and in large amount. In the
waters under surface atmospheric pressure the lime is deposited
by evaporation and therefore much more slowly. In warm waters
the deposit of lime may be much increased by the action of low
forms of vegetable life, algae, living in them, which secrete lime from
the water.

The rock thus formed by deposit of carbonate of lime
from solution is called travertine, from the old Roman
name of the town of Tivoli near Rome, where an exten-
sive formation of the substance exists. When deposited
slowly, as in the stalactites and stalagmites in caves, it is
rather hard and compact, fine crystalline, sometimes
white but usually tinted yellowish or brownish; it often
has a fibrous or concentric structure; it breaks with a



PLATE 29.



' w^f*.





A. CALCAREOUS TUFA, DEPOSITED ON VEGETATION,
COLORADO.




B. CALCAREOUS TUFA, YELLOWSTONE PARK.
(U. S. Geological Survey.)



DESCRIPTION OF STRATIFIED ROCKS 313

splintery fracture. When deposited more rapidly, as by
springs, it is softer, not evidently crystalline, and porous to
loose or earthy; when formed coating vegetation it may
be open, cellular, spongy, bladed or moss-like, as illustrated
in Plate 29.

These looser, less compact, varieties are commonly called
calcareous tufa or calcareous sinter. Deposits of traver-
tine, or calcareous tufa, are found in nearly all countries
and especially in limestone regions. Many caves are
celebrated for the number, size and beauty of the stalac-
titic and stalagmitic formations they contain. See Plate
30. Springs depositing carbonate of lime are very com-
mon, but warm carbonated waters are chiefly found in
volcanic regions or those which have recently been so,
like the celebrated Mammoth Hot Springs of the Yellow-
stone Park, and others found in California, Mexico, Italy,
New Zealand, etc. See Plate 31. The so-called Mexican
" onyx " or " onyx marble," which is extensively used
as an ornamental stone, is a travertine with a banded
structure, beautifully brought out by its varied tinting
through metallic oxides.

Marl. This name is given to loose, earthy or friable
deposits consisting chiefly of intermingled carbonate of
lime, or dolomite, with clay, in variable proportions. The
color is usually gray, but they are often yellow, green,
blue or black, and sometimes with pronounced color tones
due to some special substance, as oxide of iron or organic
matter. They show all gradations into clays and shales.
On exposure to the air or water they crumble quickly
into coarse soils. The carbonates in them are readily
detected by their effervescence in acid. According to
special substances or objects, which they may contain in
addition to those mentioned, different varieties are named;
thus sandy marl is full of grains of quartz sand and often
of other minerals; shell marl is a whitish, earthy deposit
formed of fragments of shells of various organisms formed
in enclosed basins of water, mingled with clay, etc. In



314



ROCKS AND ROCK MINERALS



the Atlantic and Southern states this name is applied to
beds which contain abundantly shells of mollusks, gastro-
pods and other shell-fish.

The chemical composition of marls varies very greatly;
the following analysis of a compact one from Colorado will
serve as an example.



CaCO 3


MgC0 3


Si0 2


A1 2 O 3


FezOa


MgO


K 2 O


H 2 O


Org.
Sub.


XyO


Total


21.6


1.7


45.9


13.2


3.9


1.3


2.3


5.4


3.5


1.2


100.0



XyO =small quantities of TiO 2 ,Na O and P Z O 8 .
Greensand Marl is described under sandstones.
PHOSPHORITE PHOSPHATE ROCKS.

Deposits of phosphate of lime, while not of great geo-
logical importance in making extensive formations, are
yet of considerable interest and, commercially, of great
value, from their use as soil fertilizers. When occurring
in stratified rocks and unconnected with igneous intrusion
they represent material of organic origin. While some
invertebrates, such as a few species of brachiopods, secrete
phosphate of lime in their shells and hard parts, it is mostly
to the bones and excrement of vertebrates that the origin,
of this material must be ascribed. Sometimes the deposit
appears to be the direct and original one, but more com-
monly it is secondary in nature, in that the phosphates
have been leached out, carried down and redeposited as
nodular, concretionary or lenticular masses in clefts and
other cavities in the rocks in which they occur.
Especially in limestones, which, being soluble, are carried
away, the less soluble phosphatic material tends to accu-
mulate in such masses. The general name of phosphate
rock or phosphorite may be used for all such material.
The appearance of these rocks is variable, sometimes com-



PLATE 30.




STALACTITES OF TRAVERTINE IN LURAY CAVERN,

VIRGINIA.
(U. 8. Geological Survey.)



DESCRIPTION OF STRATIFIED ROCKS



315



pact semi-crystalline, fibrous or concretionary, often
cavernous or spongy; sometimes in rounded mammillary
forms; in other cases more or less earthy. The color is
usually gray, but sometimes white, buff, reddish, bluish,
or even black. The simplest test for these phosphates is
to dissolve a powdered sample in nitric acid, and, after
filtering off the insoluble matter, to add an excess of
ammonium molybdate solution and ascertain by the
yellow precipitate if phosphorus is present. The general
chemical composition is shown in the following analyses
of samples from various localities in North Carolina.





1.5


31 2


22.1


23.4


Carbonate of lime


12.0


15.9


42.1


64.3


Phosphate of lime


71.8


42.1


20.5


11.2


Water and other constituents


14.7


10.8


15.3


1.1


Total


100.0


100.0


100.0


100.0













Phosphorites are widely distributed; in the United
States they are found extensively in the Carolinas, in
Florida, in Tennessee, and in some of the other states;
they occur in England and Wales, in Belgium, northern
France, and Russia.

COAL AND OTHER CARBONACEOUS ROCKS.

It is well known that, interbedded with other stratified
rocks of the different geological periods down to the present,
there occur layers of carbonaceous character, which under
the names of coal, lignite, etc., represent the remains of
former vegetable life, which once flourished where these
beds are now found. The formation of peat in modern
lakes, swamps, and bogs, and its occurrence in beds inter-
stratified in recent delta deposits with those of sands and
clays, as in the Mississippi delta, shows us how these beds
of coal were formed. For between the growing vegetation
of to-day, its change into peat, from this into lignite or
brown coal, and so on into bituminous coal, then into



316 ROCKS AND ROCK MINERALS

anthracite, and eventually into graphite or practically
pure carbon, every step of gradual transition can be
traced.

The vegetable matter composing plants consists for the most part
of carbon, hydrogen, and oxygen. Its decay in the air, like combus-
tion, is a process of oxidation; the hydrogen goes off as water, the
carbon as carbon dioxide, the oxygen of the air assisting that in the
vegetable matter to effect the change. In this process most of the
carbon is removed. If the decay takes place under water, however,
the access of the oxygen of the air is prevented and the process
becomes much like that where wood is burned with a limited amount
of air to form charcoal. Some of the carbon unites with some of the
oxygen to form carbon dioxide; some of the hydrogen unites with
the rest of the oxygen to form water; the rest of the hydrogen unites
with some of the carbon to form marsh gas (methane) and the
remainder of the carbon is left behind. This can be illustrated
by the following equation in which the formula of cellulose, which
comprises the most important part of vegetable matter, is used.

Cellulose Carb. Diox. Water Methane Carbon
s - CO S + 3H 2 O + CH 4 + 4C.



It is not intended to imply that this change takes place at once,
or is complete, under water; it goes on gradually, and as the CO 2
and CH are evolved, the residual matter becomes richer in carbon
and poorer in hydrogen and oxygen. Thus vegetable matter is
converted into peat and this by compression and further change
into brown coal or lignite. The same process goes on in coal beds,
furnishing the deadly gases known to the miners as choke-damp
(CO 2 ) and fire-damp (CH 4 ), and lignite thus changes to bituminous
coal. Folding of the strata, with compression and heat, and the
consequent rupturing and fissuring of the overlying beds, which
permits the easy escape of the gases, hastens the process, and under
such circumstances the coal is changed to anthracite, which is much
richer in carbon, or even into graphitic coal which is practically
pure carbon. Thus the degree to which lignite has advanced through
bituminous coal to anthracite depends in part on its geological age,
and in part on the conditions to which it has been subjected. .

Peat. This varies from a brown to yellowish matted
mass of interlaced fibrous material, strongly resembling
compressed tobacco, in which remains of plant leaves,



PLATE 31.




DESCRIPTION OF STRATIFIED ROCKS



317



stems, roots, etc., are still recognizable, in the upper por-
tion of the bed, to a dark brown, or black, compact, homo-
geneous mass appearing much like dark clay when wet,
in the deeper, lower parts. A dried, compact, very pure
peat from Germany is stated to have the following
composition:



Carbon.


Hydrogen.


Oxygen.


Nitrogen.


Ash.


Total.


55.9


5.8


36.4


1.0


0.9


100.0



Under enormous pressure it has been found that peat may be
artificially converted into a hard, black substance like coal. The
wide distribution of peat and its use as a fuel are too well known to
need further mention. Its purity depends on the amount of clay
and sand mingled with it in the process of formation; even the purest
peat, like coal, has a small percentage of ash resulting from the
mineral constituents in the plants.

Lignite. Brown Coal. Usually a chocolate brown in
color, but varying to yellowish or black; compact and
firm to earthy and fragile; luster dull and soft to pitchy;
often shows distinctly the texture and grain of wood or
intermatting of vegetable fibers. Hardness varies from
1-2.5, the specific gravity from 0.7-1.5. It burns readily
with a smoky yellow flame, and strong odor. The carbon
in it varies from 55 to 75 per cent. A lignite from Ger-
many is stated to have the following composition which
will serve as an illustration.



Carbon.


Hydrogen.


Oxygen.


Nitrogen.


Ash.


Total.


57.1


4.6


36.0


0.2


2.0


99.9



Lignite, belongs in the Cretaceous and Tertiary formations and
often forms considerable beds where these formations occur. It is



318



ROCKS AND ROCK MINERALS



found in small amount in the eastern United States in the Tertiary
at Brandon, Vermont, but in the Cretaceous deposits of the Rocky
Mountain states, and in the Dakotas it occurs in large and valuable
fields. It is found also on the Pacific Coast, and in Germany in
Europe; and elsewhere. Where better coal is not to be had it fre-
quently furnishes a valuable fuel.

Bituminous or Soft Coal. This is a compact, brittle
rock of a gray-black to velvet-black color. It has a
lamellar, conchoidal or splintery fracture; sometimes more
or less cubical. The luster varies from dull to pitchy;
the specific gravity from 1.2-1.5. It gives a black to
brownish-black streak. It burns with a yellow flame and
gives a strong bituminous odor. It often shows distinct
stratification through the varying luster of the different
layers. Generally there are no traces of organic struc-
tures visible to the eye. Some varieties fuse or sinter
together on heating, leaving a coherent residue or coke,
and are thus called coking coals; others fail to do this and
fall to powder. The amount of carbon in a soft coal
varies from 75-90 per cent. A coking coal from Northum-
berland in England has been found to have the following
composition.



Carbon.


Hydrogen.


Oxygen.


Nitrogen.


Sulphur.


Ash.


Total.


78.7


6.0


10.1


2.4


1.5


1.4


100.1



The sulphur in coal comes from pyrite, which is a very common
impurity. Bituminous coals vary considerably in the relative
proportions of fixed carbon to volatile matter, that is in the pro-
portion of the carbon left behind on heating to the gases, tar, etc.,
driven off; the latter may be as much as 30-40 per cent and such
coals are called fat coals and are used for the making of gas, coke,
etc. Those with 15-20 per cent volatile matter are largely used for
steam engines and are often called steam coals. They are transi-
tional to anthracite.

In addition to ordinary coal there are, depending on the physical



DESCRIPTION OF STRATIFIED ROCKS 319

characters, a number of varieties which are well recognized. Thus
cannel coal is a dense, lusterless, highly bituminous form without
structure and generally showing conchoidal fracture. Jet is some-
what similar but characterized by its high luster, intense black color,
asphaltic appearance, and toughness, which permits of its being
readily turned and worked. Its use in mourning jewelry, buttons,
ornaments, etc., is well known. It occurs in small, scattered, isolated
masses in the later formations in various places, one of the chief
localities being at Whitby in Yorkshire, England. It is regarded
by some as representing water-logged fragments of originally
coniferous wood. Bituminous coal occurs in North America in
Nova Scotia; in the Appalachian coal field of western Pennsylvania,
Ohio, West Virginia, Kentucky, Tennessee, Alabama and Georgia ; the
Central coal field of Illinois, Indiana and Kentucky; in Michigan; the
Western field of Iowa, Missouri, Kansas, Arkansas, Oklahoma and
Texas. These are of Carboniferous age. In the Rocky Mountain