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

sum, and then the salt, would be deposited, leaving a
mother liquor containing the more soluble sulphates and
chlorides of magnesium and potassium. If subsidence
and the lowering of the barrier should now occur, there
would be an influx of the lighter sea-water above, while
the heavier mother liquor would flow out below and the
basin would be charged anew with sea-water. If the
barrier is again closed, for example by waves building it up
as seen along the coast of the Carolinas, the conditions
would be repeated and fresh deposits of gypsum and salt
formed. Thus by repetitions of such a process we can
imagine that great thicknesses of salt might be locally
deposited. Finally, if no outflows occur the mother
liquor is also evaporated and the more soluble salts
deposited.

In the United States rock-salt occurs in beds in New
York, Michigan, Louisiana, Kansas and various other
states. It is found in Europe, in Germany, Austria and
Poland in vast deposits; in several counties of England and
in many other places. Interior drainages are present in
all of the continents and in connection with them there
are salt lakes and deposits of salt.

FLINT, GEYSERITE AND OTHER SILICEOUS ROCKS.

Deposition of silica, Si0 2 , from its solution in water
occurs both by simple concentration and evaporation
and by the action of organic life. It is possible that it
may also happen from chemical reactions. The deposits
thus formed, while lacking the wide extent and geologic
importance of the sedimentary formations produced by
the processes of erosion, have yet considerable interest



DESCRIPTION OF STRATIFIED ROCKS 297

and may be of local significance. On account of their
general similarity of composition they are here included
under one heading, but the group does not include the
mechanically formed siliceous sandstones. The material
composing these rocks is, mineralogically, sometimes in the
form of quartz pure crystallized, SiC>2, and sometimes
in the form of opal or chalcedonic silica uncrystallized
silica containing more or less water in combination as
hydro xyl.

Flint. This is a dark gray or black rock, so extremely
compact that it appears as a homogeneous substance.
The fracture is conchoidal and the chips have a translucent
edge like many felsites, which indeed it may closely
resemble. The hardness is 7. It consists of an intimate
mixture of quartz and opal, the coloring matter being
organic and disappearing when a chip is heated before
the blowpipe.

Flint is not a rock in the sense that it occurs in extended inde-
pendent formations. It occurs in irregular nodules or concretions
in chalk which vary widely in size, from that of a pea to extensive
layers. Similarly an impure flint, occurring chiefly in limestones
from the Cambrian up, is called chert. When these substances are
studied under the microscope they are found to contain the hard
siliceous parts of various organisms, chiefly of sponges and radio-
larians. The matter was first derived from sea-water by such
organisms, but appears secondarily to have gone into solution and
been chemically deposited around certain centers, and in certain
places, where favorable conditions obtained. The uses of flint for
savage and prehistoric implements and weapons and for striking
fire are well known. Other siliceous masses, similar in a general
way to flint and chert, sometimes of the same and sometimes of
uncertain origin, have received various names such as lydianite,
harnstone, etc. Jasper is a chemically precipitated opaline silica.
In places, as in the Lake Superior region, the jaspers are strongly fer-
rugineous and interlaminated with bands and streaks of hematite.
They constitute rock masses of considerable size, affording valuable
deposits of iron ore. They are called jaspilite. The cherty layers
are colored bright red by the iron oxide. Another variety of these
siliceous flint-like rocks are the novaculites, which occur in con-
siderable beds in Arkansas, and are greatly used in the manu-



298 ROCKS AND ROCK MINERALS

facturc of whetstones and hones. They are very dense, conchoidal
in fracture, white or pale gray in color, semi-translucent, and com-
posed of silica. Their origin is uncertain.

Geyserite. Siliceous Sinter. In volcanic regions silica
is frequently deposited by hot waters, boiling springs and
geysers. Sometimes this is produced by simple evapora-
tion and drying of the water and sometimes, as shown
by Weed, it is due to vegetable organisms, algce, which
secrete silica from the heated waters in which they live
and become coated with it. The material of the geyser
cones and basins produced by drying is hard, compact,
and opaline, while that formed by the plants is more or
less loose, spongy, and tufaceous. If pure, it is white in
color. Its formation is well illustrated in the hot spring
and geyser areas of the Yellowstone Park and New Zea-
land (see Plate 27). The material thus formed is known
as geyserite, or siliceous sinter.

Diatomaceous Earth. This is a soft, white, chalk-like,
very light rock composed of innumerable microscopic
shells of diatoms. The latter are excessively minute,
unicelled organisms which possess free motion and are
covered with a siliceous shell of great delicacy; they are
considered forms of vegetable life. In waters of suitable
character they may swarm in incredible numbers and their
shells, accumulating at the bottom, may give rise to de-
posits of considerable magnitude. Some varieties of the
rock are pale yellow, brown or gray. It is easily distin-
guished from chalk, which it may resemble, by its non-
effervescing with acid; from clay by its gritty feeling,
when rubbed between the fingers, and its weak argil-
laceous odor or the absence of it. A more positive test is
the effervescence produced when it is mixed with car-
bonate of soda and fused before the blow-pipe. The
loose, scarcely coherent material is called infusorial earth;
when more compact it is sometimes called tripolite. It
is extensively used for polishing purposes. Beds of con-
siderable magnitude occur in the United States in Mary-



PLATE 27.




o"

^ '&

l-i

PH *0

^ r^

02 O

H "



DESCRIPTION OF STRATIFIED ROCKS 299

land, Virginia, Georgia and Alabama, where they are
worked commercially, also Ln Missouri, Nevada, California
and elsewhere, often as a layer in swamps which repre-
sent the fillings of former lakes. They are also found in
Germany and other parts of Europe.

IRON ORE ROCKS.

The deposits of iron ore which occur as rocks, inter-
stratified or associated with sedimentary beds, have origi-
nated through complex processes, sometimes wholly,
sometimes partly, of a purely chemical nature and usually
more or less influenced by the agencies of organic life.
The most important set of processes has been previously
mentioned but now deserves a more detailed description.

Iron exists in the original (the igneous) rocks in the
form of silicates, such as biotite, olivine, pyroxene and
hornblende, and also as oxides, such as magnetite, hema-
tite and ilmenite, as disseminated grains. It also occurs
in the secondary metamorphic rocks as silicates and oxides.
It is also pretty generally diffused through the sedimentary
rocks, in part as coloring matter and cement, and mostly
in the form of ferric oxide, ferric hydroxides and ferrous
carbonate. In the igneous rocks it is largely in the ferrous
state and to a considerable degree also in the meta-
morphic ones. Also, to understand the concentration of
iron and formation of iron ore rocks, it must be borne in
mind that the metal forms only one carbonate, ferrous
carbonate or siderite, FeCOs which, like carbonate of
lime, is soluble in water containing carbon dioxide.

When the rocks are decomposed and broken down by
the agencies of weathering and erosion, the silicates con-
taining iron are altered; the ferrous oxide in them com-
bines in part with the carbon dioxide in the circulating
ground water to form ferrous carbonate which goes into
solution, and in part it is oxidized to ferric oxide. The
original oxides of iron react in a similar manner. The
ferric oxide thus formed or liberated would be insoluble,



300 ROCKS AND ROCK MINERALS

but in the presence of decaying vegetable matter in the
soil and organic acids leached downward into the rocks,
deoxidation of the ferric oxide ensues; it is reduced to
ferrous oxide and then becomes ferrous carbonate and
goes into solution. The reason for this is that decay of
organic matter is a process of oxidation, like slow com-
bustion; the organic matter takes oxygen from the air,
but in the presence of moisture and ferric oxide it will
take oxygen from the latter, reducing it to the ferrous
oxide which is then fitted to unite with carbon dioxide
and become the carbonate.

The iron of the rocks, which is thus brought into solu-
tion, is leached out, and in standing bodies of shallow water,
such as swamps, lagoons or estuaries, with small outlets
to the sea, it may be concentrated and give rise to exten-
sive deposits. Under some conditions these deposits may
be of the carbonate directly, but usually the solution of
the carbonate is re-oxidized, carbon dioxide escapes, and
the iron is precipitated as ferric hydroxide (limonite).
This oxidation is largely, if not wholly, performed by cer-
tain bacterial organisms which demand iron in their inter-
nal economy, and therefore, secrete the iron from the
water, and change it in their cells from the ferrous to the
ferric condition, thus rendering it insoluble. Living and
dying in unimaginable numbers, though excessively
minute, they give rise to large deposits.

The ferric hydroxide which is thus precipitated may
accumulate on the bottom as bog iron ore, or limonite, or,
as is so often the case in shallow bodies of standing water,
like swamps, etc., it may again come in contact with decay-
ing vegetable matter, and be changed back into carbonate.
Such beds of iron ore may be quite pure, or they may be
more or less mingled with clay and sand, brought in at
times of high water, and thus impure limonites, clay iron-
stones, black-band ore, etc., are formed. This also
explains the not infrequent association of stratified iron
ore and coal beds in the same series of rocks, and the reason



DESCRIPTION OF STRATIFIED ROCKS 301

why in this case the iron ore is commonly ferrous
carbonate.

The moving ground waters containing iron in solution,
as described above, may also issue as springs and give rise
to deposits of iron ore.

Certain masses of iron ore, chiefly limonite, are supposed
to be residual products of weathering and solution. This
is illustrated in the view that masses of limestone con-
taining ferrous carbonate have been dissolved and carried
away, but the iron, oxidized to the ferric condition in the
process, has become insoluble and remaining behind has
gradually concentrated. The more important iron ore
rocks may now be described.

Bog Iron Ore. Limonite. This is sometimes loose and
earthy, sometimes firm and porous. It consists mainly
of limonite, mixed more or less with humus, phosphates,
silicates of iron, clay, sand, etc. Its character has been
sufficiently described under limonite among the minerals.
It sometimes occurs in concretions. With increasing
amounts of clay it passes over into yellow ocher. It is
found in all parts of the world. In the United States it
is widely distributed, and along the Appalachian belt,
from Vermont to Alabama, deposits of limonite, most of
which are probably residual in character, have furnished
iron ore since the early settlement of the country, and in
great quantity.

Clay Ironstone. Siderite. When reasonably pure, side-
rite, or spathic iron ore, is a coarse to fine crystalline aggre-
gate of siderite grains. It is whitish to yellow, or pale
brown in color, but on exposed surfaces much darker
brown to black, owing to oxidation of the ferrous carbon-
ate to limonite, or of the manganous carbonate to manganic
oxides. It generally contains, more or less, carbon-
ates of lime, magnesia and manganese. Iron pyrites or
hematite are commonly associated minerals. For the
properties of siderite, reference may be had to its de-
scription among the minerals.



302 ROCKS AND ROCK MINERALS

An impure variety of siderite mixed with clay, sand
and limonite in variable proportions, of a compact appear-
ance, and generally of dull brown colors, is known as clay
ironstone. It is apt to occur in nodules, often as con-
cretions around some fossil, and lenticular masses which
increase until they become interstratified beds of consid-
erable thickness. Another variety which contains so
much organic, coaly matter that it is colored black is
known as black-band ore. It is especially associated
in the strata with coal beds from the Carboniferous
upward.

Carbonate ores of iron are of less importance in the
United States than the deposits of limonite and hematite.
They occur in Pennsylvania, Ohio and Kentucky, of Car-
boniferous age, and in the Lake Superior region in Michigan
and Minnesota, of Algonkian age. They occur in Europe
in England, Germany, France and Spain, in deposits of
great technical value. Black-band is found in the coal-
bearing strata of Pennsylvania, England, etc.

Hematite. Red Iron Ore. This occurs in the form of
veins, lenticular masses and beds, in various geological
formations and especially in those whose strata have been
folded. As a rock, it varies from fine grained and compact
to earthy or fibrous, is of a red to brown color or, where
crystalline, of a dark gray. Its properties as a mineral
have been previously given. It occurs pure or nearly so,
but with varying mixtures of clay, sand or silica, it passes
insensibly into ferrugineous clays, red ochers, or shales,
sandstones, cherts, etc. In this connection see jaspilite
under flint. While hematite undoubtedly occurs as a
normal sedimentary or stratified rock, interbedded with
other unchanged strata, as in the beds which have such
a wide distribution in the eastern United States in the
Clinton group of the Niagara period, it is more generally
to be considered a metamorphic rock, and as such, might
be included among the metamorphic iron rocks described
in Chapter XI, such as itabirite and hematite schists.



DESCRIPTION OF STRATIFIED ROCKS 303

Extensive deposits of hematite are found in various
parts of the United States and Canada. The greatest
amounts mined as ore come from Tennessee and the Lake
Superior Region, the vast production in the latter leading
the world in output. Large beds are also found in England
and other parts of Europe.

Iron Oolite. The iron rocks described above, and especially red
hematite, not infrequently assume a concretionary form in which
the rock is composed of rounded, sometimes polygonal, grains which
vary in size from that of fine sand to peas. An examination of
them shows that they have a concentric shelly structure. The color
varies from red to brown. Sometimes the rock is composed of
them alone and sometimes they are thickly embedded in a marly or
clayey cement. The iron ore appears in many cases to have been
deposited around grains of sand, fragments of fossils, etc., as neuclei.
The Clinton ores mentioned above frequently assume this oolitic
character and it is well known from various European localities.
Such ores have sometimes been changed into magnetite while still
retaining the oolitic structure.

LIMESTONE AND OTHER CARBONATE ROCKS.

This group of rocks has the common property of being
composed of carbonate of lime, calcite, CaCOs, or of
this substance intermingled more or less with dolomite,
MgCa (003)2- It is also a common property that, so far
as known, the carbonate of lime has primarily been
separated from water, rendered insoluble and accumu-
lated by the action of living organisms of one kind or
another. Secondarily, the deposits thus made may be
mechanically broken up and redeposited, or they may be
taken into solution, carried away and precipitated else-
where. There may be some possible exceptions to this
rule, that the carbonate of lime is primarily precipitated
by organisms, in the cases where it is concentrated in
alkaline lakes by inflowing waters and finally deposited,
or in the evaporation of shut-off portions of the sea, but
these are of small account and negligible in comparison
with the great formations produced by life agencies.



304 ROCKS AND ROCK MINERALS

Hence it is generally held that the great masses of car-
bonate rocks, even when they do not contain fossils,
are a proof of the existence of life at the time of their
original deposition.

This group of rocks is soluble in hydrochloric acid;
entirely so when pure carbonates, but generally leaving
more or less of a residue, consisting chiefly of sand, clay,
silica, etc. In some cases, where dolomite is present, the
acid must be heated. Their hardness is less than 4,
hence they may be readily scratched or cut with the
knife.

The following are the important members of the group.

Limestone. This is the most common and important
carbonate rock. It is fine grained to very dense in
texture and its color varies from whitish, through tones of
yellowish to brown, or from various shades of gray, dove-
color, bluish-gray, dark-gray to black. It is rarely of
reddish colors. The yellow and brown colors are due to
iron oxide, the gray and black to organic matter. The
gray colors are most common. Compact varieties have
an even to somewhat conchoidal fracture. It effervesces
freely with any common acid, with vinegar (acetic acid)
or lemon juice (citric acid). It is easily scratched with
the knife and many of the less compact varieties are
friable to the finger nail. The specific gravity varies
from 2.G-2.8. On exposed surfaces it is apt to be cavern-
ous and often tinted or blotched reddish or yellowish from
oxidation of small amounts of ferrous carbonate it may
contain. It occurs in individual beds of all thicknesses
up to 100 feet or more.

Some limestones consist of pure grains of calcite, others
possess a fine, clay-like cement between them. Acces-
sory minerals, which are sometimes seen, are pyrite and
quartz, the latter in minute crystals, sometimes lining
cavities.

In following analyses I, II, and III are of very pure
limestones; IV is an impure type containing considerable



DESCRIPTION OF STRATIFIED ROCKS



305



dolomite and sand and clay. Such transitions through
impurities are common; thus V for example shows one
toward the clay-ironstone previously described. Transi-
tions to dolomite are not common; an examination of a
large number of analyses shows that generally either the
rock contains very little or no magnesia, or it has much
and is a regular dolomite as described later.





SiO 2


A1 2 O 3


Fe 2 O 3


FeO


MgO


CaO


H 2 O


XyO


C0 2


Total


I....


0.4


_


0.4


_


trace


54.8


1.1


_


42.7


99.4


II...


0.6





0.4





0.4


54.2





_


44.0


99.6


III..


1.2


0.2


0.2


0.3


0.6


53.8


0.9


0.1


42.7


100.0


IV..


7.0


3.6





9.0


39.3


0.2


1.2


38.8


99.1


V...


3.2


0.1


10.9


15.2


11.0


26.6


0.5


1.5


41.1


100.1



I, Trenton Limestone, Lexington, Virginia ; II, Buff Limestone,
Hoosier Quarry, Bedford, Indiana; III, Lithographic Limestone,
Solenhofen, Bavaria; IV, Impure dolomitic Limestone, Greason,
Pennsylvania; V, Sideritic-dolomitic Limestone, Gogebic dist.,
Michigan.

XyO represents small quantities of organic matter, manganese
oxide, etc.

The strength of limestone as a rock varies very much
with the texture; that of firm compact varieties is very
high while loose porous ones are very weak. Thus a
dense variety has been shown to have a crushing strength
of over 40,000 pounds per square inch, while others
scarcely exceed 3000 pounds per square inch. The well
known white oolitic limestone of Bedford, Indiana, has an
average crushing strength of 4300 pounds. Any good
firm and compact limestone has a strength far in excess of
any load that it may be called upon to endure in modern
structures. The porosity of limestones varies considerably;
those containing the most sand are usually the most
porous; the ratio of pore space to rock volume may vary



306 ROCKS AND ROCK MINERALS

from 15 per cent to practically nothing, the ratio of the
weight of water it can absorb to the weight of rock is in
general much less than this, usually not more than one-
half as much.

There are many varieties of limestone, depending on circumstances,
especially the mode of formation. Thus in some there are abun-
dant remains of fossils which may give the rock a distinctive char-
acter. These comprise a great variety of organisms, among which
may be mentioned corals, crinoids, shells of mollusks, brachiopods,
gastropods, foraminifera, remains of sponges, etc. The " encrinal
limestone " of Silurian age in western New York is an example.
Sometimes these fossils occur in such numbers that the entire rock
is composed of masses of shells, or the hard part of one particular
organism, with just enough fine carbonate of lime between them
to act as a cement. Examples of this are seen in the layers com-
posed wholly of brachiopod shells found in the Niagara formation
of the Silurian in western New York. Such rocks are sometimes
called "shell limestones." Certain limestones composed of corals
are also examples of the same thing.

On the other hand, there are varieties which depend on the presence
of some impurity which gives a particular character to the rock.
Thus it may contain much clay and is termed an argillaceous lime-
stone or it may contain much sand of siliceous character and be an
arenaceous limestone : such rocks are transitional to shales and
sandstones. Others which are dark colored may yield a strong,
disagreeable, bituminous odor when struck and broken and are
called bituminous limestones; they contain considerable organic
matter. In some, which are termed glauconitic limestone, the rock is
more or less rilled with green grains of glauconite. Lithographic
stone is a fine, compact, somewhat schistose limestone; the flesh-
colored rock from Solenhofen, Bavaria, remarkable for the well
preserved fossils it occasionally contains, is especially used for this
purpose. It is a very pure limestone, as shown by the analysis
given above.

Limestones are very apt to contain concretions and masses of
chert, or hornstone, of the character described in a previous section ;
they often become so abundant as to form definite bands or layers
in the rock.

By the weathering of limestones the lime carbonate is
removed in solution, leaving the insoluble impurities
behind. These form clays or loams which are colored



307



deeply red or yellow by the oxidation of the iron min-
erals originally present, and commonly contain pebbles
of chert or quartzose material and masses of limonite.
Such residual soils are commonly very fertile and cover
large areas in the southern United States, and in other
parts of the world.

Uses of Limestone. The use of limestone for structural
purposes of all kinds is well known and needs no further
comment. The same is true of its manufacture, by burn-
ing, into quicklime for mortar and cements. Large
quantities are also used as a flux in smelting operations, as
in the making of iron and steel. In recent years the use of
certain impure limestones containing 15-40 per cent of
clay, or other substances consisting of silica, alumina and
iron oxide, in the manufacture of natural hydraulic
cements has risen to very large proportions.

Dolomite. The geological use of this term is not always
the same as the chemical one. Chemically, or mineralogic-
ally, by dolomite is meant a chemical compound of a
definite composition CaMg(COs)2 with CaO, 30.4 per
cent, MgO 21.7, CO 2 47.8, while geologically the term is
used for any limestone which consists dominantly of
this compound, although it may also contain a large
amount of admixed calcite, CaCOs, and in some parts of
Europe it is employed to designate limestones of a particu-
lar geological period, some of which are not dolomites
at all.

The description of the colors, texture, and other physical
characters of limestone given above, applies equally well
to dolomite. In fact it cannot ordinarily be told in the
field, or by mere inspection of a hand specimen of a rock,
whether it is a dolomite or a pure limestone.

Dolomite is somewhat harder than true limestone and
if it is a pure dolomite it will not dissolve with efferves-