ECONOMIC GEOLOGY
In this chapter it is our purpose to briefly consider geology in its
direct relations to the arts and industries. When we realize that the
value of strictly geologic products taken from the earth each year
in the United States alone amounts to billions of dollars, we can
better appreciate the practical application of geological science.
Such products include coal, petroleum, natural gas, many valuable
metal-bearing minerals, and many nonmetalliferous minerals and rocks.
In most cases these valuable products of nature have been slowly
accumulated or concentrated at many times and under widely varying
conditions throughout the millions of years of known geological time.
To trace the extent of, and most advantageously remove, such deposits
for the use of man is always invariably impossible unless geological
knowledge is brought to bear. In many cases the problems involved
are intricate, and only the trained geologist is able to at all
successfully cope with them. In such cases it is necessary not only
to have a thorough knowledge of minerals and rocks as such, but also
of their origin and structure. Much of the practical application of
geology is carried out by the mining engineer who should have, above
all, a thorough knowledge of the great principles of geology.
Our plan of discussion is to consider, first, coal, petroleum, and
natural gas; then the most important metalliferous deposits of ores;
and finally nonmetalliferous minerals and rocks of exceptional
commercial importance. Underground waters have already been discussed
from the practical standpoint in the chapter on "Waters Within the
Earth." Certain minerals have already been sufficiently considered from
the economic standpoint in the chapter on "Mineralogy."
COAL, PETROLEUM, AND NATURAL GAS
Coal. Most valuable of all geological products is coal. Although it is
not, strictly speaking, a mineral, both because of its organic origin
and lack of definite chemical composition, coal is generally classed
among our mineral resources. Some idea of the national importance of
coal in the United States may be gained when we realize that the energy
derived from a single year's output is equivalent to that of hundreds
of millions of men working full time through the year. The uses of coal
are too well known to need mention here.
Coal is, beyond question, of organic (plant) origin as shown by its
very composition; perfect gradations between plant deposits like peat
and true coal; and the presence of microscopic plant remains and spores
in the coal. An excellent summary of just what happens during the
transition of ordinary vegetable matter into coal has been given by
D. White as follows: "All coal was laid down in beds analogous to the
peat beds of to-day. All kinds of plants, especially such species as
were adapted to the particular region where the deposit was located, in
whole or in part went into the deposit.
"Plants are composed chiefly of cellulose and proteins. The former,
comprising by far the larger bulk, constitute the framework, whereas
the latter are concerned in the vital functions. With these are
associated many other substances, among which are chiefly starch,
sugars, and fats and oils, constituting reserve foodstuffs; waxes,
resin waxes, resins, and higher fats, performing mainly protective
functions.... These components differ widely in their resistance to
various agencies. Those substances involved in the life function
and the support of the plant are relatively very stable under the
conditions imposed upon them.
"At the death of the plants, governed by conditions imposed in the
bog, a partial decomposition, maceration, elimination, and chemical
reduction begins, brought about by various agencies, chiefly organic,
mainly fungi at first and bacteria later. The most labile are removed
first, the more resistant next, and so on, as the conditions require,
leaving the most resistant behind in a residue called peat.
"The process of decomposition, elimination, and chemical reduction
begun in peat, chiefly by biochemical means, is taken up and continued
by dynamochemical means into and through the various successive
later stages, and results in the various grades of coal, as lignite,
sub-bituminous, and cannel coal, and anthracite."
The principal chemical elements involved in the changes which take
place are carbon, oxygen, and hydrogen, as shown by the following
analyses of about average samples of each member of the so-called "coal
series."
=========================+========+========+==========+=========
The "coal series" | Carbon | Oxygen | Hydrogen | Nitrogen
-------------------------+--------+--------+----------+---------
Wood (cellulose) | 50 | 43 | 6 | 1
Peat | 59 | 33 | 6 | 2
Lignite | 69 | 25 | 5.5 | 0.8
Bituminous coal | 82 | 13 | 5 | 0.8
Anthracite coal | 95 | 2.5 | 2.5 | trace
Graphite | 100 | .. | .. | ..
-------------------------+--------+--------+----------+---------
From this table it is seen that the oxygen relatively diminishes while
the carbon relatively increases, though, of course, all three elements
actually decrease during the chemical change from cellulose to coal.
These three elements disappear mainly in the form of gases, such as
water vapor, marsh gas, and carbonic acid gas. The final or graphite
stage is almost reached by the graphitic anthracite of Rhode Island,
which is so nearly pure carbon as to be really useless as coal.
The conditions under which successive layers of vegetable matter (later
turned into coal) become embedded in the earth's crust have been
outlined in the chapter on the "Evolution of Plants." The most perfect
conditions for prolific plant growth, and accumulation as great beds
in the earth's crust, were during the Pennsylvanian period of the late
Paleozoic era in many parts of the world, but especially in the United
States, China, Great Britain, and Germany. Most of the world's great
supply of coal comes from rocks of Pennsylvanian Age, while next in
importance are Cretaceous rocks, and some comes from strata of other
ages later than the Pennsylvanian, even as late as the Tertiary.
The United States not only has the greatest known coal fields, but
it also produces far more coal than any other country. In 1918 the
production was 678,000,000 tons, the greatest in our history, or
enough, if loaded into cars of forty tons capacity, to fill a train
which would reach around the earth at the equator about six times!
Equally amazing is the fact that this coal was nearly all consumed by
this one nation! In 1919 the production fell to 544,000,000 tons. Is
there real danger that our supply of coal will soon run out? Hardly so
when we consider, first, the fact that probably not more than 1 per
cent of the readily available coal has thus far been removed, and,
second, the high probability that rate of increase in coal production
for the last twenty years will not continue. In fact, during the last
two or three years the production has fallen off considerably. But even
so, coal, which is our greatest natural resource, and which can never
be replaced, should be scientifically conserved. In the case of the
very restricted anthracite coal fields what might be called a crisis
has already been reached, because a very considerable part of the
available supply has been taken out.
Something like 350,000 square miles of the United States are underlain
with one or more beds of workable coal (not including lignite)--in
some areas five to twenty or more beds one above the other. There are
also about 150,000 square miles of country underlain with the more or
less imperfect coal called lignite. It has been estimated that there
are more than a trillion tons of easily accessible coal, and another
trillion tons accessible with some difficulty in the principal coal
fields of the United States.
The greatest production of coal by far is from the Appalachian Mountain
and Allegheny Plateau districts, from the western half of Pennsylvania
to Alabama, where all the coal is bituminous of Pennsylvanian Age.
Here as well as elsewhere the coal beds are interstratified with
various kinds of sedimentary rocks, most commonly with shales and
sandstones. In the Appalachian field the strata including coal beds are
more or less folded toward the east, while they are nearly horizontal
toward the west. The famous Pittsburgh coal bed is probably the most
extensive important single coal bed known. It covers an area of over
12,000 square miles and is workable, with a thickness of five to
fifteen feet, over an area of 6,000 square miles of parts of western
Pennsylvania, Ohio, and West Virginia.
[Illustration: Fig. 76.--Map of the United States, showing the
principal coal fields. Cross-lined areas represent lignitic coals.
(After U. S. Geological Survey.)]
The greatest production of anthracite coal by far is from
central-eastern Pennsylvania, where strata of Pennsylvania Age,
including a number of anthracite beds, are mostly highly folded. Most
remarkable of all in this district is the so-called "mammoth bed" of
anthracite, nearly everywhere present, with a thickness up to as much
as fifty or sixty feet. Less than 500 square miles are there underlain
by workable anthracite coal.
Next to the greatest production of coal in the United States is from
the two large areas in the middle of the Mississippi Valley. It is
all bituminous coal, associated with nearly horizontal strata of
Pennsylvanian Age.
The scattering areas through the Rocky Mountains yield all types of
coal--anthracite, bituminous, and lignite. In some of these areas the
coal beds have been but little disturbed from their original horizontal
position, but usually they are more or less folded along with the
inclosing strata, the crustal disturbances affecting the coal beds
having taken place late in the Mesozoic era and early in the Cenozoic
era. Practically all of these coals are of Cretaceous and Tertiary
Ages, the best being Cretaceous. Very little of the Rocky Mountain coal
is anthracite.
On the Pacific Coast coal production is relatively very small. The
coals are there bituminous to lignitic of Tertiary Age, usually folded
in with the strata.
In Alaska there are widely distributed, relatively small coal fields,
but they have been little developed. Alaskan coals range in age from
Pennsylvanian to Tertiary, and in kind from anthracite to lignite.
Petroleum. Crude oil or petroleum is an organic substance consisting
of a mixture of hydrocarbons, that is, it is made up very largely of
the two chemical elements carbon and hydrogen, in rather complex and
variable combinations. It is practically certain that petroleum has
been derived by a sort of slow process of distillation from organic
matter--animal or vegetable or both--in stratified rocks within the
earth. Many strata, as for example carbonaceous shales, are more or
less charged with dark-colored decomposing organic matter. The chemical
composition itself, the kinds of rocks with which it is associated, and
certain optical (microscopic) tests all point to the organic origin of
petroleum. In southern California at least, certain of the oils have
quite certainly been derived from the very tiny oily plants called
diatoms which fill many of the strata.
[Illustration: Fig. 77.--Profile and structure section showing folding
of strata, with included coal beds, across one of the anthracite coal
fields of eastern Pennsylvania. Length of section, a little over 2
miles. (After U. S. Geological Survey.)]
During the last twenty years petroleum has come to be one of the most
important and useful natural products. Among the many substances
artificially derived from petroleum are kerosene, gasoline, naphtha,
benzine, vaseline, and paraffine. The United States leads in the
production of petroleum, while southern Russia and Mexico are very
important producers. In the United States the principal areas
underlain with petroleum-bearing strata are the northern Appalachian
field (through western Pennsylvania to central West Virginia); the
Ohio-Indiana field (central Indiana to northwestern Ohio); the
mid-continental field (southeastern Kansas and northeastern Oklahoma);
the southeastern Texas-Louisiana field; and the southwestern California
field. The total areas underlain with oil total about 10,000 square
miles. In the Appalachian, Ohio-Indiana, and mid-continental fields the
strata carrying oil range in age from Ordovician to Pennsylvanian, and
they are mostly but little disturbed from their original horizontal
position. The Texas-Louisiana oils come mainly from Cretaceous and
Tertiary strata which gently downtilt under the Coastal Plain toward
the Gulf. In California the oil-bearing strata are of Tertiary Age and
generally considerably disturbed and folded.
Under proper conditions below the earth's surface the derived oil
accumulates in porous or fractured rocks. There must, of course, be a
source from which the petroleum is derived or distilled; a porous or
fractured rock formation to take it up; a cap rock or impervious layer
to hold it in; and a proper geologic structure to favor accumulation.
The most common porous (containing) rock is sandstone, and the most
common cap rock is shale. Oil is rarely found without gas, and
saline water is likewise often present. If the containing strata are
horizontal, the oil and gas are usually irregularly scattered, but
if tilted or folded, and the beds porous throughout, they appear
to collect at the highest point possible. It was the result of
observations along this line that led I. C. White to develop what is
known as the "anticlinal theory." According to this theory, in folded
areas the gas collects at the summit of the fold (anticline), with the
oil immediately below, on either side, followed by the water. It is,
of course, necessary that the oil-bearing stratum shall be capped by a
practically impervious one.
[Illustration: Fig. 78.--Map showing the principal petroleum and
natural gas fields of the United States. (After U. S. Geological
Survey.)]
[Illustration: Fig. 79.--A vertical (structure) section showing a very
common type of oil-bearing structure. In this anticline, water, oil,
and gas arranged in order of their specific gravities. Removal of the
gas would allow the oil and water to rise higher toward the apex of the
porous layer. (After Indiana Geological Survey.)]
[Illustration: Plate 19.--General View in the Appalachian Mountains
Along New River, Virginia. This is a typical portion of the great area
which, during Mesozoic time, was reduced by erosion to the condition
of a low-lying plain ("peneplain"). Since early Cenozoic time the
peneplain has been upraised and New River has carved out its V-shaped
valley to its present depth, while tributary streams have carved out a
series of valleys along belts of weak rocks nearly at right angles to
the main valley. The remarkably even sky line marks approximately the
old peneplain surface. (_Photo by Hillers, U. S. Geological Survey._)]
[Illustration: Plate 20.--(_a_) A Big Glacial Bowlder of Plutonic
Igneous Rock Carried Miles from Its Parent Ledge by the Ice Sheet Which
Passed Over the Adirondack Mountains During the Ice Age. (_Photo by the
author._)]
[Illustration: Plate 20.--(_b_) A Long, Winding Ridge of Sand and
Gravel (Called an "Esker") Deposited by a Stream in a Channel in the
Ice Near the Margin of the Great Glacier During Its Retreat from the
Adirondack Mountains. (_Photo by the author._)]
"If the rocks are dry, then the chief points of accumulation of the oil
will be at or near the bottom of the syncline (downfold), or lowest
portion of the porous bed. If the rocks are partially saturated with
water, then the oil accumulates at the upper level of saturation. In a
tilted bed, which is locally porous, and not so throughout, the oil,
gas, and water may arrange themselves according to their gravity in
this porous part." (Ries.)
Although the term "oil pool" is commonly used, there is really no
actual pool or underground lake of oil, but rather porous rock
saturated with oil. It has been estimated that in an oil field of
average productiveness a cubic foot of the porous rock contains from
six to twelve pints of oil. The life of a well drilled into an "oil
pool" varies from a few months to twenty or thirty years, or sometimes
even more, but a heavy producer (especially a "gusher") almost
invariably falls off very notably in production in a few months, or at
most a few years. The typical Pennsylvanian oil well is said to last
about seven years. The fact that the United States is still able to
increase oil output is because new fields are found and developed, the
most recent being in the interior and northern parts of Texas. It is
practically certain, however, that the climax of oil production in the
United States will be reached before many years--long before that of
bituminous coal.
It is a well-known fact that oil, as well as natural gas, is usually
under more or less pressure within the earth. The pressure is so
great in some cases that where, in the course of drilling, oil or
gas accumulated under proper conditions, as for example those shown
by Figure 79, are encountered the pressure may be hundreds of pounds
per square inch, or enough to blow to pieces much of the drilling
outfit. It is under such conditions that great "gushers" are struck.
A wonderful case in point was the famous Lakeview gusher, struck
in California in 1910. "Within a few days the well was far beyond
control. It continued to flow (for a time shooting high into the air)
for eighteen months, finally stopping after it had produced over
8,000,000 barrels of oil, about 6,000,000 of which had been saved.
The daily production of the well varied greatly, reaching a maximum
of 65,000 barrels." (Pack.) One very common cause of oil pressure is
the expansive force of the associated imprisoned gas which steadily
increases as the gas is generated. Another cause which is seemingly
applicable in many cases is hydrostatic pressure, where under certain
structural conditions the pressure of water in a long-tilted layer is
exerted against oil accumulated toward the top of an anticline (or
upbend) in the strata.
The world's output of petroleum for 1917 was nearly 559,000,000
barrels, of which the United States produced nearly 338,000,000
barrels, Mexico 87,000,000 barrels, and Russia 34,000,000 barrels of 42
gallons.
Natural Gas. The most perfect fuel with which nature has provided
us is natural gas. Not only is it easily transported even long
distances through pipes, but also as a fuel it is easily regulated,
leaves no refuse, and is less damaging to boilers than coal. It is
a colorless, odorless, free-burning gas, consisting very largely
of the simple hydrocarbon called marsh gas or fire damp. Petroleum
nearly always has more or less natural gas associated with it, but in
some cases considerable quantities of gas may exist alone. Natural
gas, like petroleum, is of organic origin--a product of slow natural
distillation of vegetable or animal matter, or both, within the
earth's crust.
One of the most common modes of occurrence of gas is at the top of an
anticline (upfold) in porous rock (like sandstone) between impervious
layers (like shale). Figure 79 well illustrates the principle, the
gas lying above the oil, and the oil above the water; that is, the
three substances are arranged according to specific gravity. Gas may
also exist in considerable quantities in irregular bodies of porous or
fractured rocks. Natural gas is nearly always under pressure within
the earth, hundreds of pounds per square inch being common, while more
exceptionally, as in certain West Virginia wells, pressures of over
1,000 pounds have been registered.
The United States is by far the greatest world producer of natural
gas, the output for 1918 having been 720,000,000,000 cubic feet. West
Virginia easily headed the list, with Oklahoma and Pennsylvania next in
order. Areas underlain with natural gas are, in the main, the same as
for petroleum, and they total more than 10,000 square miles. During the
last forty years the waste of natural gas in the United States has been
appalling. In many cases wells in quest of oil have encountered gas and
often such abandoned wells have been allowed to play millions of cubic
feet of gas daily into the air for years. A striking example was the
Murraysville well of western Pennsylvania, which shot 20,000,000 cubic
feet of gas per day into the air for six years!
METAL-BEARING (ORE) DEPOSITS
Iron. Without question the most useful of all metals is iron. As
such it is rare in nature, but in chemical combination with other
substances it is extremely widespread and very common. Iron makes up
about 5 per cent of the weight of the earth's crust, but in the form
of ore (i.e., a metal-bearing mineral or rock of sufficient value to
be mined) it is notably restricted in occurrence. The three great ores
of iron are the minerals hematite, magnetite, and limonite, whose
composition and characteristic properties the reader will find stated
in the preceding chapter on "Mineralogy."
One of the worst impurities in iron ore is phosphorus, which makes
iron "cold short," i.e., brittle when cold. Ore for the manufacture of
Bessemer steel must contain very little phosphorus (less than 1/1000 of
the metallic iron content of the ore). Sulphur as an impurity in the
ore tends to make the iron "hot short." Silica (quartz) is bad because
it necessitates the use of more lime for flux in the furnace.
Iron ores occur in rocks of most of the great geologic ages, but in
the United States principally in the pre-Paleozoic and Paleozoic. The
United States is by far the greatest producer of iron ore in the world,
the output for 1917 having been about 75,288,000 tons, the greatest in
the history of this or any other country. This one year's output loaded
into cars of 40 tons capacity would have made a train about 15,000
miles long! All but about 5,000,000 tons of this tremendous production
was hematite ore. In 1919 the output of iron ore dropped to about
60,000,000 tons.
[Illustration: Fig. 80.--Drawing showing details of part of an
ore-bearing vein at Pinos Altos, New Mexico. The chalcopyrite and
sphalerite are the ores. Somewhat reduced in size. (After Paige, U. S.
Geological Survey.)]
We shall now very briefly consider several of the greatest iron-mining
districts of the United States, giving some idea of the modes of
occurrence and origin of the ores. Greatest of all is the Lake Superior
region, not far west and south of the lake in Minnesota, Michigan,
and Wisconsin. Considerably more than one-half the iron ore mined
in the United States comes from the single State of Minnesota, and
about one-fourth of it from Michigan. Most of the Minnesota ore by
far is obtained from the so-called "Mesaba Range," which in 1917
produced 41,000,000 tons of hematite ore. The ore deposits are there
of irregular shape, lying at or near the surface (usually covered only
by glacial deposits). None of them extend downward more than a few
hundred feet. The soft, high-grade ore is removed by steam shovels in
great open pits. In the several districts of northern Michigan and
Wisconsin the ores (nearly all hematite) are associated with more or
less highly folded rocks at considerable depths. The Lake Superior iron
ores all occur in rocks of Archeozoic and Proterozoic Ages. According
to the best explanation of their origin the iron of the ores was once
part of a sedimentary series of rocks in the form of iron carbonate and
silicate, interstratified with layers of a flintlike rock associated
with slate, quartzite, etc. After these rocks were raised into land and
subjected to weathering the old iron compounds were altered to oxides,
mainly hematite, and somewhat concentrated. Further concentration of
the ore was caused by dissolving out the flintlike layers of the old
rocks.
The Birmingham, Ala., region is the second most important iron ore
producer in the United States, with an output of nearly 6,000,000 tons
in 1918. The ore is hematite, forming part of the famous Clinton iron
ore deposits of Silurian Age. This deposit, named from Clinton, N.
Y., extends through central New York and in more or less interrupted
parallel bands through the Appalachian Mountains to near Birmingham
where the richest deposits occur. This ore appears to be an original
bed (or locally several beds) of fairly rich iron ore deposited on the
shallow Silurian sea bottom and then covered by other strata. At the
time of the Appalachian Mountain revolution the iron ore was more or
less highly folded in with other strata throughout the Appalachians. A
remarkable fact regarding the Birmingham district is that in the near
vicinity of the ore there are both coal for fuel and limestone flux for
smelting the ores.
The next most important mining region of the United States is the
Adirondack Mountain region of northern New York, where about 1,000,000
tons of ore are obtained yearly. Magnetite is the ore, and it occurs
in more or less irregular lenses and bands in granite and closely
associated rocks of pre-Paleozoic Age. One view regarding the origin
of this ore is that it segregated during the process of cooling of the
molten granite, and another view (recently advocated by the author) is
that it was derived from an older iron-rich igneous formation by either
the molten granite or very hot solutions from it and concentrated into
the ores. About 2,500,000 tons of magnetite were mined in the United
States in 1916, nearly one-half of it in the Adirondacks.
The third important iron ore is limonite, nearly 2,000,000 tons of
which were produced in the United States in 1916. Most of it came from
the Appalachian Mountains. All of this limonite is of secondary origin;
that is, it has been derived from certain early Paleozoic iron-bearing
limestones either by weathering or solution, and concentrated into ore
deposits.
Copper. This is one of the most useful of all metals. Several of its
very important uses are as a conductor of electricity in the form
of wire; in making alloys such as brass and bronze; in copperplate
engraving; and in roofing and plumbing. Various minerals containing
copper are found in many parts of the world, but only about six of them
are really important as ores. These are native copper, chalcopyrite,
chalcocite, azurite, malachite, and cuprite, most of which are
described in the chapter on "Mineralogy." The number of places where
they may be profitably mined as ore is distinctly limited. Fifteen or
twenty countries produce more or less copper, but the United States is
by far the greatest producer, with an output of nearly 2,000,000,000
pounds of copper in 1916, the output having fallen off some in 1918.
This was two-thirds of the world's output and ten times as much as
the nearest competitor. The other leading countries are Japan, Chile,
Mexico, Spain, and Canada. In 1918 the four leading States in order
were Arizona, Montana, Michigan, and Utah, with production ranging from
nearly 765,000,000 pounds to about 230,000,000 pounds.
In Arizona several great copper-mining districts lie in the
southeastern one-fourth of the State. Almost invariably the ores are
directly associated with limestone and an igneous rock (granite), both
of late Paleozoic Age. The ores are almost always near the border
between the two rocks, mostly as great irregular deposits within the
limestone, and less commonly as veins within the granite. The original
ores were carried in solution and deposited by hot liquids (or vapors)
from the cooling granite. At lower levels the ores are mainly sulphides
of copper (e.g., chalcopyrite and chalcocite), while at higher levels
they are mostly carbonates (malachite and azurite) and oxides (e.g.
cuprite). The difference is due to the fact that the ores nearer
the surface have been subjected to weathering and altered from their
original condition.
The region around Butte, Mont., is next to the greatest copper
producer. Nearly all the ores are sulphides of copper (mainly
chalcocite) which occur with quartz in a great system of nearly
parallel veins in granite of Tertiary Age. "It is supposed that in the
copper veins the hot ore-bearing solutions ascended the fractures in
the granite, replacing the rock by ore, and resulting in an intense
alteration of the walls." (Ries.)
Third in rank among the copper-producing States is Michigan, the mines
being located on Keweenaw Peninsula, which extends into Lake Superior.
For fully fifty years this district has been one of the most famous and
important copper producers in the world. A unique feature is that the
ore is native copper, associated with some native silver. The rocks
containing the ore are steeply tilted lava sheets and conglomerate
(cemented gravel) strata of Proterozoic Age. Openings in porous
lava and spaces between the conglomerate pebbles have been filled
by metallic copper, which was carried off in hot solutions from the
cooling lavas. Certain of the mining shafts have been sunk more than
5,000 feet below the surface, these being next to the deepest in the
world.
Utah ranks fourth among the copper producers, the greatest mining
district being at Bingham Canyon, southwest of Salt Lake City. The
rocks are late Paleozoic strata, pierced by a large body of igneous
rock. Some of the sulphide ores (mainly chalcopyrite) occur in veins
in the igneous rock and some in large tabular masses in the adjacent
limestone. Hot solutions from lower portions of the uncooled igneous
rock carried the ore in solution into the limestone and into cracks in
the upper cooled igneous rock.
Lead. Lead must surely be counted among the five or six most useful
metals. As in the case of nearly all the other most important natural
resources, the United States is the world's greatest producer of
lead, the output of metallic lead having been 552,000 tons in 1916
and somewhat less in 1918. Most of this came from Missouri, Idaho,
Utah, and Colorado. The leading other countries are in order--Spain,
Germany, Mexico, and Australia. Nearly all the lead comes from the
mineral galena (a sulphide of lead), which is described in the chapter
on "Mineralogy." Among the many uses of lead are the following:
manufacture of certain high-grade paints from lead compounds; making
alloys such as pewter, type metal, solder, babbit metal; in plumbing;
in glass making; and in the manufacture of shot.
The greatest lead-mining district is in the vicinity of Joplin, Mo.,
where the ore (galena), associated with much zinc ore, occurs as veins
and great irregular deposits in limestone of early Paleozoic Age. It
is generally agreed that underground waters dissolved the ores out of
the limestone in which they were disseminated as tiny particles and
deposited them in concentrated form at lower levels.
In the famous Coeur d'Alene district of northern Idaho the great output
of lead is really obtained from a lead-silver ore; that is, galena
rich in silver. This ore is in composition a lead-silver sulphide. It
occurs in great fissure veins, mostly following fault fractures in
highly folded strata of Proterozoic Age. Igneous rocks cut through the
strata, and it is believed that hot ore-bearing solutions given off
from the highly heated igneous rocks rose in the fissures and deposited
the ores.
The Park City and Tintic districts of Utah are great producers of lead.
The lead ore (galena) is usually rich in silver. It occurs mainly in
veins and irregular deposits in limestone of Paleozoic Age closely
associated with certain igneous rocks.
One of the most famous mining districts in the world is that around
Leadville, Col., where ores of four metals--gold, silver, lead, and
zinc--have been extensively mined. The salient points in the rather
complex geology are the following: Paleozoic strata, including much
limestone, rest upon a foundation of pre-Paleozoic granite. Sheets
of igneous rock are interbedded with the strata and many dikes of
igneous rocks cut through the whole combination. After the last igneous
activity all the rocks were somewhat folded and notably faulted in
many places. The ores were dissolved out of the igneous rock and
deposited in large masses mostly in the limestone and in fissure veins,
especially along and near the fault zones.
Zinc. Another of the few most useful metals is zinc. It never occurs
in metallic form in nature, but most of it by far is obtained from the
ore mineral sphalerite (sulphide of zinc) described in the chapter on
"Mineralogy." A red oxide of zinc ore, called zincite, assumes great
economic importance in New Jersey. In 1917 the United States produced
686,000 tons of metallic zinc and was easily the world's leader. Since
1917 the production has fallen notably. The four greatest producing
States are Missouri, Montana, New Jersey, and Colorado. Germany and
Belgium are the greatest foreign producers.
Most important of all in the United States is the district around
Joplin, Mo., where the ore is closely associated with lead ore. The
mode of occurrence and origin of these ores are above referred to in
the discussion of lead.
In Montana some of the great east-west fissure veins in granite are
rich in silver ores in the upper levels, and in zinc ores (mainly
sphalerite) at depths of from some hundreds of feet to nearly 2,000
feet, that is as far down as they have been mined. They, like the
great copper veins of the same general district, were carried by
hot solutions which rose from the lower still very hot granite and
deposited the ores in fissures of the same cooler rock higher up.
Two great ore bodies in the general vicinity of Franklin, N. J., are of
unique interest, because they are mostly the red oxide of zinc called
zincite. The ore deposits occur in white limestone along or close to
its contact with metamorphosed (altered) strata and granite of early
Paleozoic Age. It is not definitely known how the ore originated, but
it was probably derived in solution from the hot granite and deposited
in the limestone by replacement of the latter.
In Colorado the principal zinc mines are around Leadville, where lead
ore is nearly always directly associated with the zinc ore. This
district is above described in the discussion of lead.
Among many uses of zinc are for galvanizing; for making certain
high-grade paints; brass and white metal; and for roofing and plumbing.
Gold. This precious metal has been used and highly prized by man for
thousands of years. The discovery of gold in California in 1848 was
one of the most important events in the history of the mining world.
As early as 1852 that State reached its climax of production with an
output of at least $81,000,000 worth of the metal. The Transvaal region
of South Africa has for two decades been the world's greatest gold
producer. Though long known, the metal has there been worked only since
1886. In 1915 the peak of gold production in the world ($468,700,000)
was reached and nearly maintained in 1916, but since that time there
has been a great falling off. In 1916 South Africa produced gold to
the value of about $200,000,000; the United States over $90,000,000;
Australia over $40,000,000; Russia over $26,000,000; and Canada over
$19,000,000.
In tiny amounts gold is really very widespread. It occurs in many
stream gravels where so-called "color" may be obtained by washing
gravel, and it is even dissolved in sea water. Gold-mining localities
are also numerous in many parts of the world, but relatively few of
them only have ever paid. The total amount of money spent in actual
gold-mining operations; in hopeless but honest operations; and for
stock in fake gold mines has no doubt exceeded the actual value of gold
produced. In many a case acceptance of a report based upon a very brief
examination of the ground by a competent geologist would have saved
the cost of hopeless expenditure of money. Some one in nearly every
community has a so-called "gold mine."
Most of the commercially valuable gold occurs in nature as native
gold, either mixed with gravel and sand (i.e., placer deposits) along
existing or ancient stream beds, or in veins mechanically held in the
mineral pyrite (described in the preceding chapter) in submicroscopic
form, or visibly mixed with quartz in vein deposits. Another kind of
ore which assumes considerable importance, as in parts of Colorado, is
in the form of telluride of gold always found in veins. In deep vein
deposits it is quite the rule to find free or native gold mechanically
and visibly mixed with quartz in the upper levels, while deeper down
the gold is mechanically, but invisibly, held in combination usually
in pyrite, which latter is associated with quartz. This difference is
due to the fact that the lower level ores are now just as they were
formed, while in the upper levels the ores have been weathered, and the
gold set free and often more or less further concentrated by solutions.
Vein deposits, including also telluride ores, are found in many kinds
of rocks--igneous, sedimentary, and metamorphic--of nearly all ages
generally directly associated with igneous rocks. In nearly all cases
the best evidence indicates that the vein fillings were formed by hot
ore-bearing solutions from the igneous rock, which solutions deposited
the ore plus quartz in fissures in either the igneous or adjacent
rocks. Among the many localities where fissure veins of the kind just
described are of great economic importance are the "Mother Lode" belt
of the Sierra Nevada Mountains of California; Cripple Creek (telluride
ore), Georgetown and the San Juan region of Colorado; Goldfield,
Tonopah, and Comstock Lode of Nevada; and near Juneau, Alaska.
Placer deposits, that is, free gold mixed with gravel and sand, also
yield much gold. They are most prominently developed in California
and Alaska. These gold-bearing "gravels represent the more resistant
products of weathering, such as quartz and native gold, which have
been washed down from the hills on whose slopes the gold-bearing
quartz veins outcrop, and were too heavy to be carried any distance,
unless the grade was steep. They have consequently settled down in the
stream channels, the gold, on account of its higher specific gravity,
collecting usually in the lower part of the gravel (placer) deposit."
(Ries.) Such gold occurs as grains, flakes, or nuggets. When a chunk of
gold-bearing vein quartz, with crevices filled by thin plates of the
metal, is carried downstream pieces are gradually broken away, and the
tough, very malleable gold bends or welds together into a single mass
called a "nugget." Nuggets varying in weight up to over 2,000 ounces
have been found. Many placer deposits are along existing drainage
channels, while others occur in abandoned and even buried former
channels.
Most of the gold of South Africa comes from Witwatersrand district
where the native metal occurs in a unique manner in beds or layers
of conglomerate associated with other strata, all the rocks being
considerably folded and somewhat faulted. Some of the mines are more
than a mile deep (vertically), the deepest in the world. The gold
either accumulated in placer form with gravel which later consolidated
into conglomerate, or it was introduced into spaces between the pebbles
subsequently by ore-bearing solutions.
Silver. For many years the United States and Mexico have been the
world's greatest silver producers, sometimes one and sometimes the
other leading, with Canada third, and Australasia fourth. In 1918 the
United States produced nearly 68,000,000 ounces of silver and Mexico
over 62,000,000 ounces. In the United States in 1918 the four leading
States were Montana, Utah, Idaho and Nevada with outputs ranging from
over 10,000,000 to over 15,000,000 ounces each.
In Montana most of the silver is in the native form, more especially
in the upper portions of the great veins rich in copper and zinc ores
near Butte. These ores and their origin are described above under the
captions "Copper" and "Zinc."
The two greatest silver districts of Nevada are Tonopah and Comstock
Lode where silver and gold minerals are associated as ores in Tertiary
igneous rocks, the ores having been deposited in veins by hot
ore-bearing solutions from the igneous rocks.
In Idaho the Coeur d'Alene district produces most of the silver, the
ore there being a silver-bearing lead ore (galena). The nature and
origin of these deposits are described above under the caption "Lead."
In Utah the silver is also obtained from silver-bearing galena
especially in the Tintic, Cottonwood Canyon, and Bingham Canyon
districts where the ores occur mainly as irregular deposits and
in fissure veins in Paleozoic strata (chiefly limestone) directly
associated with igneous rocks, hot ore-bearing solutions from the
igneous rocks having furnished the ores.
Tin. Production of tin in the United States has never amounted to much,
a little mining having been carried on from time to time in South
Carolina, Black Hills of South Dakota, and southern California. About
one-half of the world's supply of tin (121,000 long tons 1918) comes
from the Malay Peninsula and two small islands near by. The only other
great producer is Bolivia, though a number of other countries produce
from 1,000 to 9,000 tons each.
The only important ore of tin is the mineral cassiterite (oxide of tin)
described above in the chapter on "Mineralogy." In the Malay region
the ore all occurs in placer deposits and is, therefore, of secondary
origin, the source of the ore not being known. In Bolivia the tin ore
occurs in veins in and close to granite, the ore having been carried
by very hot vapors or liquids which were derived from the still highly
heated granite.
Tin is used chiefly in the making of tin plate, bronze, pewter, gun
metal, and bell metal.
Aluminum. The mineral called bauxite (a hydrous oxide of aluminum)
is the great ore from which aluminum is obtained by an electrical
process. Bauxite is noncrystalline, relatively light in weight, white
to yellowish in color, and in the form of rounded grains, or earthy or
claylike masses. The United States and France are the only two great
producers of bauxite, most of which is treated for metallic aluminum.
In 1918 the United States produced more than 100,000 tons of aluminum.
In the United States the principal deposits are in Georgia, Alabama,
and Arkansas. Bauxite is probably always a secondary mineral formed
by decomposition of igneous rocks rich in certain aluminum silicate
minerals. In some cases, as in the Georgia-Alabama region, the bauxite
appears to have been formed and concentrated in deposits by hot
solutions from uncooled igneous rocks.
Aluminum is most used in the manufacture of wire for electric current
transmission. It is also mixed with certain other metals like copper,
zinc, magnesium, and tungsten to form special types of alloys, some of
which possess remarkable tensile strength up to nearly 50,000 pounds
per square inch. Aluminum is used in powdered form to generate very
high temperatures in certain welding processes. It is also made into
many kinds of utensils and instruments.
Mercury. This metal, commonly known as "quicksilver," is of special
interest because it is the only one which exists in liquid form at
ordinary temperatures. The metal occurs in only small quantities in
nature, most of it by far being obtained from the red mineral cinnabar
described in the chapter on "Mineralogy." In order of importance the
greatest quicksilver producing countries in 1916 were Italy, United
States, Austria, and Spain. In the United States, California is by far
the leading State, while Texas and Nevada are the only other important
producers.
In California most of the ore occurs in veins and irregular deposits
in metamorphosed strata of Mesozoic and Cenozoic ages usually closely
associated with igneous rocks. There, as well as in other parts of the
world, hot vapors from igneous rocks carried the volatile ore upward
and deposited it in fissures.
Among the many uses of mercury are in making fulminate for explosives;
making certain drugs and chemicals, pigments, electrical and physical
apparatus; silvering mirrors; and in the amalgamation process of
extracting gold and silver.
OTHER ECONOMIC PRODUCTS
Building Stones. Some of the principal features which should be
considered in building stones are power to resist weathering, power to
withstand heat, color, hardness, and density, and crushing strength.
Building stones representing rocks of nearly all important geologic
ages are widely distributed throughout the world.
_Granite_, including certain other closely related rocks, is one of
the oldest and most useful building stones. The New England States
are the greatest producers, while the Piedmont Plateau district (east
of the Appalachians) from Philadelphia to Alabama also contains
important granite quarries. In the Adirondack Mountains, in Wisconsin
and Minnesota, through the Rocky Mountains, and the Sierra Nevada
Mountains there are extensive areas of granite which are relatively
little quarried. The granite occurs only in regions of highly disturbed
rocks, usually in mountains or hills, where great volumes of the molten
rock were forced into the earth's crust, cooled, and later laid bare by
erosion.
_Marble_, according to geological definition, is a metamorphosed
limestone, that is a limestone which has been crystallized under
conditions of heat, pressure, and moisture within the earth. More
loosely in trade any limestone which takes a polish may be called
marble. The greatest marble-producing districts of the United States
are western New England (especially Vermont) and the Piedmont Plateau
and Appalachian Mountains in rocks of Paleozoic age. In northern New
York and the mountains of the west there are relatively few marble
quarries.
Ordinary _limestones_ are widely distributed in many States where they
range in age from early Paleozoic to Tertiary. Most of the quarries
supply stone for near-by markets. The so-called Bedford limestone
of Indiana has, for many years, been perhaps the most widely used
limestone for building purposes in the United States.
_Sandstones_, which are stratified rocks consisting mainly of
rounded quartz grains cemented together, are widely used in building
operations. Like limestones, they are very widespread in formations
of all ages except the very old. There are many sandstone quarries
supplying more or less local markets throughout the country. Two of
the best known and most widely used sandstones are the so-called
brown-stone of Triassic Age extending interruptedly from the
Connecticut Valley of Massachusetts to North Carolina, and the Berea,
Ohio, sandstone of light gray color and uniform texture.
_Slate_ is mostly a metamorphosed shale, that is a shale which has been
subjected to great pressure within the earth so that the stratification
has been obliterated and a well defined cleavage has been developed
at right angles to the direction of application of the pressure.
Good slate is fine-grained, dense, and splits readily into wide thin
plates. It occurs only where mountain making pressure and metamorphism
have been brought to bear upon the strata. Most of our great slate
quarries are located in early Paleozoic rocks from New England through
the Piedmont Plateau. Some quarries are also located in Arkansas,
Minnesota, and westward to California.
Clay. "Clay, which is one of the most widely distributed materials
and one of the most valuable, commercially, may be defined as a
fine-grained mixture of the mineral kaolinite with fragments of other
minerals, such as silicates, oxides, and hydrates, and also often
organic compounds, the mass possessing plasticity when wet and
becoming rock-hard when burned to at least a temperature of redness."
(Ries.)
Most clays originate by the weathering of rocks, particularly igneous
and metamorphic rocks rich in the mineral feldspar. As a result of the
decomposition of the feldspar, much clay is formed, the main substance
of which is kaolin. Both feldspar and kaolin are described in the
preceding chapter. When the resulting clay rests upon the rock from
which it has been derived it is called residual clay. Much of the clay
is, however, carried away, mainly by streams, and deposited in lakes or
the sea, or on river flood plains. Some clay deposits are of wind-blown
origin, and still others are formed by the grinding action of glaciers.
Clays are very widespread, and they are directly associated with rocks
of all geologic ages.
Among the many important uses of clay are the following: manufacture of
common brick, fire brick, pottery, chinaware, porcelain ware, tiles,
terra cotta, and Portland cement.
Lime and Cement. Limestone, which is one of the most common and
widespread of all stratified rocks, forms the basis for the manufacture
of the important substances lime (or "quicklime") and Portland cement.
Lime results when pure limestone (carbonate of lime) is "burned" or
heated to a temperature high enough to drive off the carbonic acid
gas. The greatest use of lime is for mixing with water and sand to
make mortar. A few of its other numerous uses are in plastering;
whitewashing; purifying certain steel; in making gas, paper, and soap;
and as a fertilizer.
Certain limestones containing clay of the right kind and proportion
are called natural cement rocks because, after being "burned," they
develop the property of "setting," like cement when mixed with water.
The "setting" of a cement is due to the fact that certain chemical
compounds formed during the heating crystallize when mixed with water,
and the hard, tiny interlocking crystals of the newly formed silicate
minerals give rigidity to the mass. Of recent years Portland cement
has largely superseded the natural rock cements. "Portland cement is
the product obtained by burning a finely ground artificial mixture
consisting essentially of lime, silica, alumina, and some iron oxide,
these substances being present in certain definite proportions."
(Ries.) The necessary ingredients are generally obtained by grinding
and burning carefully selected mixtures of limestone in some form,
and clay or shale. The great and growing uses of cement need not be
detailed here.
Salt. Most of the common salt (the mineral "halite") of commercial
value occurs in nature in sea or salt lake water; or in beds or strata
of rock salt associated with other strata; or as natural brine in
openings or pores in certain rocks. Considerable salt is obtained by
evaporation of tidewater, as around San Francisco Bay, and of salt lake
water, as at Great Salt Lake, Utah. It has been estimated that the
Great Salt Lake, whose area is about 2,000 square miles and greatest
depth 50 feet, contains several hundred million tons of common salt.
This salt has been washed out of the rocks of the surrounding country
and gradually accumulated in the lake because it has no outlet.
Most important of all sources of salt is the rock salt which occurs
in the form of strata within the earth's crust. Such strata are found
in rocks of nearly all ages from the early Paleozoic to the present.
They resulted from the evaporation of salt lakes or salty more or
less cut-off arms of the sea, after which other strata accumulated on
top of them. Thus in the Silurian system of strata underlying all of
southwestern New York State there occur almost universally from one to
seven beds of salt. The strata including the salt dip gently southward
so that at Ithaca, New York, seven salt beds were struck in a well
at a depth of about 2,200 feet. Northward the salt comes nearer and
nearer the surface. One well penetrated a layer of solid salt 325 feet
thick. Some of this salt is being mined much like coal, but most of it
is obtained by running water into deep wells to dissolve the salt, the
resulting brine being pumped out and evaporated.
Under portions of southern Michigan there are both salt beds and
natural brines charging certain porous rock layers. Both the salt beds
(of Silurian Age) and the brines (of Mississippian Age) supply great
quantities of salt from brines pumped out and evaporated.
In 1918 the United States produced 51,000,000 barrels (280 lbs. each)
of salt. Michigan (17,000,000 barrels) and New York were the leading
States, followed by Kansas, Ohio, West Virginia, and California. Some
of the uses of common salt are given in the description of halite in
the preceding chapter.
Gypsum. The composition and properties of this common and useful
mineral are given in the chapter on "Mineralogy." Rock gypsum is
the variety of great commercial importance. It is widespread, being
quarried in many States, and occurs interstratified with rocks of many
ages where it has originated by evaporation or partial evaporation of
salt water lakes or more or less cut-off arms of the sea. Salt beds
are often associated with gypsum.
For about ten years the average yearly production of gypsum in the
United States has been approximately 2,500,000 tons, or about ten
times that of the nearest foreign competitor (Canada). New York, Iowa,
Michigan, and Ohio are the chief producers. In New York the rock gypsum
(usually four to ten feet thick) lies between shale and limestone
strata of Silurian age, and it is quarried from the central to the
western part of the State. In Michigan the rock gypsum beds, commonly
five to twenty feet thick, lie in Mississippian strata in the southern
portion of the State. A great bed of exceptionally pure rock gypsum
underlies about twenty-five square miles of Webster County, Iowa, in
rocks of late Paleozoic Age. The Kansas gypsum deposits extend across
the central part of the State in rocks of Permian Age.
Rock gypsum is mainly used in making "plaster of Paris," as a retarder
in cement, and as a fertilizer (so-called "land plaster").
GLOSSARY OF COMMON
GEOLOGICAL TERMS
Names of subkingdoms and important classes of fossil plants and
animals, and mineral species, are not included; these being briefly and
systematically discussed in chapters 17, 18, 19, and 20, respectively.
By using the index the reader can quickly locate the page where any
one of these names is discussed. Some definitions in this glossary are
taken from U. S. Survey Bulletin No. 613.
_Anticline._--A kind of folded structure in which strata have been
bent upward or arched.
_Archeozoic._--The earliest known era of geologic time.
_Basalt._--A common lava of dark color and of great fluidity when
molten. Basalt is less siliceous than granite and rhyolite, and
contains much more iron, calcium, and magnesium.
_Base-level._--The lowest level to which a stream can cut (erode) its
channel. A whole region may be base-leveled by erosion.
_Cambrian._--The first or earliest period of the Paleozoic era of
geologic time.
_Cenozoic._--The present era of geologic time. It began at least
several million years ago.
_Chalk._--A soft, fine-grained, white limestone consisting mainly of
tiny shells.
_Conglomerate._--A sedimentary rock consisting of consolidated or
cemented gravel. Often sandy.
_Cretaceous._--The last period of the Mesozoic era of geologic time.
_Crystal._--A regular polyhedral form, possessing a definite internal
molecular structure, which is assumed by a substance in passing
from the state of a liquid or gas to that of a solid. Nearly
every mineral, under proper conditions, will crystallize.
_Crystalline Rock._--A rock composed of closely fitting mineral
crystals that have formed in the rock substance, as contrasted
with one made up of cemented grains of sand or other materials, or
with a volcanic glass.
_Crystallography._--The study of crystals.
_Devonian._--The middle one of the seven periods of the Paleozoic era
of geologic time.
_Dike._--A mass of igneous rock that has solidified in a fissure or
crack in the earth's crust.
_Drift._--Commonly called glacial drift. The rock fragments--soil,
gravel, and silt--carried by a glacier. Drift includes the
unassorted material known as till (ground moraine) and deposits
made by streams flowing from a glacier.
_Drowned River Valley._--When a land surface sinks enough to
permit tidewater to enter the lower ends of its valleys to form
estuaries, a good example being the lower Hudson Valley.
_Era._--A name applied to one of the broadest subdivisions of
geologic time (e.g. Paleozoic era).
_Erosion._--The wearing away and transportation of materials at and
near the earth's surface by weathering and solution, and the
mechanical action of running water, waves, moving ice, or winds
which use rock fragments as tools or abrasives.
_Exfoliation._--The splitting off of sheets of rock of various sizes
and shapes due to changes of temperature. It is a process of
weathering.
_Fault._--A fracture in the earth's crust accompanied by movement of
the rock on one side of the break past that on the other. If the
fracture is inclined and the rock on one side appears to have slid
down the slope of the fracture the fault is termed a normal fault.
If, on the other hand, the rock on one side appears to have been
shoved up the inclined plane of the break, the fault is termed a
reverse or thrust fault.
_Fault-block._--A part of the earth's crust bounded wholly or in part
by faults.
_Fault-scarp._--The cliff formed by a fault. Most fault scarps have
been modified by erosion since the faulting.
_Fissure._--A crack, break, or fracture in the earth's crust or in a
mass of rock.
_Flood-plain._--The nearly level land that borders a stream and is
subject to occasional overflow. Flood-plains are built up by
sediment left by such overflows.
_Fold._--A bend in rock layers or beds. Anticlines and synclines are
the common types of folds.
_Formation._--A rock layer, or a series of continuously deposited
layers grouped together, regarded by the geologist as a unit for
purposes of description and mapping. A formation is usually named
from some place where it is exposed in its typical character.
_Fossil._--The whole or any part of an animal or plant that has been
preserved in the rocks or the impression left on rock by a plant
or animal. Preservation is invariably accompanied by some change
in substance, and from some fossils the original substance has all
been removed.
_Geography._--The study of the distribution of the earth's physical
features, in their relation to each other to the life of sea and
land, and human life and culture.
_Geology._--The science which deals with the history of the earth and
its inhabitants as revealed in the rocks.
_Glacier._--A body of ice which slowly spreads or moves over the land
from its place of accumulation.
_Gneiss_ (pronounced nice).--A metamorphic, crystalline rock with
mineral grains arranged with long axes more or less parallel,
giving the rock a banded appearance. Derived from either igneous
or stratified rocks well within the earth under conditions of
pressure, and usually also heat and moisture.
_Igneous Rocks._--Rocks formed by the cooling and solidification
of a hot liquid material, known as magma, that has originated
at unknown depths within the earth. Those that have solidified
beneath the surface are known as intrusive rocks, or if the
cooling has taken place slowly at great depth, as plutonic rocks,
e.g. granite. Those that have flowed out over the surface are
known as effusive rocks, extrusive rocks, or lavas, e.g., basalt.
Volcanic rocks include not only lavas, but bombs, pumice, tuff,
volcanic ash, and other fragmental materials or ejecta thrown out
from volcanoes.
_Joints._--Nearly all rocks, except very loose surface materials, are
separated into blocks of varying size and shape by a system of
cracks called joints. They may be caused by earth-crust movements,
contraction during solidification of molten rocks, or contraction
during drying out of sediments.
_Jurassic._--The middle one of the three periods of the Mesozoic era
of geologic time.
_Lava._--An igneous rock which in molten condition has poured out
upon or close to the earth's surface, e.g. basalt.
_Limestone._--A sedimentary rock consisting essentially of carbonate
of lime which generally represents accumulation of shells of
organisms, but in some cases precipitates from solution. Often
impure.
_Loess_ (pronounced lurse with the r obscure).--A fine homogeneous
silt or loam showing usually no division into layers and forming
thick and extensive deposits in the Mississippi Valley and in
China. It is generally regarded as in part at least a deposit of
wind-blown dust.
_Marble._--A crystalline limestone, usually a metamorphic rock, the
limestone having been altered by heat, pressure, and moisture
within the earth.
_Meander._--To flow in serpentine curves. A loop in a stream. Most
streams in flowing across plains develop meanders.
_Mesa._--A flat-topped hill or mountain left isolated during the
general erosion or cutting down of a region.
_Mesozoic._--Next to the present era of geologic time. _ Metamorphic
Rock._--Any igneous or sedimentary rock which has undergone
metamorphism, that is notable alteration from its original
condition. (See Metamorphism.)
_Metamorphism._--Any change in rocks effected in the earth by heat,
pressure, solutions, or gases. A common cause of the metamorphism
of rocks is the intrusion into them of igneous rocks. Rocks that
have been so changed are termed metamorphic. Marble, for example,
is metamorphosed limestone.
_Mineral._--An inorganic substance of definite chemical composition
found ready made in nature, e.g. calcite, quartz.
_Mississippian._--A period of the Paleozoic era of geologic time--in
order of age, the third from the last of the era.
_Moraine._--Glacial drift carried on, within, or under a glacier and
deposited at the end, along the sides, or under the glacier.
_Oil-pool._--An accumulation or body of oil in sedimentary rock that
yields petroleum on drilling. The oil occurs in the pores of the
rock and is not a pool or pond in the ordinary sense of these
words.
_Ordovician._--Next to the earliest period of the Paleozoic era of
geologic time.
_Ore._--A metal-bearing mineral or rock of sufficient value to be
mined.
_Outcrop._--That part of a rock formation which appears at the
surface. The appearance of a rock at the surface or its projection
above the soil. Often called an exposure.
_Paleontology._--The study of the world's (geologically) ancient
life, either plant or animal, by means of fossils.
_Paleozoic._--An old era of geologic time--third back from the
present.
_Peneplain._--A region reduced almost to a plain by the
long-continued normal erosion of a land surface. It should be
distinguished from a plain produced by the attack of waves along a
coast or the built-up flood plain of a river.
_Pennsylvanian._--Next to the last period of the Paleozoic era of
geologic time.
_Period._--A name applied to one of the subdivisions of an era of
geologic time, e.g. Cambrian period.
_Permian._--The last period of the Paleozoic era of geologic time.
_Petrology._--The study of rocks, including igneous, sedimentary, and
metamorphic rocks.
_Physiography._--The study of the relief features of the earth and
how they were produced.
_Placer Deposit._--A mass of gravel, sand, or similar material
resulting from the crumbling and erosion of solid rocks and
containing particles or nuggets of gold, platinum, tin, or other
valuable minerals, which have been derived from rocks or veins.
_Plutonic Rock._--An igneous rock solidified from a molten condition
well within the earth. (See Igneous Rocks.)
_Proterozoic._--Next to the earliest known era of geologic time.
_Quartzite._--A metamorphic rock composed of sand grains cemented by
silica into an extremely hard mass.
_Quaternary._--The later of the two periods of the Cenozoic era of
geologic time.
_Rejuvenated._--Any region which has been subjected to erosion for
a greater or less length of time and then reelevated so that the
streams are renewed in activity.
_Rock._--Any extensive constituent of the crust of the earth, usually
consisting of a mechanical mixture of two or more minerals, e.g.
granite, shale. Less commonly a rock consists of a single mineral
(e.g. pure marble), or of organic matter (e.g. coal).
_Sandstone._--A sedimentary rock consisting of consolidated or
cemented sand. Often shaly or limy.
_Schist._--A rock that by subjection to heat and pressure and usually
moisture within the earth has undergone a change in the character
of the particles or minerals that compose it and has these
minerals arranged in such a way that the rock splits more easily
in certain directions than in others. It is a metamorphic rock
derived from either sedimentary or igneous rock, more commonly the
former.
_Sedimentary Rocks._--Rocks formed by the accumulation of sediment
in water (aqueous deposits) or from air (eolian deposits). The
sediment may consist of rock fragments or particles of various
sizes (conglomerate, sandstone, shale); of the remains or products
of animals or plants (certain limestones and coal); of the product
of chemical action or of evaporation (salt, gypsum, etc.); or of
mixtures of these materials. Some sedimentary deposits (tuffs)
are composed of fragments blown from volcanoes and deposited on
land or in water. A characteristic feature of sedimentary deposits
is a layered structure known as bedding or stratification. Each
layer is a bed or stratum. Sedimentary beds as deposited lie flat
or nearly flat, but subsequently they have often been deformed by
folding and faulting.
_Shale._--A sedimentary rock consisting of hardened thin layers of
fine mud.
_Silurian._--A period of the Paleozoic era of geologic time--in order
of age, the third from the beginning of the era.
_Slate._--A rock that by subjection to pressure within the earth has
acquired the property of splitting smoothly into thin plates. The
cleavage is smoother and more regular than the splitting of schist
along its grain. It is a metamorphic rock nearly always derived
from shale.
_Soil._--The mantle of loose material resting upon bedrock, either in
its place of origin or transported by water, wind, or ice.
_Strata_ (or stratified rocks).--Sedimentary rocks which, by the
sorting power of water (less often by wind), are arranged in
more or less definite layers or beds separated by stratification
surfaces.
_Stratification._--The separation of sedimentary rocks into more or
less parallel layers or beds.
_Stratigraphy._--The branch of geologic science that deals with the
order and relations of the strata of the earth's crust.
_Structure._--In geology, the forms assumed by sedimentary beds and
igneous rocks that have been moved from their original position by
forces within the earth, or the forms taken by intrusive masses
of igneous rock in connection with effects produced mechanically
on neighboring rocks by the intrusion. Folds (anticlines and
synclines) and faults are the principal mechanical effects
considered under structure. Schistosity and cleavage are also
structural features.
_Syncline._--A kind of folded structure in which strata have been
bent downward. It is an inverted arch--the opposite of an
anticline.
_Talus_ (pronounced t[=a]y'lus).--The mass of loose rock fragments
that accumulates at the base of a cliff or steep slope.
_Terrace._--A steplike bench on a hillside. Most terraces along
rivers are remnants of valley bottoms formed when the stream
flowed at higher levels. Other terraces have been formed by waves.
Some terraces have been cut in solid rock, others have been built
up of sand and gravel, and still others have been partly cut and
partly built up.
_Tertiary._--The earlier of the two periods of the Cenozoic era of
geologic time.
_Triassic._--The earliest period of the Mesozoic Era of geologic time.
_Unconformity._--A break in the regular succession of sedimentary
rocks, indicated by the fact that one bed rests on the eroded
surface of one or more beds which may have a distinctly different
dip from the bed above. An unconformity may indicate that the beds
below it have at some time been raised above the sea and have been
eroded. In some places beds thousands of feet thick have been
washed away before the land again became submerged and the first
bed above the surface of unconformity was deposited. If beds of
rock may be regarded as leaves in the volume of geologic history,
an unconformity marks a gap in the record.
_Vein._--A mass of mineral material that has been deposited in or
along a fissure in the rocks. A vein differs from a dike in that
the vein material was introduced gradually by deposition from
solution, whereas a dike was intruded in a molten condition.
Quartz and calcite are very common vein minerals.
_Volcanic Rocks._--Igneous rocks erupted at or near the earth's
surface, including lavas, tuffs, volcanic ashes, and like material.
_Weathering._--The group of processes, such as the chemical action of
air and rain water, and of plants and bacteria, and the mechanical
action of changes of temperature, whereby rocks on exposure to the
weather change in character, decay, and finally crumble into soil.
Transcriber Note
All illustrations splitting paragraphs were moved before or after the
paragraph. All simple typos were corrected (i.e., Reudemann to Ruedemann,
pryoxene to pyroxene).