The Elements of Geology

on the earth from without, and all are set in motion by an energy external to the earth, namely, the radiant energy of the sun. All, too

their broad plains and great plateaus and mountain ranges above the sea. Either, then, the earth is very young and th

waters from the shores. At times also these forces have aided in the destruction of the lands by gradually lowering them and bringing in the sea. Under the action of forces resident within the earth the crust slowly rises or sinks; from time to time it has been folded and broken; while vast quantities of molten rock

IONS OF

a long time is needed to produce perceptible changes of level, and which leave the strata in nearly their original horizontal attitude. These movements are most c

grounds, with the stumps still standing, are now overflowed by the sea, and fragments of their turf and wood are brought to shore by waves. Assuming that the sea

clays, which, as their marine fossils show, were outspread beneath the sea. Their presen

table crust now in progress along many shores. Some are emerging from the sea, some are sinkin

elative upheaval whatever is now found in place above sea level and could have been formed only at or beneath it,

emergence to the amount of their present height above tide. No less conclusive is the presence of sea-laid rocks which we may find in

peat beds, now submerged beneath the sea. In the deltas of many large rivers, such as the Po, the Nile, the Ganges, and the Mississippi, buri

n. To this class belong Narragansett, Delaware, Chesapeake, Mobile, and San Francisco bays, and many other similar drowned valleys along the coasts of the United States. Less conspicuous are the SUBMARINE CHANNELS which, as soundings show, extend from the mouths

portions are stationary or moving in the opposite direction. In this way a land surface becomes WARPED. Thus, while Nova Scotia and New Brunswick are now rising from t

rate of emergence has not been uniform. The old strand line, which stands at five hundred and seventy-five feet

osite directions are swift and shallow. At the western end of Lake Erie are found submerged caves containing stalactites, and old meadows and forest grounds are now under water. It is thus seen that the water of the lak

t between the tributaries of the Mississippi and Lake Michigan is but eight feet above the mean stage of the lake. If the canting of the region continues at its present rate, in a thousand years the waters of the lake will here overflow the divide. In three thousand five hundred years all the lak

he various agencies which are to be chiefly concerned m the wear of any land,-whether streams or glaciers, weathering or the wind,-and the degree of their efficiency. The lands must be uplifted before they can be eroded, and since they must be eroded before their waste can be deposited, movements of elevation are a prerequisite condition for sedimentation also. Subsidence is a necessary condition

ined upon the land, and here in several seaboard towns streets by the shore are still submerged. The rate of oscillation increases also from the coast inland. On the other hand, along the German coast of the Baltic the only historic fluctuations of sea level are tho

y fills, the highest standing two hundred and thirty feet above the river. Here the Saguenay is eight hundred and forty feet in depth, and the tide ebbs and flows

166). Three marble pillars are still standing. For eleven feet above their bases these columns are uninjured, for to this height they were protected by an accumulation of volcan

S OF TH

on either side of the valley V, we find outcrops of layers tilted at high angles. Sections along the ridge r show that it is composed of layers which slant inward from either side. In places the outcropping strata

ere outspread in nearly level sheets upon the ocean floor. Since that time they must have been deformed. Layers of solid rock several miles in thickness have been c

is most easily done by taking the angle at which the strata are inclined and the compass direction in which they

layer is 90 degrees. The direction of the dip is taken with the compass. Thus a geologist's notebook in describing the attitude of outcropping strata contains many such entries as these: d

he horizontal plane is the STRIKE. The str

e made with the shelf by the side of the book, while the strike is represented by a line run

an ANTICLINE. A downfold, where the strata dip from either side toward the axis of the trough, is called a SYNCLINE. There is sometimes

ds the dips of the rocks on each side the axis of the fold are equal. In UNSYMMETRICAL folds one limb is steeper than the other, as in the anticline in Figure 167. In O

along the limbs. Where strong rocks such as heavy limestones are folded together with weak rocks such as s

has been thrown into a series of parallel folds whose axes run from northeast to southwest (Fig. 175). In Pennsylvania one may count a score or more of these earth waves,- some but from ten to

produced by a heave, a series of folds, including overturns, fan folds, and folds thickened on their crests at the expense of their limbs, could only be mad

e produced the results which we see. Rocks are brittle, and it is only when under a HEAVY LOAD and by GREAT PRESSURE SLOWLY APPLIED, that they can thus be folded and bent instead of being crus

by folding to lateral pressure, and flow instead of breaking. Indeed, at such profound depths and under such inconceivable weight no cavity can form, and any fractures would be healed at once by the welding of gra

VELOPED IN CO

as well. A hand specimen of slate, or even a particle under the microscope, may show plications similar in form and origin to the foldings which have produced ranges o

ince it is most perfectly developed in fine-grained, homogeneous rocks, such as slates, which cleave to the thin, smooth-surfaced plates with which we are familiar i

lel planes, along which the rock naturally splits more easily than in any other direction. The irregular grains of the mud which has been altered to slate have been squeezed flat by a pressure exerted at right angles to the plane of cleavage. Cleavage is found only in folded rocks, and, as we may see in Figure 176, the strik

r cleavage in the fact that any inclusions in them, such as nodules and fossils, have b

geneous material of fine grain, such as beeswax, when subjected to heavy

s which had the right of way across the region were able to hold to their courses, and as a circular saw cuts its way through the log which is steadily driven against it, so these rivers sawed their gorges through the fold as fast as it rose beneath them. Streams which thus maintain the course which they had antecedent to a deformation of the region are known as ANTECEDENT streams. Examples of such are th

ed more rapidly, processes of

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the stronger are its slopes, the faster is it worn away. Even while rising, a young upfold is often thus unroofed, and instead of appearing as a long, Smooth, boat-shaped ridge, it commonly has had opened along the rocks of the axis, when these are

ndeed, are not the result of folding. Some, as we shall see, are due to upwarps or to fractures of the crust; some are piles of volcanic material; s

rinkled into a fold; or into a series of folds, forming a series of parallel ridges and intervening valleys; or a number of folds have been mashed together into a vast u

e found expression in any individual surface features. As the result of these deformations deep under ground the surface was broadly lifted to mountain height, and th

the folds of the Alps were smoothed out, they would occupy a belt seventy-four miles wider than that to which they have been compressed, or twice their present width. A sec

ich mountains have undergone accounts fully for their height,

lded strata rested on an unyielding foundation, and that what was lost in width was gained i

in mountain scenery, excepting in the very youngest ranges, is due solely to erosion. We may therefore classify mountains according to the degree to which they have been dissected. The Juras are an example o

knife-edged crests, deep valleys with ungraded slopes subject to frequent landslides, are all features of Alpine scenery typical of a mountain range at this stage in its life history. They represent the survival of the hardest rocks and the strongest structures, and the destruction of the weaker in their long struggle for existence against the

gged crests of their earlier life are smoothed down to low domes and rounded crests. The southern Appalach

since been wasted to low-lying lands. Such a section as that of Figure 67 illustrates how ancient mountains may be leveled to their roots, and represents the final stage to which even the Alps and the Himalayas must sometime arrive. Mountains, perhaps of Alpine height, once stood about Lake Superior; a lofty rang

mention here some of the conditions which have com

irty thousand feet thick, while the same formations thin out to five thousand feet in the Mississippi valley. The folds of

e most part the shallow-water deposits of continental deltas. Mo

be laid only in a gradually sinking area. A profound subsidence, often to be r

it forms, is filled with sediments, which at last come to be many thousands of feet thick. The downward movement finally ceases. A slow but resistless pressure se

DISLOCATIONS

ure, like brittle bodies, instead of by folding and flowing, like plastic solids. Whether rocks bend or break depends on the character and

r this name are included many division planes caused by cooling and drying; but it is now generally believed that the larger and more regular joints, esp

alleys, by the sea in driving back its cliffs, by glaciers in plucking their beds, and how they are enlarged in soluble rocks to form natural passageways for un

al distance between the ends of a parted layer, is the throw. The angle which the fault plane makes with the vertical is the HADE. In Figure 184 the right side has gone down relatively to the left; the right is the side of the downthrow, whil

urfaces are polished and grooved by the enormous friction which they have suffered as they have ground one upon t

he downthrow; the hanging wall has gone down. The total length of the strata has been increased by the displacement. It seems that t

ly associated. Under lateral pressure strata may fold to a certain point and then tear apart and fault along the surface of least resistance. Under immense pressure strata also break by shear without folding.

ounger beds; and when the fault planes are nearly horizontal, and especially when the rocks have been broken into

egion have been driven east for seven miles on a thrust plane,

ng from northeast to southwest, along which the older strata have been pushed westward over the younger. The longest continuous fault has

ents. It may occur also that strata which as a whole yield to lateral thrust by folding include beds of brittle rocks, such as thin-layered limestones, which a

by the consolidation of volcanic cinders, of angular waste at the foot of cliffs, or of fragment

URES DUE TO

rface by a scarp, because the face of the upthrown block

w side, emerge in a line of cliffs. Where a fault is so old that no abrupt scarps appear, its general course is sometimes marked by the line of division between highland and lowland or hill and plain. Great faults have sometimes brought ancient cry

line of dislocation may be alike in their resistance to erosion and therefore have been worn down to a common slope. The fault may be entirely concealed by the

ateau is a crust block ninety miles long and thirty-five miles in maximum width, which has been hoisted to nine thousand three hundred feet above, sea level. On the east it descends four thousand feet by a monoclinal fold, which passes into a fault towards the nor

dstone and other stratified rocks formed from the waste of those long-vanished mountain ranges. Remnants of sandstone occur in places on the north of the great fault, and are here seen to rest on the worn and fairly even surface of the crystallines. We may infer that these ancient mountains were reduced along their margins to low plains,

w, a slice inclosed between two fissures may sink below the level of the crust blocks on e

hundred feet below that level in parts of the Dead Sea. South of the Dead Sea the floor of the trough rises somewhat above sea level, and in the Gulf of Akabah again sinks below it. This uneven floor could be accounted for either by the profound warping of a valley of erosion or by the unequal depression of the floor of a rift valley. But that the trough is a true valley of fracture is proved by the fact th

ft valley at the time of its formation to ent

e down- faulting of the blocks about one which is relatively stationary, mountains known as block mountains are produc

arrow, tilted crust blocks intercepted between the fissures give rise to the numerous north-south ranges of the region. Some of the tilted blocks, as those of southern Oregon, are as yet but moderately carved by erosion, and shallow lakes lie on the waste that has been washed into the depr

ure of the crust. This crude view has long since been set aside. A map of the plateaus of northern Arizona shows how independent of the immense faults of the region is the course of t

p-sunk valleys and lofty mountain ranges, and faults whose throw is to be measured in thousands of feet, are slow and gradual. They a

shafts by shifting the upper portion across the lower. Along one of the faults of this region it is estimated that there has been a movement of at least four hundred feet since the Glacial epoch. More conspicuous are the instances of active faulting by means of sudden slips. In 1891 there

ada Mountains. In the Owens valley, California, the throw amounted to twenty-five feet in places, with a horizontal movement

other: folds pass into faults; in a deformed region certain rocks have bent, while others under the same strain, but under different conditions of plasticity and load, have broken; folded mountains have been worn to their roots, and the peneplains to which they have

locks have moved, by the position of the two parts of some well-defined layer of limest

ade of 15 degrees, the ori

e original fault scarp worn away, showing cliff

degrees, showing cliffs due to harde

a hade of 80 degrees, w

of coal AB (Fig. 193). At B it is found broken by a fault f which hades tow

is pierced twice at different levels because of a fault. Dr

esistant stratum dislocated by a fault. Is the fault a STRIKE FAULT, i.e. one running parallel w

ght of the block. Is the fault a strike or a dip fault? Draw a second diagram showing the same block after denudation has worn it down below the center o

tions both of A and of B, Figure 196, after deform

e erosion of the valleys on the right-hand side of the mountain? With the deposition of the sediments? Do you find any remnants of the original surface baf produced by the dislocation? From the

long a time elapsed between the formation of the two faults as measured in the w

e draw a similar section of strata with a dip of 30 degrees outcropping along a horizontal line normal to the strike one thousand f

NFOR

on surface which thus parts older from younger strata is known as an UNCONFORMITY, and the strata above it are said to be UNCONFORMABLE with the rocks below, or to rest unconformably upon them. An unconformity thus records movements of the crust and a consequent break in the deposition of the strata. It denotes a period of land erosion of greater or less length, which may some

osion, depression, the deposit of b; and finally the uplift which has brought the rocks to open air and permitted the dissection by which the unconformity is revealed. From this section infer that during early Silurian times the area was sea, and thick sea muds were laid

the crust. In Figure 203, for example, the deformation which upfolded the range of mountains took place after the deposit of the series of strata a

ut their flanks have been infolded by later crumplings with the original mountain mass, and have been repeatedly crushed, inverted, faulted, intruded with igneous rocks, and denuded. The structure of great mountain ranges thus b

of the canyon are among the most ancient known, and are overlain unconformably by a mass of tilted coarse marine sandstones b, whose total thickness is not seen in the diagram and measures tw

ief after their deformation? To which surface were they first worn down, mm' or nm? Describe and account for the surface mm'. How does it differ from the su

w since the earliest geological ages and recent

of these sediments draw inferences as to the land mass from which they were derived. Was it rising

sandstones were deposited upon it? When was it tilted? Draw a diagram showing the atti

hat has made the surface nn' so even? How does it come to cross the hard crystalline rocks a and the weaker sandstones b at the same impartial level? How did the sediments of c come to be laid upon it? Give now the entire history recorded in the section, an