becomes flexed by its own weight if a slab be supi^orted by only one
end. There is no doubt that if ice covered a lake to a thickness
of a dozen or more feet, and a slowly-accumulating pressure to a
sufficient amount could be brought to bear against one side of it,
the ice might be plicated over its surface as boldly and numerously
as the formations of the Appalachians.

Fractures have usually been produced in the course of the flexing
of the earth's crust ; a violent exertion of pressure under such cir-
cumstances would naturally produce them on a grand scale ; but
they are not an inevitable result of the process of plication. If the
rocks were moist, ā€” as has been the case during these upturnings, ā€”
the plication would take j)lace the more readily. If they were
heated also, and if by this means they were penetrated by super-
heated water or steam, the mobility of the particles would be still
greater, and they might even have, as observed on p. 708, a degree
of plasticity.

In general, the arenaceous and argillaceous beds which have been
folded were not at the time firmly consolidated, but derived their
consolidation from the heat which escaped from below during the
progress of the movement, and which was the cause of metamor-

47



722 DYNAMICAL GEOLOGY.

phism where the plications were most numerous. Limestone is
always in solid layers unless quite imj)ure.

4. Formation of valleys. ā€” The plication of the earth's crust pro-
duces alternating depressions and elevations, unless the folds are
pressed together into a close mass. The depressions are synclinal
valleys. The minor valleys of this kind are generally obliterated
by subsequent denudation ; and often even the summits of ridges,
under this latter agency, may consist of the rocks of a synclinal
axis. Besides synclinal valleys, there are often also monoclinal valleys
(p. 720). In addition, there are wider depressions lying between
distant ranges of elevations which were produced through a gentle
bending of the earth's crust (made uj) of plicated strata or not) ;
and these great valleys or depressions (like the Mississippi and
Connecticut valleys) maybe cdlXed geoclinal, the inclination on which
they depend being in the mass of the crust, and not in its strata.

5. Elevatioyi of mountains. ā€” The force engaged in producing the
great systems of plications over the earth is sufficient for the ele-
vation of mountains of all heights.

Mountain-chains are not made of igneous ejections, except occa-
sionally in some small portions.

They are not a result of the mere accumulation of a series of
sedimentary beds ; for, when the last layer of such a series is
laid down, the whole is still under water, and some force is re-
quired to raise them above the ocean, so as to entitle them to a
place among the earth's mountains. And generally there are
plications and metamorphism attending upon such elevation, due,
directly or indirectly, to the same powerful agency. While, then,
they consist mostly of sedimentary beds, altered or unaltered, they
have been raised to their places by an adequate force. Mountains
lifted by lateral pressure or tension within the crust would be sup-
ported as raised ; they would not be resting on a sea of unstable
vapors, but would have a solid basis, ā€” that by the movement of
which they were elevated.

6. Epochs of elevation separated hy long intervals. ā€” Mountain-chains
are not the work of the earlier periods of the globe alone, when, it
is believed, the earth's fires were most active, but of jDarticular
epochs in the course of all its ages ; and the loftiest of the globe
received much the larger part of their altitude after the close of
the Mesozoic era (p. 503). Plications and disturbances of strata,
and metamorphism, have also occurred at intervals in all ages, and
the two sets of phenomena were partly cotemporaneous.

The special epochs of great uplifts and foldings in eastern North
America have been shown to be ā€” (1) the later part (or close of the



CHANGES OF POSITION AND LEVEL. 723

Laurentian period) of the Azoic age ; (2) probably, the close of the
Lower Silurian, for part of the Green Mountains ; (3) the close of the
Palseozoic era, for the greater part of the Appalachian region,
between Labrador and Alabama. It appears, then, that the ten-
sion within the crust continued accumulating through long inter-
vals, before it reached that degree which was sufficient to bring on
an epoch of plication, uplift, and metamorphism. No one will
pretend to count the thousands of centuries between the Azoic era,
or the close of the Lower Silurian, and the close of the Palseozoic
era. In Europe, and probably in western America, the intervals
were less ; moreover, great uplifts, plications, and metamorphism
took place in these regions after the Palseozoic ; but in every case
the period during which tension was accumulating, preparatory for
the epoch of disturbance, was a long one ; for the epochs of the
elevation of mountains, even in Europe, are but few in number in
the whole course of past time.

7. Oscillations and minor uplifts. ā€” But during this period of accu-
mulating tension other and minor effects were apparent. Oscilla-
tions of the crust, causing changes of level, were going on unceas-
ingly, and they are yet in progress. The alternations of level
through the Palaeozoic in North America require no other explana-
tion. They were part of the indications of that living and growing
force which was to exhibit its grandest results after the Carboni-
ferous age had ended.

8. The water-line of the ocean liable to variations from oceanic subsidences.
ā€” As all parts of the earth, oceanic as well as continental, must
have participated in the changes of level, the water-level was ever
fluctuating like the land-level ; and hence it is not safe to measure
the latter always by the former, as is too commonly done. Many
of the apparent elevations may have been due to a deepening of
the oceanic basin, ā€” which has nearly three times the area of the
land (p. 10), ā€” and some of its apiDarent subsidences may have been
caused by an elevation of its bottom. It is probable that at least
1000 feet of the height of the continents ā€” the average height of
the land of the globe ā€” has arisen from the increase in the depth
of the ocean which took place during the successive Palseozoic,
Mesozoic, and Cenozoic eras.

9. Mountains small elevations compared with the extent of the globe. ā€” It
should be remembered, in this connection, that mountains arq
relatively to the size of the earth but little ridgelets on its surface.
A chain 10,000 feet high would stand up only one-tenth of an inch
on a globe 110 feet in circumference, or 35 feet in diameter, ā€” as
large as many a capacious house ; and one-hundredth of an inch would



724 DYNAMICAL GEOLOGY.

correspond on such a globe to the mean height of the continents.
If the Eocky Mountains on a globe of this size were given their
actual slope (equal on the east side to two or three feet in 5000
feet of length), they would be hardly recognizable. The liighest
peaks of the Appalachians would have a height of only one-sixteenth
of an inch, and the highest of the globe, of only three-tenths of an inch.
A change of level in the crust of 100 feet, which might, in the ear-
lier geological ages, have lifted a large part of a continent out of
the sea, would be represented by one-thousandth of an inch on the
same globe. The movements for such effects would relatively,
therefore, be exceedingly small. Considering the length of time
which must have elapsed since the crust of the globe was first formed
and through which contraction has been effecting its changes, and
the vastness of the force that would thus be produced in the crust
of a globe 25,000 miles in circumference, it may rather occasion
surprise that the highest summits stand only 30,000 feet above the
ocean's level, and less than 100,000 feet above the lowest depths of
the oceanic basin.

10. Courses of elevations in a region the same in different periods. ā€” The
elevations and strike of the rocks in northern New York, which
date from the Azoic age, ā€” the first emergence of the Green Moun-
tains, dating from the close of the Lower Silurian, ā€” the plications,
elevations, and metamorphism of the larger part of New England,
dating from the close of the Palaeozoic era, ā€” the formation of the
trap ridges of the Connecticut River valley, dating from the middle
Mesozoic era, ā€” have the same general direction. The Mesozoic
trap ridges and the plications and uplifts of the Appalachians in
Pennsylvania are also nearly parallel ; and the same is true of the
corresponding elevations in Virginia. These few examples are
sufficient to illustrate the principle stated.

11. Courses of elevations in a region not the same in different periods. ā€”
Europe contains many examples of this diversity of direction in
the same region : on page 533 a case of this kind in the Alps is
mentioned. It seems natural that the elevating force should vary
somewhat its direction with the progress of time, or, if remaining
the same, that it should encounter a difference of resistance which
should lead to a result unlike those in former periods.

12. Courses of elevations different in the same period. ā€” The Mesozoic
trap ridges and sandstone of Nova Scotia trend nearly northeast,
those of the Connecticut valley north-by-east, those of Pennsylvania
east-northeast, and those of Virginia northeast-by-north ; and yet
there is every reason to believe that they belong to the same period
of origin. The Appalachian chain varies much in directions



ORIGIN OF FRACTURES AND FAULTS. 725

southwest of New York ; and there is no evidence that this dififer-
ence is attributable to a difference of age.

While the plications of'the rocks of New England are in the
main nearly north-by-east in course, as shown by the strike of the
rocks, there is a region in southern Connecticut where the strike is
transverse to this direction, or parallel to Long Island and the
course of the Appalachians in Pennsylvania ; from which it may be
inferred that, cotemporaneously with the action of forces from the
eastward producing the prevailing plications of New England, a
force was also acting from the southward, or at right angles to its
southern coast. The complexity of directions in the White Moun-
tains may not be owing to a difference of age, but to the combined
action of forces from these two directions.

13. The theory here adopted not in the main hypothetical. ā€” In attri-
buting the plications of the earth's crust and the elevation of most
mountains to a lateral pushing movement or tension within the
crust, there is nothing that is hypothetical. The statement is the
expression simply of a fact. The conclusion that this tension is
due to the contraction of a cooling globe has not yet been fully
established. It is here adopted because no other that is at all
adequate has been presented. The cause must have been one
which would have produced an increasing amount of tension
through the passing periods, causing oscillations of the crust and
minor uplifts in the course of those long periods, and then a
great catastrophe, or an epoch of plications, metamorphism, and
grander uplifts, as a result of the great increase ; then another
slow increase and another catastrophe ; then others ; and a series
of similar but more or less independent catastrophes in distant
parts of the globe, raising, as late as the Tertiary period, many of
the earth's great mountain-chains, ā€” but one which should cause
only minor oscillations and uplifts in more recent times, since the
earth has now a degree of stability unusual in the past ages (p. 586).
And no cause answers to these demands, so far as known, but
the one mentioned, ā€” the contraction of a cooling globe.



2. FRACTURES, FAULTS, AND STRUCTURAL PECULIARITIES.

1. Fractures.

The following are some of the causes of fractures : ā€”
1. Drying through the heat of the sun, as in the formation of
cracks in mud or earth. ā€” Such cracks are usually but a few
inches deep, ā€” though in the soil of some prairies they occasionally



726 DYNAMICAL GEOLOGY.

extend to the depth of a yard or more, and are two or three inches
wide at top.

2. Baking effect from the heat of igneotis ejections. ā€” The adjoining
rock, especially if argillaceous, is often cracked into small columns
of five or six sides, or more, while at the same time hardened.

3. Loss of heat. ā€” Contraction from the loss of heat often pro-
duces a reticulation of vertical cracks, which are usually too narrow
even for the insertion of a knife-blade, unless the rock contain
considerable moisture. To this cause is to be attributed the divi-
sion of basalt or trap into columnar forms. (The size of the
columns is, however, dependent on concentric crystallization within
the mass, as explained on p. 98.)

4. A removal of the support of rocks hy undermining or other causes.

5. Pressure of a column of liquid rock, as in volcanoes.

6. Expansive force of vapors, especially when suddenly evolved, as in
volcanic regions.

The preceding are local causes of fracture. The last-mentioned
is an exception to this, according to those geologists who attribute
to vapors the elevation of mountains.

7. Tension withiji the earth'' s crust, ā€” the same agency which has
been explained on a preceding page as the true source of its plica-
tions and of the uplifting of mountains. Fractures have been
made, through this means, of all extents, from those intersecting
single layers, to profound, breaks reaching down to regions of inter-
nal fires. In the plication of the rocks, fractures are most likely
to be produced along the axes of the folds where the flexure is
greatest, ā€” those of the upward or anticlinal flexure opening up-
ward, and those of the downward or synclinal flexure opening
downward. If the latter extend through to the surface, they may
give exit to melted rock. In periods of metamorj)hism, the lateral
pressure causing the plications appears in general to have so closed
up the fractures made, that igneous ejections were rare. It is not
certain that any took place during the metamori^hism of the
Appalachian region ; though subsequently, after the rocks had
been stiffened by crystallization, the sinking of the geoclinal val-
leys occupied by the Mesozoic sandstone formation gave origin to
a great profusion of trap ejections (p. 430).

Direction of fractures. ā€” Fractures, in a region elevated by any
method of pressure, tend to form, as shown by Hopkins, in a direc-
tion at right angles to the line of greatest strain or pressure, and
also, in many cases, in a second direction transverse to this, making
two systems, a primary and a subordinate.



ORIGIN OF FRACTURES AND FAULTS. 727



2. Faults.

The several causes which may produce fractures through layers
or strata may also cause faults, and the profounder the fractures
the more extensive the faults that may result.

In general, there is a dropping down of the rock on one side of
the fracture by gravity ; and, where the fault is a sloping one (as
in fig. 978), the mass on the upper side usually slides down the
sloping plane. But, under the action of the pressure which has
plicated the earth's crust and lifted mountains, the reverse move-
ment has not been uncommon. The figure referred to is an exam-
ple ; and, by the upthrow, rocks of the Lower Silurian have been
carried up to the level of those of the Subcarboniferous. Similar
faults may occur along the axes of plications as described by the
Professors Rogers.

In faulting, there may be either a vertical or lateral slide, or an
oblique one. The inequality of the faulted parts of the veins repre-
sented in the figures on page 121 is accounted for on the ground
of a lateral or oblique slide.

The strata sometimes have a diflFerent dip on the sides of a fault (figs. 96,
97). This may arise in different ways.

1. The plane of fracture may not have the same slope in its different parts,
so that in either a vertical or lateral slide parts of unequal dip are brought
together.

2. The rocks may open at the fault, and the parts be adjusted together by
wear of the sides during the downthrow or uplift; and any portion of the
fissure remaining open may be filled by the rubbish thus produced.

3. Wedge-shaped plates, larger below, may separate and fall, leaving the
rock either side of the vacated space to be pressed together by the breaking-
force.

4. Fractures converging downward may separate wedge-shaped masses;
and the rock on one side or the other may fall off some degrees, while the wedge
settles into its new position by gravity, and is adjusted to it by friction in the
descent. If the rock to be faulted has a considerable dip transverse to the direc-
tion of the force, lateral slides would be of very common occurrence.

5. Plications and uplifts may take place,^after a profound fracture in the
rocks, on one side of the fracture, and not on the other. The abrupt transition
in many places between the plicated region of the Appalachians and the
slightly-tilted rocks of the country northwest of them can have no other ex-
planation.

3. Structural peculiarities: slaty cleavage and jointed

structure.

1. Slati/ cleavage. ā€” Slaty structure (exemplified in figs. 89 to 91, p.
101) has been shown, by Sharpe, Sorby, Darwin, and others, to be



728 DYNAMICAL GEOLOGY.

a result of the pressure in action during an uplift. The slates are
transverse to the force. The pressure tends to turn all pebbles or
particles so as to place their flatter side in this transverse position,
and any bubbles present become flattened out in the same way.
Again, in all such action, force taking place in oscillations would
tend to cause transverse lamination.

The laminated structure of ice has been explained by Tyndall
on the same principle (p. 675). Tyndall has proved by experiment
that slaty cleavage may be produced by means of pressure in white
wax, clay, and similar substances, when they are left free to expand
in directions transverse to the pressure.

When argillaceous and arenaceous layers alternate, the former
may receive the slaty structure and the latter not ; because the
arenaceous layers, if not too firmly solidified, can accommodate
themselves to the new condition by motion among their partially
adhering particles of sand ; if firmly consolidated, only joints will
be produced, though in some varieties they may be so numerous as
to occasion a coarsely laminated structure.

2. Joints. ā€” Joints are due to the same cause as slaty cleavage,
and may occur in slaty as well as other rocks.

Two systems at right angles to one another often result from
one action, but that in the line of movement is much the least
distinct.

Subjection to pressure from different directions, in different
periods, would produce different systems of joints, and, in slates,
sometimes a new direction of cleavage.

3. EARTHQUAKES.

1. General characteristics. ā€” Earthquakes are vibrations of the
earth's crust. The vibrations, begun at a line of fracture, or by
a sudden movement or shock of whatever kind, are conveyed in
the rocky crust, just as the sound of a scratch at one end of a log
is carried to the other. If the ear be placed near the ice in
winter, it will hear a crack Inade in it, although miles off*. If the
earth's crust suffer an abrupt fracture somewhere in its dej^ths
where tension has long been increasing and has finally forced a
relief, the vibration may move on through a hemisphere, and will
be almost regardless of the mountains on the surface.

Earthquakes are of two kinds : ā€”

(1.) A simple vibratory movement, without any permanent dis-
placement of the rocks.

(2.) A vibration accompanying an uplift.



EARTHQUAKES. 729

The latter is far the most violent, as the simple impulse of
vibration has an additional onward progression equivalent to the
uplift or displacement.

Besides these wave-movements, there are also, in most cases,
the very rapid wave which gives sound to the ear. The sound-
wave may be felt before the translation-wave, and may travel
farther. At the shock of St. Vincent, in 1812, sounds like thun-
der were heard over several thousand square miles in the Caraccas,
the plains of Calaboso, and on the banks of the Rio Apure. At
the Lima earthquake, in 1746, a subterranean noise, like a thun-
der-clap, was heard at Truxillo, where the earthquake did not
reach.

The rate will vary with the elasticity of the rock, and somewhat,
also, with the elevations over the surface.

2. Regular progression in earthquakes. ā€” Regular progression
may be a usual fact, although not generally observed. Professor
Rogers has shown that an earthquake, on the 4th of January,
1843, traversed the United States from its northwestern military
posts, beyond the Mississippi, to Georgia and South Carolina, along
an east-southeast course, Natchez lying on the southern border
and Iowa about the northern. The rate of travel ascertained was
thirty-two to thirty-four miles a minute.

3. Phenomena attending earthquakes. ā€” (1) Fractures of the
earth, sometimes of great extent; (2) subsidences or elevations
of extended regions, and draining of lakes ; (3) displacements of
loose rocks, and, where a mass overlies another and is not attached
to it by its precise centre, a partial revolution, resulting from an
onward impulse ; (4) destruction of life in the sea, on the same
principle that a blow on the ice of a pond will stun or kill the fish
in the waters beneath ; (5) production of forced waves in the ocean ;
(6) destruction of life on the land. Destructions of cities and of
human life have been too often recounted to need special illustra-
tion in this place.

The elevations that take place are sometimes spoken of as
effects of an earthquake, although not properly so. Vibration may
be attended by fractures and uplifts ; but these effects result from
the cause that produces the shaking.

Some of the elevations and subsidences that have attended
earthquakes are mentioned on page 588.

4. Earthquake oceanic waves have been alluded to on page
655. One or two additional examples of their effects may here be
added. In 1755, accompanying the Lisbon earthquake, the sea
came in in a wave 40 feet high in the Tagus, 60 at Cadiz, 18 on the



730 DYNAMICAL GEOLOGY.

shores of Madeira, 8 to 10 on the coast of Cornwall. One in 174G,
on the coast of Peru, deluged the sea-port Callao, and the city of
Lima seven miles from the coast, sunk 23 vessels, and carried a fri-
gate several miles inland. Two hundred shocks were experienced
in 24 hours. The ocean twice retreated, to rush in a lofty wave over
the land. The shock to a vessel from an earthquake wave is as if
it had received a heavy blow or had struck a rock.

According to Professor Bache, the oceanic waves, produced by
the great earthquake at Simoda (Japan) in 1854, crossed the Pacific,
and were registered, as to their number, intervals, and forms, on the
self-registering tide-gauges of the Coast Survey along the coast of
Oregon and California ; and from the data thus afforded he was
enabled to calculate the mean depth of the intervening ocean,
stated on page 12.

5. Causes of earthquakes. ā€” (1.) The tension and pressure hy xvhich
the great oscillations and plications of the earth's crust have been produced. ā€”
The effects of this tension have not yet wholly ceased. This is
probably the most general cause of earthquakes.

The uplifting of the formations, moreover, must have always left
the interior of the crust in a state of unstable equilibrium ; and
any incipient slide in the progress of time along an old fracture,
or between tilted beds, would be attended by an earthquake-shock.

All are familiar with the cracking sounds occurring at inter-
vals in a board floor of a house, and arising from change of tem-
perature, especially in a room in winter that is heated during
the day ; and with the more common sounds of similar character
from the jointed metallic pipe of a stove or furnace given out after
a fire is first made, or during its decline. In each case, there is a
strain or tension accumulating for a while from contraction or
expansion, which relieves itself, finally, by a movement or slip
at some point, though too slight a one to be perceived ; and the
action and effects are quite analogous to those connected with the
lighter kind of earthquakes.

(2.) Any cause of extensive fracture or movement, ā€” as the under-
mining of strata, the sudden evolution of vapors, etc.