results from the billowy motion peculiar to the earthquake. This is not
simply a violent horizontal motion, but an equally violent vertical one.
It is a series of waves,--a succession of alternate elevations and
depressions of the solid crust. The height of these waves can only be
judged of by their effects; but it is difficult to account for some of
these effects, without supposing the waves to have been several yards in
height, and their velocity, in the few instances in which the time has
been accurately determined, was twenty miles a minute.
That such earthquake waves actually exist there can be no doubt. During
the earthquake in Calabria, in 1783, the flagstones in many of the towns
were lifted from their places and thrown down inverted, and trees bent
so that their tops touched the ground. During the great earthquake in
Chili, in 1835, the walls of houses, which were parallel to the line of
oscillation, were thrown down, while those that were at right angles to
it, though greatly fractured, were often left standing. Wherever careful
observations have been made, during and after severe earthquakes,
analogous facts have been noticed. Persons are generally affected with
sea-sickness. The sea is violently agitated. It often retires to an
unusual distance, and then returns upon the shore with most destructive
waves. Incredible, therefore, as it may seem, that the solid crust of
the earth should be thrown into such wave-like undulations, the fact is
well established.
With a velocity of twenty miles an hour, the successive waves may be
some miles apart, and yet be sufficient to account for all the
phenomena. It is evident, therefore, that the curvature of the wave
will be very slight, and yet enough to break into fragments all the
rocks thus curved. During the earthquake in Chili, before referred to,
"the ground was fissured, in many parts, in north and south lines. Some
of the fissures near the cliffs were a yard wide. Many enormous masses
had fallen on the beach. The effect of the vibrations on the hard
primary slates was still more curious. The superficial parts of some
narrow ridges were as completely shivered as if they had been blasted by
gunpowder." Similar phenomena seem everywhere to be exhibited by
earthquakes.
It may be presumed that almost all parts of the earth have, at different
periods, been subject to these earthquake waves. Accordingly, we find
that the crust of the earth is nowhere in an entire state, but is
divided by irregular lines into comparatively small fragments. By this
means, the deep fissures produced by fractures opening upwards would be
filled with fragments of rock shattered from the uplifted edges. In this
way the boulder masses were originally loosened from their parent beds,
and exposed to the action of ice, or any other transporting agencies. In
the same way the rocky bed of the ocean is, to a considerable depth,
reduced to a disintegrated mass. In this condition it will be rapidly
removed by marine currents, more or less broken, worn and comminuted, by
the movement, and deposited elsewhere. The materials have thus been
furnished for a very large proportion of the sedimentary rocks, and
especially of those which are composed of distinct fragments of other
rocks. By this means, also, wherever the rock formations come to the
surface, they are so broken that limestone, sandstone or granite,
suitable for architectural purposes, is seldom found, except at
considerable depths. This fragmentary condition of the surface rock is
such as exposes it to be acted upon readily by any powerfully abrading
causes, or to be more rapidly disintegrated by atmospheric and aqueous
causes.
4. We have already assumed that one principal division of rocks--the
unstratified--is of igneous origin. We have the proof of actual
observation, that lavas, and the accompanying _tufas_ and _grits_, are
volcanic products. The peculiarities of these products, in situation,
structure, and form, and in the imbedded minerals, are so great, that
whenever we find these peculiarities in the rocks of a country not now
volcanic, we still regard these rocks as of volcanic origin. We thus
have lavas, as well as stratified rocks, of different ages. There has
probably been no time in the earth's history when they have not been
forming.
The _trappean rocks_ are also of igneous origin. It is evident, from
their occurring in the form of dikes, that they have been in a melted
state. As they rest upon rocks of a sedimentary origin, they must have
been thrown up by volcanic forces. Yet they differ from ordinary lavas.
They are not vesicular in their structure, are more crystalline, and
there is in no case evidence that they have flowed from craters. If we
regard them as the lavas of submarine volcanoes, we shall have
conditions which will account for all their peculiarities. At a certain
depth the pressure of the water would be sufficient to prevent the
formation and escape of vapor, and therefore the lavas thus ejected
would not be vesicular. As the rapid cooling of lavas depends, in a
great degree, upon the escape of watery vapor, submarine lavas would
cool slowly, in consequence of the pressure. The liquidity depending in
part upon the retention of the heat, and in part upon the retention of
the aqueous vapor, they would consequently remain in a liquid state much
longer than the lavas of sub-aërial volcanoes. They would therefore take
a more highly crystalline form. All the loose materials thrown out
during the eruption would be removed by oceanic currents, and hence no
cone would be built up around the orifice of eruption. We may therefore
regard the trappean rocks as the lavas of submarine volcanoes. The
present volcanoes of this kind are necessarily producing the same kind
of rocks, though there will be no other proof that they exist, except
the existence of the volcano, till the bed of the sea becomes dry land.
The _granitic rocks_ are also the product of igneous causes. Granite is
the most abundant of these crystalline rocks; and the others, such as
crystalline limestone, are so intimately associated with granite that
they must have had the same origin. Granite is everywhere found to send
off dikes into the overlying rocks, and must therefore have been in a
state of fusion; that is, it must have existed as lava beneath the
surface. It is obvious that fluid lava always exists in great quantity
beneath areas of energetic volcanic activity.
Portions of this lava must in succession take the solid form. Wherever
the surface is elevated along a line of fracture, the lava which is
accumulated beneath rises above the level of the general reservoir of
lava, and will therefore part with its heat more rapidly. On cooling, it
becomes the granitic nucleus of the mountain. We ought also to suppose
that, by the extremely slow process of the transmission of heat to the
surface, the crust of the earth is everywhere increasing in thickness;
that is, the upper portion of the great lava mass is solidifying.
Sir James Hall has shown, by experiment, that earthy substances, reduced
to a state of fusion, become more highly crystalline as they are allowed
to cool more slowly, and are subjected to greater pressure. It is
difficult to conceive of these conditions existing in a higher degree
than they do in the cooling masses of lava below the stratified rocks.
These lavas must therefore take the highly crystalline form which the
granitic rocks are found to have.
All the igneous rocks have therefore existed as subterranean lavas. The
volcanic rocks have become vitreous, the granitic are crystalline, and
the trappean are intermediate in structure, coinciding with the
circumstances of pressure and rate of cooling under which they have
severally been formed.
5. _The Elevation of Mountains_ is another result of volcanic action.
The height of mountains depends, in part, upon general elevation. Yet
there is a different action, upon which the existence of the mountain,
as such, depends. Whenever igneous action becomes intense under any
portion of the earth's surface, and the elastic force greater than the
repressive, the solid crust will be broken and raised up, and along this
line of fracture the lava will rise above its general level elsewhere.
This lava, thus lifted out of the general mass, in time solidifies, and
forms the nucleus of a mountain. At successive periods the elevating
force is renewed, and adds somewhat to the mountain mass before
supplied. In this way the mountain is ultimately formed.
So far as observations have been made, the elevation of mountains seems
not to be gradual, but spasmodic; and yet the elevating force probably
accumulates constantly and uniformly. The repressing force consists of
the weight of the strata above, which may be regarded as constant, and
their strength, which is variable. When the elevating force becomes
greater than both the repressing forces, the crust is fractured. The
strength of the strata then becomes nothing, and the repressing force is
the weight alone. The elastic mass below at once expands, and the
requisite space is furnished by the uplifting of the strata along the
line of fracture. As the ridge of lava which fills this additional space
cools, it recloses, in part, the original fracture, and the repressing
force again consists of the two elements,--weight and strength. There
will therefore be no further elevation till the elevating force is again
superior to these two forces. Thus the elevating force, though it may
accumulate at a uniform rate, will manifest itself only at considerable
intervals.
As the accumulation of lava along the line of fracture is the cause of
the upheaval, every mountain must have a central granitic axis.
Sometimes this granitic mass is pushed up through the fissure, as in the
case of Mont Blanc. At other times, the stratified rock, which formed
the original surface, is carried up so as to form the surface rock
nearly to the top. In either case, the strata are lifted along the line
of fracture, and left in an _inclined position_. In this position the
older rocks are always found, wherever there has been any considerable
amount of igneous disturbance.
In some instances, the additional space required by the expansion of the
igneous mass below is furnished, not by the uplifting of the strata, but
by their compression into folds between two lines of upheaval. The
igneous rock is elevated but little above the stratified through which
it had burst; but the stratified rocks have taken the undulatory form,
and the widening of the igneous mass along the lines of fracture has
compressed the undulations, until the planes of _the strata have become
vertical_. Fig. 82 will give an idea of the successive changes by which
the vertical position of the strata has been produced.
[Illustration: Fig. 82.]
The force by which mountains are elevated being the elasticity of the
vapor diffused through the subjacent lava, it may happen, if the lava
have a high degree of fluidity, that this vapor will collect in large
masses, and rise as far as the lava is in a fluid state. The irregular
flow of lava from craters during an eruption is undoubtedly due to the
rapid ascent of such steam bubbles through the lava. Such an
accumulation of vapor under a mountain mass, if it cannot escape, would
support it as long as the temperature remained unchanged. But, upon a
reduction of temperature, the mass which had been upheaved by it would
be unsupported, and liable at any time to sink. Instances of
_subsidence_ on a comparatively small scale will admit of explanation in
this way. Papandayang, one of the loftiest volcanic mountains of Java,
sunk down four thousand feet in the year 1772. The area engulfed was
sixteen miles long and six broad. The crater of Kilauea, in one of the
Sandwich Islands, was evidently formed in this way. It is situated on
the side of a mountain, and consists of a chasm eight miles in
circumference and a thousand feet in depth. Liquid lava can always be
seen boiling in the small craters at the bottom; and at times it rises
so as to overflow them, and fill the chasm to within four hundred feet
of the top, when lateral subterranean passages are opened, by which it
is discharged. The same explanation--a depression of the central
portion--may be given of the formation of the large craters in the
Canary and Grecian islands. It is also probable that Lake Avernus and
others, in Italy, and some in Germany, have had a similar origin.
The subsidence of Papandayang is of importance as a historical fact; and
it is not at all unreasonable to suppose that larger chasms of great
depth were also sudden subsidences of a similar character. Lake Superior
has a depth considerably greater than the elevation of its surface above
the level of the sea. The bottom of the Dead Sea is two thousand six
hundred feet below the surface of the Mediterranean. And at one place in
the Atlantic Ocean a sounding was attempted with more than six miles of
line, without reaching bottom. These sunken areas, however, though of
great extent, occupy only an insignificant portion of the entire surface
of the earth.
6. _The Elevation of Continents._--The causes of change of level which
have been given will not explain those _slow vertical movements_ which
are now taking place in Greenland and the north of Europe, or those by
which the present continents have been elevated and the bed of the sea
depressed. Any cause which will account for these movements must be one
operating for long periods, under large areas, and with great
uniformity.
The cause which fulfils all these conditions most satisfactorily is a
_variation of temperature_ in the mass of rock underlying the portion of
the surface whose level is changing. It has before been shown that the
temperature increases as we descend below the surface; but there is also
reason to suppose that it undergoes great variations. The volcanic grits
interstratified with the silurian rocks of England show that at the
silurian period volcanic fires were active below that portion of the
surface. When the early fossiliferous rocks of this country were
deposited, the Alleghany Mountains had not been elevated; but before the
tertiary period they had taken nearly their present form. Some portion
of the intermediate period was therefore one of volcanic upheaval. The
trappean rocks are also evidence of intense volcanic action existing
here. France, during the tertiary period, was a highly volcanic country;
but all volcanic activity has now subsided. The Andes have been mostly
elevated since the tertiary period, and are still rising. It is evident,
then, that at different periods volcanic heat may vary from its highest
to its least degree of activity, below any portion of the earth's
surface.
This variation of temperature must be followed by variation of volume of
the earth's crust; that is, it _must produce expansion or contraction_.
Experiments have been made, under the direction of the United States
government, to determine the expansion of the several kinds of rock used
in our public works. It was found that granite expands nearly one two
hundred thousandth of its length for every degree of increased
temperature, limestone somewhat more than that, and sandstone about
twice as much. Taking the expansion of the granite as the basis of
calculation, and supposing the crust for a hundred miles in thickness to
be undergoing change of temperature, there would be a resulting
difference of level exceeding two and a half feet for each degree of
change in temperature, or more than two thousand five hundred feet for a
change of one thousand degrees.
This calculation is made upon the supposition that the law of expansion
is the same for all temperatures, and that no new conditions are
introduced at high temperatures by the presence of aqueous particles. We
know, however, that solids expand more rapidly at high temperatures than
at low, and the elasticity of aqueous vapor at high temperatures must
increase the rate of expansion of the rock through which it is diffused.
Although we are not able to introduce, numerically, the effect of these
two circumstances, yet it is obvious that they must be considerable.
The mean elevation of land above the level of the sea is about nine
hundred feet, the mountain masses above that level not being included;
and the estimated mean depth of the ocean, not including its chasms,
does not exceed two thousand six hundred feet. The _total elevation of
the continental masses_, for which it is necessary to account, does not
therefore exceed three thousand five hundred feet. This amount of
vertical movement may evidently be produced by the expansion and
contraction resulting from changes of temperature.
These changes of level must, however, be very gradual. Any diminution of
temperature must result from the transfer of heat to the surface; and
the conducting power of rocks is very imperfect. The lava in a crater is
often so cooled on the surface that it can be walked on, while but a few
feet below it is still liquid. Lava currents continue in gradual motion
long after the surface is nearly cold. This was the case with one of the
currents from Ætna for more than nine months after its eruption, and
with another for ten years. Humboldt visited Jorullo forty years after
it was thrown up, when the lava around the mountain was still in a
heated state, the temperature in the fissures being on the decrease from
year to year; but twenty years after its ejection the heat was still
sufficient to light a cigar at the depth of a few inches. If so long a
period is insufficient to solidify a comparatively small quantity of
melted rock when the circumstances for cooling are most favorable, we
may well suppose that centuries would be required to abstract sufficient
heat from the earth's crust to produce any material change in the areas
of continents.
If this account of the elevation and subsidence of continents is
correct, it would seem that they ought to be constantly undergoing
change of level. And their _apparent stability_ may be regarded as an
objection to it. If in any place there is absolutely no vertical
movement, then those conditions must exist in which, for the time being,
there is no change of temperature.
But it is doubtful whether there ever is absolute stability of any
portion of the surface for long periods of time. Of the minor vertical
movements of the interior of continents, there can, from the nature of
the case, be no evidence whatever. Changes of level, where they are
known to be taking place, are so slow, that they are hardly perceptible
in the period of a human life. Such changes had been going on for
centuries in Sweden before they were suspected. As accurate observations
have increased in number, and historical records become available, it is
becoming known that a very large amount of the seaboard is undergoing
change of level. It becomes probable, then, that these extremely slow
changes of level are constantly and everywhere taking place.
That portion of the crust of the earth constituting the present
continents, being further removed from the centre, would part with its
heat more rapidly, and receive heat from the central mass more slowly,
than that portion which at present constitutes the bed of the sea. The
continents are therefore in a situation to undergo contraction and
depression, and the bed of the sea is most favorably situated for
rising. If the distribution of water through the mass has any influence
in promoting its expansion, then the bed of the sea would receive this
supply most abundantly, and the continents the least so. We see, then,
in nature, those provisions for an alteration of level, which, from the
character of the several rock formations, we know to have taken place.
When any portion of the earth's surface is covered with the sea, the
conditions exist which will at length elevate it. When it becomes dry
land, the conditions exist which will in time depress it below the level
of the ocean. Hence, those impressions in regard to the land, as stable
beyond the possibility of change, we ought to abandon; and those
vertical movements, which, when proved, we are accustomed to regard as
extraordinary, we shall, at length, consider as only particular
instances of one of the most general laws of nature.
7. _Variations of Climate._--The only sources of heat by which climate
can be affected are the sun and the heated interior of the earth.
If the former melted condition of the entire mass of the earth be
assumed, the temperature of the surface must have been increased, by
conduction of heat from within, for long periods after the superficial
stratum had become solid. It is, however, susceptible of proof, that the
present climates are not sensibly affected by interior heat, though at a
little more than a mile below the surface the temperature is equal to
that of boiling water. At any time, therefore, after the waters had
become condensed, collected into oceans, and become sufficiently cool to
support the animal life of which the remains are now found, it is not
probable that the climate was, to any considerable extent, influenced by
the heat conducted from the interior.
Still, there have been great changes of climate since those early
organic forms existed; and, since we have no ground for supposing that
the temperature of the sun's rays has suffered any reduction, we have to
inquire whether the means of retaining the heat from the sun could at
any time have been different. _The relative position of land and water_
depends, as we have seen, upon igneous causes, and has been very
different at different times. We shall find that climate must have been
greatly modified by these changes; for the land radiates and absorbs
heat freely, and water possesses this power in a very low degree.
Let us suppose the zone comprised between the tropics to be occupied by
land, and the portions without these limits to be covered with water.
Under these conditions, the land, having a nearly vertical sun the
whole time, would accumulate heat to a degree scarcely compatible with
the existence of animal life. This is sufficiently proved by the
oppressive tropical climates of the present time, influenced as they are
by polar lands and contiguous seas.
Under the same conditions, the sea would be heated by contact with the
land, and the heat would be distributed by marine currents to the polar
regions. But the water thus distributed would not part with its heat,
because it has but little radiating power, and nowhere comes in contact
with polar land. It follows, then, that both land and water would be
subjected to a very high temperature.
But, if we suppose the land confined to the polar regions, and the sea
to the equatorial, the opposite results would follow. The equatorial sea
would absorb but a small proportion of the solar heat which would be
thrown upon it. The land would receive the sun's rays too obliquely to
receive much elevation of temperature, as the present polar climates
show. Hence, the temperature of the earth would differ but little from
that of the planetary spaces, which is fifty-eight degrees below zero, a
temperature too low to allow of any considerable development of organic
life.
These are the conclusions to which we are led by considering the
different powers of land and water to absorb and radiate heat, and we
shall find that the existing climates are in accordance with these
conclusions. America has a lower temperature than Europe in the same
latitudes. It has also a smaller proportion of land in the equatorial
regions, and a greater proportion in the north polar regions. The
eastern continent is colder in Asia than in Europe in the same
latitudes. It has also less equatorial and more polar land. The southern
is colder than the northern hemisphere at equal distances from the
equator. There is also less land near the equator on the south side, and
probably as much land around the south as the north pole.
Hence, we see that there may have been such a relation of land and water
as to account for all the variations of temperature which are known to
have existed. We cannot say that such actually has been the case. We can
tell, with some degree of accuracy, what portions of the present
continents were land at the several geological periods; but
three-fourths of the surface of the earth is covered with water, and of
the condition of this portion during those periods we have no means even
of conjecturing. We can only say, that, by the operation of known
causes, the relative position of land and water may have been such as to
produce the climates known to have existed at former periods of the
history of the earth.
INDEX.
Page
A.
Abundance of vegetable products of the coal period, 59
Accumulation of vegetable matter, 117
Actinolite, 15
Action of internal heat, 128
Action of waves, 107
in forming harbors, 108
Advantages of geological changes, 91
Ætna, 26, 73
Agate, 14
Age of rocks, doubtful--
from change of lithological character, 61
from distance, 61
from disturbance, 61
Alternation of coarse and fine material, 115
Aluminium, 12
Amethyst, 14
Amygdaloidal structure, 17
Ancient volcanic rocks, 29
Andes, granite veins in, 25
Angle of inclination, 71
Anoplotherium, 55
Anticlinal axis, 71
Aqueous causes, 103
Aqueo-glacial action, 120
Argillaceous schist, 20, 31
Arrangement of materials in the crust of the earth, 21
Artesian wells, 92
Asbestos, 15
Atmospheric causes, 95
Atolls, 81
Augite, 15
Auvergne, volcanic district of, 28
B.
Basalt, 18
Bed of the sea--
sunken areas in the, 142
why elevated, 145
Belemnites, 52
Breccia, 19
Brine springs--
in Silurian rocks, 35
in the carboniferous formation, 43
in the new red sandstone, 47
C.
Calamite, 47
Calcium, 13
Cambrian system, 32
Carbon, 11
Carbonate of lime, 15
Carbonate of magnesia, 19
Carbonic acid a cause of disintegration of rocks, 95
Carboniferous formation, 39
essential to national wealth, 43
extent of, 47
a prospective arrangement, 43
faults in, 41
not always disturbed by faults, 42
Carboniferous limestone, 39
sometimes becomes a coal-bearing rock, 42
fossils of the, 40
Carnelian, 14
Cause of internal heat, 128
Cause of stratification, 114
Caverns, 69
Cephalaspis, 39
Cephalopoda, 36
in oölite, 50
Chalcedony, 14
Chalk, 52
Changes of climate, 88
how produced, 146
Changes in the crust of the earth, 67
of temperature a disintegrating agent, 96
at the surface, 85
Chemical action, 97
in solids, 99
in crystallization, 97
Chlorine, 12
Chlorite, 15
Chlorite slate, 32
Classification of rocks, 21
Clay, 19
Clay slate, 20
Cleavage structure, 68, 98
Coal, 16
varieties of, 42
mode of quarrying, 42
origin of, 116
conversion of vegetable matter into, 117
now forming, 118
Coal measures, 131
fossils of the, 44
Coal plants, tropical character of, 88
Coal and iron associated, 43
Clouded marble, 33
Columnar structure, 18, 99
Compact limestone, 19
Concretionary formations, 99
Conglomerate, 19
of old red sandstone, 38
Connecticut valley--
one of denudation, 87
trap of, 30
Continents--
mean elevation of, 144
total elevation of, 144
elevated gradually, 144
why depressed, 145
Contorted strata, 72
Coral formation, 81, 102
extent of, 102
Coral reefs--
fringing, 81
barrier, 81
Coral rag, 49
Corals in silurian rocks, 35
Copper mines of Lake Superior, 47
Creation, a progressive work, 62
Cretaceous formation, 52
Cretaceous formation, fossils of, 52
geographical range of, 53
Crinoidea in Silurian rocks, 36
Crust of the earth, 16
expansion and contraction of, 143
D.
Delta deposits, 114
Denudation of igneous rocks, 85
Denudation of sedimentary rocks, 85
Denudation produced by earthquake waves, 136
Deposition of sediment, 113
Diluvium, 20
Dike, 69, 133
Divisional planes, 68, 98
Dolomite, 13, 19
Drift, 20
extent of, 53
connected with striated surface of the rocks, 54
connected with subsidence, 54, 126
E.
Earth in a state of change, 97
Earthquakes, 130
wave-like motion of, 136
Earthquake waves, rocks shivered by, 137
Effect of atmospheric agencies, 95
Electrical discharges, effect of, 96
Elementary substances, 11
Elevation and subsidence, 73
Elevation and subsidence several times repeated, 82
Elevation of mountains, 73
cause of, 139
spasmodic, 140
gradual, 75
Elevation of different mountains at different times, 75
Elevation of continents, 76
cause of, 142
Elevation of North America, 76
Elevation of the coast of Maine, 76
Elevation of Europe, 77
Elevation of South America, 78
Encrinites, 36, 50
F.
Fault, 41, 69
Felspar, 14
Filling up of lakes, 76
Fingal's cave, 30
Fissile structure, origin of, 68
Flint, 14
in chalk, 52, 99
Fluorine, 12
Folded axes, 61, 72
Formation of soils, 87
Fossils--
definition of, 57
how preserved, 57
mineralization of, 58
use of, 60
order in which animals appeared, shown by, 58
animal and vegetable, created together, 59
as a record of climate, 88, 101
Fossiliferous rocks, 32
classification of, 32
Fractures, 42, 68, 130
opening downward, 133
G.
Garnet, 16
Geological causes, how far uniform, 94
Geological causes, slow operation of, 95
Geological investigations aided by displacement of strata, 92
Geological periods, prolonged, 63
shown by amount of strata, 63
shown by duration of species, 64
shown by amount of organic matter, 64
shown by microscopic accumulations, 65
Geology and Revelation, 65
Giant's Causeway, 30
Glacial period, 90
Glacial theory, 124
Glaciers--
how formed, 120
cause of motion, 121
when they decrease, 122
earthy matter on them, 122
lateral moraines, 123
surfaces grooved by, 123
terminal moraines, 123
Gneiss, 18, 30
Gorge, 69
Graham Island, 131
Granite, 16
varieties of, 16
thickness of, 23
structure of, 23
formation of, 67
igneous origin of, 138
Granite veins, 24
in granite, 24
Granite of different ages, 25
Granitic axes of mountains, 24, 140
Greensand, 19, 52
Greenstone, 18, 30
Grooved surfaces of rock, 54, 87, 126
Gypsum, 15
in new red sandstone, 47
beds, how produced, 99
H.
Hall, Sir James, experiments, 135, 139
Heterocercal tails of fishes, 48
Homocercal tails, 48
Hornblende, 14
Hornblende slate, 20, 32
Hydrogen, 11
Hypersthene, 15
Hypersthene rock, 17, 25
I.
Icebergs--
how formed, 124
earthy materials in, 125
motion of, 125
size and number of, 125
effect in distributing drift, 126
grooving the surface, 126
Iceland spar, 19
Iceland, volcanic eruption in, 26
Ichthyosaurus, 50
Igneous causes, 127
Iguanodon, 51
Inclined position of strata produced by upheaval, 70, 140
Increase of temperature below the surface, 127
Iron, 12
J.
Jasper, 14
Jorullo, 131
K.
Kilauea, 26, 142
L.
Lakes, filling up of, 76, 113
Lava, 17, 25, 137
varieties of, 25
tertiary, 27
elastic vapors contained in, 130, 132
great quantity of modern, 26
Lead-bearing strata, 35, 40
Lepidodendron, 46
Lias, 19, 49
Limestone, 15, 19
as a primary rock, 25
metalliferous, 40
Local changes of climate, 90
M.
Magnesian limestone, 19, 47
Magnesium, 13
Mammoth, 56
Man--
recently created, 59
as an agent in producing geological changes, 101
impressions of the feet of, 59
skeleton of from Guadaloupe, 59
Manganese, 12
Marine currents, 108
Marine currents, cause of, 109
abrading power of, 111
Marl, 19
Mastodon, 55
Megatherium, 55
Metamorphic changes, 67
Metamorphic rocks, 30
amount of, 67
origin of, 134
order of superposition, 31
upper limit variable, 13
localities of, 32
Metallic ores, 92
Mica, 14
slate, 18, 31
Millstone grit, 40
fossils of, 41
becomes coal measures, 42
Mineral--
definition of, 13
Mineral veins, 69
formation of, 100
Modern formation, 57
why but little known, 57
fossils, 57
Moisture of the atmosphere a disintegrating agent, 96
Monte Nuovo, 26
Mount Loa, eruption of, 26
N.
Neocomian system, 52
New red sandstone, 47
fossils of, 48
ores of, 47
geographical range of, 49
Niagara Falls, how preserved, 106
Nitrogen, 11
Nummulite rock, 54
O.
Oceanic mountains, 75
Ocean level, nearly permanent, 74
Old red sandstone, 38
fossils of, 39
extent of, 39
Oölite, 19
Oölitic structure, 49
Oölite system, 49
calcareous, 49
fossils of, 50
localities of, 51
Opal, 14
Organic causes, 101
Orthoceras, 36, 40
Outcrop, 71
Oxide of iron, 16
Oxygen, 11
P.
Paleotherium, 55
Papandayang, 95, 142
Permian system, 47
Plesiosaurus, 51
Porphyritic structure, 17
Potassium, 12
Primary limestone, 19
Pterodactyle, 51
Pumice-stone, 17
Pyroxene, 15
Q.
Quartz, 14
rock, 32
R.
Raindrops, impressions of, 48
Raised beaches, 76
Ravine, 69
Recent elevation in Europe, 79
Recent formation, 57
Ripple marks, 48
Rivers--
beds of, raised, 113
continued into the sea, 112
abrading action of, 105
abrading action of promoted by foreign substances, 106
Rock crystal, 14
Rock salt, 16, 118
Rocks, denned, 16
Rose quartz, 14
S.
Saccharine limestone, 19, 32
Saliferous system, 47
Saline properties of the ocean, how obtained, 104
Salt beds--
where found, 118
how formed, 119
Sandstone, 19
Schorl, 16
Scoriæ, 17, 133
Sediment--
amount of in rivers, 107
deposition of, 113
sorted by rivers, 112
Selenite, 16
Serpentine, 15
a primary rock, 25
Shale, 20
Siberia, remains of elephants in, 89
Sigillaria, 45
Silicium, 12
Sinking of wharves, towns, &c., 79
Silurian system, 34
tabular arrangement of, 34
divisions of, 35
fossils of, 36
geographical range, 38
Slaty structure--
in the gold washings of Chili, 98
produced by electric currents, 98
Slope of mountains, 73
Soapstone, 15
Sodium, 13
Solidification of rocks, 68, 119
Soluble materials of rocks, 103
Solution of mineral substances--
promoted by heat, 104
promoted by an alkali, 104
promoted by carbonic acid, 105
Sources of the sedimentary materials, 103
Sources of the sediment of rivers, 97
Species--
disappearance of, 62
causes of the disappearance of, 62
Springs, 92
Stability of continents only apparent, 145
Statuary marble, 19
Stigmaria, 44
Strata--
horizontal, 70, 115
permeable and impermeable, 92
irregular, how produced, 115
Striated surfaces, 87
Submerged forests, 79
Subsidence of land, 79, 145
Subsidence of land in Greenland, 82
Subsidence and elevation in the Pacific, 80
Sulphate of lime, 15
Sulphur, 12
Sun-cracks, 48
Sunken areas, 142
Syenite, 17
Synclinal axes, 72
T.
Taconic system, 32
Talc, 15
Talcose slate, 20, 32
Temperature at great depths, 127
Temple of Jupiter Serapis, 83
Tertiary system, 53
age, how determined, 53
fossils, 54
divisions of, 53
geographical range, 56
Tilestones of the old red sandstone, 38
Trachyte, 18
Tracks in new red sandstone, 49
Transportation of sediment, 111
Trappean rocks, 17
localities of, 29
Tremolite, 15
Trias, 47
Trilobite, 37, 40
V.
Valley, 69
of elevation, 71, 75
of subsidence, 72
Valley of denudation, 87
Vein of segregation, 69
Verd-antique marble, 15
Volcanic rocks, 17, 25
of different ages, 26
in what states ejected, 25
Volcanic mountains, dimensions, 26
Volcanic activity--
regions of, 129
water essential to, 130
Volcanic cones--
formation of, 131
lateral, 132
Volcanic cinders, scoriæ, glass, 132
Volcanic action, effects of, 133
Volcanic origin--
of trappean rocks, 138
of granitic rocks, 138
Volcanoes--
number, 26
linear arrangement of, 131
near the sea, 129
new, 131
of the tertiary period, long active, 28
W.
Watt on fusion of basalt, 100
Waves, action of, 107
Wealden, 49
Wind a geological agent, 96
Z.
Zechstein, 47
QUESTIONS
TO
ELEMENTS OF GEOLOGY
QUESTIONS.
CHAPTER I.
SECTION I.
How many elementary substances are known?
In what combinations is oxygen found? What proportion of the earth's
crust consists of it?
In what combinations does hydrogen occur? Nitrogen? Carbon? Sulphur?
Chlorine? Fluorine? Iron? Manganese?
How does silicium occur? Aluminium? Potassium? Sodium? Calcium?
Magnesium?
How are these elementary substances classified? (Silicium, or silicon,
has but a doubtful claim to be regarded as metallic.)
SECTION II.
What is a simple mineral? How many are known?
What are the physical properties of quartz? How are the several
varieties distinguished?
What are the physical properties of felspar? Mica? Hornblende? How are
its varieties distinguished? Augite? Hypersthene? Talc? How are its
varieties distinguished? Serpentine? Carbonate of lime? Gypsum? Its
varieties?
What other minerals are mentioned?
SECTION III.
Define the crust of the earth. Rocks.
What are the unstratified rocks?
What is the structure of granite?
How are the varieties distinguished?
What is the porphyritic structure?
Describe hypersthene rock.
What are volcanic rocks? Lava? Scoriæ? Pumice-stone?
How is the vesicular structure produced?
What are volcanic breccias? Volcanic grits?
What is the composition of the trappean rocks?
What is the amygdaloidal structure?
What are the three varieties of trappean rocks, and how are they
distinguished?
[Illustration: Fig. 1.]
Name the stratified rocks. Describe gneiss. Mica slate. Sandstone.
Conglomerate. Greensand. Describe the varieties of limestone.
What is dolomite? Of what does clay consist? Clay slate? What
modifications does clay slate present? What is diluvium?
CHAPTER II.
SECTION I.
What is the primary division of rocks?
Upon what principle are the unstratified rocks divided?
Upon what principle are the stratified rocks divided?
Why are the non-fossiliferous called metamorphic rocks?
Name the four classes of rocks.
SECTION II.
What is the most abundant plutonic rock?
How is its thickness ascertained?
What is its amount?
Where is it found?
What is its ordinary structure?
What peculiarity of structure facilitates the cleavage of granite?
[Illustration: Fig. 2.]
The granitic masses are generally deep below the surface; in what other
position does granite appear?
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
In what classes of rocks are granite veins found?
Were they all produced at the same time?
How is this demonstrated?
What is the relative position of the older and newer granites?
What other plutonic rocks occur in considerable quantities?
SECTION III.
Of what do volcanic rocks consist?
In what states are they ejected?
What are the principal varieties of lava, and how are they
distinguished?
Why is the basaltic lava the last to be ejected?
How is the age of the volcanic rocks determined?
What are the three divisions of the volcanic rocks, as dependent upon
age?
What is the proportion of the volcanic to other rocks?
How many active volcanoes exist?
Describe the eruptions of Kilauea.
Describe the eruption in Iceland in 1783.
What are the dimensions of Mount Ætna, and how has it been produced?
How are the tertiary lavas known to be such?
Where have they been most studied?
What is the evidence that these rocks in France are volcanic?
Have these lavas been produced within the historic period?
[Illustration: Fig. 5.]
Were they produced at an early period in the earth's history?
Give the evidence that their activity was long-continued.
[Illustration: Fig. 6.]
What is the form of the earlier volcanic rocks?
What circumstances distinguish the trappean from other volcanic rocks?
What are some of the prominent localities of the trappean rocks?
How do they occur in the islands west of Scotland?
How in the valley of the Connecticut river?
SECTION IV.
What is the lowest metamorphic rock?
Describe it.
How does mica slate differ from gneiss?
Is it well distinguished from argillaceous slate?
What is the third rock in the metamorphic series?
Why is it difficult to determine the upper limit of this series?
Why do the principal rocks of this series occur in the order here given?
What other rocks may take the place of these principal rocks?
Where do the metamorphic rocks occur?
What is their thickness and amount?
SECTION V.
How many systems of fossiliferous rocks are there, and what are they?
What other system is provisionally introduced?
What is its position?
[Illustration: Fig. 7.]
Describe it.
What materials of value are obtained from this system?
[Illustration: Fig. 8.]
What fossils does it contain?
In what localities is it found?
What explanation, in reference to these rocks, is given by those who
deny that they constitute a distinct system?
[Illustration: Fig. 9.]
[Illustration: Fig. 10.]
In what respects does the State of New York present the best facilities
for studying the Silurian system?
Describe the Champlain division.
[Illustration: Fig. 11.]
The Ontario division.
The Helderberg division.
[Illustration: Fig. 12.]
[Illustration: Fig. 13.]
[Illustration: Fig. 14.]
[Illustration: Fig. 15.]
[Illustration: Fig. 16.]
Describe the Erie division.
What are the fossils of this system?
[Illustration: Fig. 17.]
Describe the Crinoidea.
The Cephalopoda, and the two forms.
[Illustration: Fig. 18.]
The Trilobite.
What higher forms of animal life existed during the silurian period?
The geographical range of the system?
Of what does the Old Red Sandstone consist?
Describe its three divisions.
What are its fossils?
[Illustration: Fig. 19.]
Describe the fishes of that period.
What was the peculiarity of the Pterichthys?
Of the Cephalaspis?
Where are the rocks of this system found?
How is the carboniferous system divided?
Describe the carboniferous limestone.
What ores are found in it?
Describe, its fossils.
Describe the millstone grit.
Of what do the coal measures consist?
How does the ironstone occur?
Describe the coal beds.
How is the continuity of the strata interrupted?
What variations from this general type occur in the formation?
[Illustration: Fig. 20.]
[Illustration: Fig. 21.]
Describe the several varieties of coal.
How is the coal quarried?
[Illustration: Fig. 22.]
What mineral springs occur in this formation?
To what uses is coal applied?
(The coal was deposited thousands of years ago, and has served no useful
purpose, that we know of, till very recently. Its formation was planned
and completed to meet a want which was not to be felt till the lapse of
many ages. It is a notable instance of the wisdom and forethought, as
well as of the benevolence, of God.) In what does this prospective
arrangement consist? What are the character and position of the fossils
of the coal measures?
[Illustration: Fig. 23.]
[Illustration: Fig. 24.]
[Illustration: Fig. 25.]
[Illustration: Fig. 26.]
What are the four most abundant forms?
Describe the Stigmaria. The Sigillaria. The Lepidodendron. The Calamite.
Where are the beds of coal found?
What is the fourth formation of rocks?
Into what two portions is it divided?
Of what does the Permian portion consist?
The Trias?
What minerals are found in this formation?
What springs?
[Illustration: Fig. 27.]
[Illustration: Fig. 28.]
[Illustration: Fig. 29.]
What fossils?
How are the fishes of the earlier and later portions distinguished?
What peculiarity of the red sandstones is mentioned?
By what kinds of animals were the tracks, which they contain, made?
Give localities of the new red sandstone.
What are the three divisions of the Oölitic system?
[Illustration: Fig. 30.]
[Illustration: Fig. 31.]
[Illustration: Fig. 32.]
Describe the Lias.
The Oölite.
The Wealden.
What are the general peculiarities of the system?
[Illustration: Fig. 33.]
What are the fossil animals of the system?
By which class of fossil animals is the system characterized?
[Illustration: Fig. 34.]
Describe the Ichthyosaurus. The Plesiosaurus. Pterodactyle. The
Iguanodon.
Where is the system developed?
What are the divisions of the Cretaceous system?
How are the layers of chalk separated?
What is the geological position of the Neocomian system, and the
greensand of this country?
What are the fossils of this system?
[Illustration: Fig. 35.]
[Illustration: Fig. 36.]
What the geographical range?
How are the tertiary deposits distinguished from the older formations?
Upon what principle is the tertiary system divided?
What are these divisions called, and what does each, name signify?
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
[Illustration: Fig. 40.]
[Illustration: Fig. 41.]
[Illustration: Fig. 42.]
[Illustration: Fig. 43.]
In what portion of the tertiary period was the drift deposited
What is the geographical range of the drift?
Of what does it consist?
What is the latest tertiary deposit?
What are the fossils of the tertiary system?
Describe the Paleotherium. The Anoplotherium. The Megatherium. The
Mastodon. The Mammoth.
What other animals belonged to this period?
Where are the tertiary deposits found?
What formations are regarded as recent?
What formations of this class are accessible?
What others are in progress?
What are the fossils of this formation?
SECTION VI.
What is a fossil?
In what ways are they preserved?
When is a fossil said to be mineralized?
Describe the process of mineralization.
How is it proved that the removal of the organic matter and substitution
of mineral particles are simultaneous?
Were animals created before vegetables?
How is this shown?
At what period was the vegetable growth the greatest?
What forms of animal life were most abundant during the earlier periods?
What vertebrated animals belonged to these periods?
What advance is made in the new red sandstone period?
During what period do the mammalia first appear in abundance?
During what geological period was man created?
How are the footprints and skeletons of human beings hi solid rocks
accounted for?
Why are not fossils distributed uniformly through all the formations,
and through all the parts of each formation?
In what does the importance of fossils consist?
How are the fresh-water and marine formations distinguished?
What circumstances render it difficult to identify rocks of the same age
in different localities?
How are formations identified?
Was the work of creation one of short duration?
What was the last work of creation of which we have any geological
evidence?
Why may we presume that no more species will be created?
Do all the animal and vegetable species which have been created still
exist?
[Illustration: Fig. 44.]
[Illustration: Fig. 45.]
[Illustration: Fig. 46.]
[Illustration: Fig. 47.]
What causes are operating to destroy species?
SECTION VII.
How long has it been since the creation of the earth?
How does the amount of stratified rock indicate the great antiquity of
the earth?
How does the stratification show the same thing?
What is the proof that the principal strata were deposited before the
creation of man, and how does this fact bear upon the question of the
antiquity of the earth?
Give the argument drawn from the successive creations and disappearance
of animal and vegetable species.
The argument drawn from the amount of organic matter in the stratified
rocks.
The argument from slow accumulation.
What is the general conclusion from these facts?
Why is this conclusion an important one?
What objection to it has been raised?
How is this objection answered?
What additional explanation is given?
CHAPTER III.
SECTION I.
What is the deepest geological change of which we have any knowledge?
What are the reasons for supposing that the lowest stratified rocks are
undergoing fusion?
Why are the lowest stratified rocks regarded as of mechanical origin?
What changes have they undergone?
SECTION II.
In what state were the stratified rocks deposited? What change have they
undergone in this respect? How is the fissile structure produced? How is
the cleavage structure produced?
[Illustration: Fig. 48.]
What is the third class of changes?
What do fractures at the surface become by the erosion of water? How are
caverns formed?
Describe a vein of segregation. A dike. A mineral vein. What is a fault?
Were the inclined strata thus deposited?
How is it proved that they have taken the inclined position since they
were deposited?
What is the direction of the dip?
What lines form the angle of inclination?
What is the outcrop of inclined strata? The strike?
[Illustration: Fig. 49.]
[Illustration: Fig. 50.]
[Illustration: Fig. 51.]
[Illustration: Fig. 52.]
[Illustration: Fig. 53.]
Describe an anticlinal axis. A synclinal axis. A valley of elevation. A
valley of subsidence.
[Illustration: Fig. 54.]
[Illustration: Fig. 55.]
[Illustration: Fig. 56.]
When are strata unconformable?
What other disturbances have taken place in the strata?
When did these various disturbances take place?
How is it known that there has been no period of universal disturbance?
SECTION III.
How is it known that the mountains have been covered by the ocean?
[Illustration: Fig. 57.]
[Illustration: Fig. 58.]
Were the granitic ridges thus covered?
Has the level of the sea been, to any considerable extent, fluctuating?
How, then, have the rocks, of which the mountain masses consist, been
covered by sea?
Give the evidence that different mountains were elevated at different
times.
Has the process of upheaval been sudden or gradual?
How are the mountain valleys, which have the direction of the mountain
ranges, been produced?
[Illustration: Fig. 59.]
How is the existence of submarine mountains shown?
What is the movement by which continents are elevated?
State the evidence of the elevation of continents from the existence of
elevated sea-beaches.
The evidence of the elevation of the coast of Maine.
The evidence of elevation from the existence of lakes.
From the geographical range of the older strata.
The evidence of the recent elevation of South America.
Of the rising of the north of Europe.
State the proof of subsidence from the occurrence of submerged forests.
Why are these changes but little observed?
[Illustration: Fig. 60.]
What are the grounds for asserting that a change of level is taking
place over a large area in the Pacific and Indian Oceans?
[Illustration: Fig. 61.]
[Illustration: Fig. 62.]
[Illustration: Fig. 63.]
[Illustration: Fig. 64.]
What is the present state of the coast of Greenland in this respect?
Have the changes of level of the same place always been in the same
direction?
Give the evidence of elevation and depression in South America. In
Italy.
[Illustration: Fig. 65.]
What general conclusion may we draw in respect to the stability of the
earth's surface?
To what extent can we ascertain the geography of past epochs?
What former relations of land and water are suggested as not improbable?
SECTION IV.
How can we estimate the denudation which the igneous rocks have
suffered?
How do faults indicate the denudation of the stratified rocks?
How do valleys indicate denudations?
Describe the instance in Scotland.
What is the evidence of denudation in the Connecticut valley? How are
valleys produced?
[Illustration: Fig. 66.]
What is the condition of the surface rock in the colder portions of the
temperate zones?
[Illustration: Fig. 67.]
[Illustration: Fig. 68.]
With what is the surface rock generally covered?
How are soils formed?
How may soils be improved?
What is necessary to render soils fertile?
SECTION V.
What means have we of judging of the climate of former periods?
What was the climate of the coal period?
What animal fossils indicate a former warm climate?
What evidence that Siberia once enjoyed a milder climate?
Do similar indications appear in the southern hemisphere?
When has the climate of the earth been most uniform?
Has the climate been growing gradually colder to the present time?
What is the evidence of a somewhat recent period of intense cold?
What recent local changes of climate are mentioned as having occurred?
SECTION VI.
State the general advantages of geological changes.
By what changes have the coal-beds and other stratified rocks become
accessible?
What advantage from these elevating forces in reference to the granitic
rocks?
[Illustration: Fig. 69.]
How do these changes affect our means of knowing the structure of the
earth?
Explain the origin of springs, wells, and artesian wells?
By what changes have the metallic ores become accessible?
In what light, then, are we to regard disturbances of geological
structure?
CHAPTER IV.
What is the object of the preceding chapters?
How can we arrive at a knowledge of the causes which have produced
geological phenomena?
Have geological causes always operated with the same intensity?
How are the means of forming correct geological theories increasing?
SECTION I.
How does oxygen become an agent in the disintegration of rocks?
How does carbonic acid operate in the disintegration of rocks?
What is the effect of moisture and rain?
What is the effect of variations of temperature?
What other atmospheric causes are mentioned?
How do these causes become important?
What are some of their effects?
SECTION II.
What are the changes which are to be referred to chemical agency?
Mention some of the disturbances which give rise to chemical changes.
What are the principal effects of chemical action?
How is the cleavage structure accounted for?
[Illustration: Fig. 70.]
[Illustration: Fig. 71.]
Mention instances which show that a cleavage may be established in a
body in a solid state.
In it a crystalline arrangement of the particles of the mass?
What other divisional planes exist in rocks?
Mention instances of concretionary formations.
Why may not these concretions have been deposited as nodules?
How have these concretions been formed?
Mention instances of segregation without the concretionary structure.
How was the segregation in these instances effected?
How is the columnar structure produced?
What is the origin of the mineral veins which are first mentioned?
How is it shown that other veins are not injected?
How were these veins formed?
What is the force by which these molecular changes have been effected?
SECTION III.
In what ways are geological changes produced by human agency?
Of what are the organic remains, in rocks, the record?
What rocks contain organic materials in large quantity?
What is the most abundant organic product?
Explain the mode of growth of corals.
Give instances of extensive coral reefs.
What is the total amount of surface covered by the coral reefs?
SECTION IV.
What degree of importance is attached to water as a geological agent?
What are the sources of the sediment which water deposits?
Why is not the formation of the sedimentary rocks capable of being
observed?
What is the first mode in which solid matter is taken up by water?
Why are the waters of the ocean saline?
What effect has the temperature of water in the solution of silex?
What effect has an alkaline condition of water?
[Illustration: Fig. 72.]
What rock is soluble in water charged with carbonic acid?
Give an instance of limestone formation from such solutions.
How do rivers furnish sediment for the stratified rocks?
What determines the position of rapids in rivers?
What is the effect of waterfalls on the abrading action of rivers?
What is the peculiarity of rock at Niagara which has prevented the fall
from becoming a succession of rapids?
What other circumstance increases the abrading action of rivers?
What is the principal source of the sediment which is transported by
rivers?
What is the annual amount of sediment furnished by the Kennebec? The
Merrimac? The Mississippi? The Ganges?
What is the general tendency of these abrading forces?
What is the effect of waves upon the coast, when it consists of
unsolidified materials?
Describe their effect upon rocky coasts.
How is the encroachment upon such coasts shown?
[Illustration: Fig. 73.]
What is the effect of waves of less power?
How are marine currents produced?
How are they increased by the evaporation of the torrid zone?
What are the most important marine currents?
Which class of currents have the greater depth?
Upon what does the power of deep currents depend?
How would the effect of these currents be increased by earthquakes?
Where will the effects of these currents be greatest?
Mention instances of these effects.
[Illustration: Fig. 74.]
What must be the effect of such currents as the Gulf-stream and
Mozambique channel?
Mention, generally, the effects of these currents.
Why does detrital matter remain suspended in the water of rivers?
How is the coarse and fine sediment separated?
Why do river currents extend some distance into the sea?
What effect does this have in distributing the sediment which the rivers
furnish?
Upon what does the transporting power of marine currents depend?
When a river enters a lake, why is its sediment deposited?
Describe the effect.
When is sediment deposited in the beds of rivers?
Describe the effects of this deposition.
Where is most of the sediment deposited?
Give the area of some delta deposits.
How do the deep-sea deposits now forming compare in extent with the
earlier formations?
State the several circumstances by which a succession of deposits would
be arranged in strata.
How are those differences produced upon which the separation into
independent formations depends?
Why are marine deposits nearly horizontal?
[Illustration: Fig. 75.]
How are the irregular stratifications produced?
What peculiarity in the fossils will distinguish the lacustrine and
marine deposits?
What peculiarity in reference to fossils will characterize the deep-sea
deposits?
How is coal shown to be of vegetable origin?
Why will the drift wood of the sea accumulate in particular localities,
and why will it sink?
Why will it become buried beneath earthy matter?
How is it known that wood thus buried will, at length, become lignite?
How is lignite converted into mineral coal?
What is the proof of it?
Have beds of coal been formed at other periods, besides the
carboniferous?
Is it probable that coal-beds are now forming?
How did the flora of the carboniferous period differ from the existing
flora?
[Illustration: Fig. 76.]
Are the alternations of the earthy and coal strata satisfactorily
explained?
In what portions of the geological series are the deposits of salt
found?
Where is saline matter principally stored?
Explain the conjectural formation of salt in the Mediterranean Sea.
What form do rocks take when deposited from a chemical solution?
How is sand or gravel solidified by the infiltration of mineral waters?
What is the effect of drying upon the solidification of rocks?
What is the effect of pressure?
What of heat?
SECTION V.
What is a glacier?
What is the extent of the glaciers of the Alps?
What change does the mass of snow in the higher valleys of the glacier
mountains undergo?
What is the source of supply to the glacier?
What is the cause of the motion of the glacier?
What is the usual annual motion?
Why will the glacier melt but little at its under side?
Where will the waste at the surface just equal the addition?
What circumstances vary the position of the terminus of the glacier?
[Illustration: Fig. 77.]
What, besides snow and ice, enters into the composition of a glacier?
How are these materials supplied?
How is a lateral moraine formed?
What effect has the motion of the glacier on the rocky surface over
which it passes?
What is the material by which this effect is produced?
How is the terminal moraine produced?
How may the moraines on the Jura Mountains be explained?
How has it been proposed to explain the striated surfaces of rocks found
in the north of Europe and America?
What is the objection to this extension of the glacier theory?
How does the ice accumulate along the coast in high latitudes to form
icebergs?
Why does it ultimately separate from the shore?
How does it become freighted with earthy matter?
In what direction do the icebergs float, and why?
What are the dimensions of an iceberg, estimated from the part that is
visible?
[Illustration: Fig. 78.]
Where does the mass of ice increase, and where diminish?
What will be the effect of its melting?
How is it supposed that icebergs may have striated the rocky surface?
What is probably the condition of the bed of the seas over which
icebergs now float?
Has the north of Europe and America been so depressed, during a period
comparatively recent, as to admit of this explanation of the drift
phenomena?
SECTION VI.
What is the condition of the interior of the earth with respect to heat?
How do the observations made in deep mines and wells prove this?
How far is the temperature influenced from the surface?
What is the general law of increment of temperature?
At what depth would most mineral substances be melted?
How is this conclusion confirmed?
What was probably the original state of the mass of the earth?
What other explanation may be given of this interior heat?
What is the elastic force upon which volcanic phenomena depend?
Upon what does the fluidity of lava depend?
Upon what does its porous structure, when cooled, depend?
Why are volcanoes situated near the sea?
Describe the principal lines of volcanic activity.
What are the forces tending to repress the elasticity of the mass below?
What will be the effect when the elastic is greater than the repressing
force?
What produces the phenomena of the earthquake?
What is a volcano?
Why are volcanoes generally arranged a linear direction?
Under what circumstances will a new volcano be formed?
What instances are cited?
How is a volcanic cone formed?
Why are lateral cones produced?
How are volcanic cinders formed? Scoriæ? Volcanic glass?
[Illustration: Fig. 79.]
Give instances of fractures as results of volcanic action. How are dikes
formed?
[Illustration: Fig. 80.]
By what agency have the changes in the metamorphic rocks been effected?
Give the instance of metamorphic action from intrusive granite in
Norway.
What instance is given as occurring in New Hampshire?
Give the experiment by which it is shown that these changes will result
from a high temperature.
[Illustration: Fig. 81.]
What must be the condition of the lowest stratified rocks in regard to
temperature?
Why is not the stratification destroyed?
What changes are produced by this high temperature?
Explain the connection of denudation and earthquake action.
What is the evidence that the surface of the earth is thrown into
undulations during earthquakes?
What is the velocity of these undulations?
Give the instance which occurred in Chili.
To what parts of the earth are these undulations limited?
What condition of the surface may be regarded as resulting from this
cause?
What is the class of rocks most obviously referable to volcanic agency?
How do the trap rocks differ from ordinary lavas?
Why are they not vesicular?
Why more crystalline?
Why were cones never formed?
What is the proof that the granitic rocks have once been in a melted
state?
Why does not the mass of melted rock below the surface retain
permanently its liquid form?
Why does it, on cooling, become more crystalline than lava?
State the process by which mountains are formed.
By what law does the elevating force accumulate?
Why, then, is the process of elevation spasmodic, and not constant?
How is the inclined position of strata produced?
How are strata brought into a vertical position over large areas?
Why do subsidences occasionally follow these movements of elevation?
Mention instances.
[Illustration: Fig. 82.]
What explanation is suggested of deep and extensive chasms?
What conditions must exist together, in the force by which continents
are produced?
What cause fulfils these conditions?
What is the proof that the temperature under given localities is
variable?
What will be the result of these variations?
What is the law of expansion of rocks, as obtained by experiment?
What amount of change of level may be thus accounted for?
What circumstances would probably increase this amount?
What amount of vertical movement must be accounted for?
Why must these changes of level be very slow?
Under what conditions would there be no change of level?
Is it probable that these conditions exist to any great extent?
Why, then, are not the changes of level observed?
Why is the bed of the sea most likely to experience the change of
elevation?
Why are the continents most favorably situated to undergo depression?
What are the sources of heat upon which climate depends?
Does the interior temperature sensibly affect the present climates?
What cause may be assigned for the changes of climate which are known to
have taken place?
What are the relations of land by which the highest temperature would be
produced?
How would this distribution of land affect the temperature of the waters
of the ocean?
What would result if the opposite relations of land and water existed?
What confirmation of these conclusions is drawn from the existing
climates of different parts of the earth?
Is there any reason to suppose that the relations of land and water
which would have produced a warmer climate in former times did not
exist?