* If we suppose that the whole of this Carbon was diffund Thro' the armoephere in the shapes of carbonic acid, prior To the creation of GEOLOGY. Organiz) krings, wushall

44

of containing less than

dee that the atingefcherewashed ca acid dial presen

bulk

The 1/1000!" part
would be liable to be blown or
away, were it not for
the grassy turf which covers them. Marine plants are ex-
tremely perishable, and exert no perceptible influence on the
earth's crust. Terrestrial plants are of a very different
character both in point of size, number, and material; and to
them are chiefly owing the vegetable deposits in all ages of the
world.

82. Trees and plants are annually carried down by rivers, and deposited along with the layers of sand and mud which have already been noticed. The rafts of the Mississippi are frequently several miles in length, and from 6 to 10 feet thick, being composed of trees, roots, and brushwood. All marshes and shallow waters give birth to innumerable aquatic plants, which grow and decay from year to year, till, in the course of centuries, their remains form thick accumulations of peat. Peat bogs, of many miles in surface, and from 4 to 20 feet in thickness, are frequent in Scotland, Ireland, and other countries, and contain trees and the remains of animals which once inhabited the country. Vegetable growth is greatly influenced by climate, being more prolific and gigantic in warm than in cold regions, and being also entirely different in character. While, therefore, peat is forming in the bogs of Ireland, the Mississippi is carrying down the pines of America, and the Ganges the palms, canes, and tree-ferns of the Indian jungle.

83. Of the vegetation of past eras we can only judge from the fossil remains found in the solid rocks; and, comparing that of the coal strata with what now exists, we are warranted in concluding that the earth has at certain times nourished a more luxuriant and gigantic vegetation. Indeed coal, as will hereafter be shown, is just as much a mass of altered plants and trees as peat is; and when the student is told of many beds of coal lying one above another, some of which are ten, fifteen, and twenty feet in thickness, he may readily conjecture what an immense mass of vegetation has been compressed into this one formation. The present formation of vegetable deposits, and the dependence of plants upon temperature and climate, are facts which it is necessary to bear in mind; otherwise it will be impossible to account for many appearances which are presented in the stratified crust of the globe.

84. Animal life is also an active agent in adding to the solid material of the globe. Generally speaking, the remains of animals are very perishable; hence, though their bones, teeth, and scaly coverings are numerously found as fossils, yet these form a mere fraction of the rocks in which they are imbedded. hare containid a quantity which it is not easy to estimate. but which, nas portals in the proportion of 3,4,5,6, 8 per cent. We and unsured by the experiments of M. Sh. di sure that Carbonic acids, far from being deirimonial to

vyctation is positivity favourable to its when plants one exhissed
to the lin's light. His highly preballe, difference, in the ten-
-stitution of the farmosphere may therefore, be regarded as one if
ORGANIC AGENCIES.
the causes influencing most powerfully the more active, & Bery
It is not in this light, therefore, that animal existence may be
said to be influential in modifying the crust of the earth; and
we may reckon of slight importance all the skeletons of the
larger animals which are deposited along with the mud, sand,
and gravel in the bottoms of existing lakes and seas. It is the
minutest forms of life which are mainly instrumental in form-
ing deposits of this class; such as the coral-insect, shell-fish,
and some crustaceous animals.

85. By the labours of the coral animalcule are formed those
extensive reefs of solid coral, or limestone, well known to the
navigators of the Pacific. These reefs rise in masses of various
shapes; sometimes as islets, at other times as circular belts
enclosing a lagoon or lake of salt water, but more frequently
in long abrupt ridges from 20 to 100 feet in thickness. The
great reef, which follows the line of the northern coast of
New Holland, is more than 1000 miles in length, in the course
of which there is one continued portion exceeding 350 miles,
without a break or passage through it. The animalcule is
scarcely so large as a pin's head; it is star-shaped, is of a soft
gelatinous structure, and myriads of them unite in their
operations to form a single branch of coral. By examin-
ing a piece of coral, its surface will be found dotted with
small star-like openings: each of these contains a single ani-
mal, and the space between them is covered by the membrane
above referred to. These animalcules have the power of
secreting limy matter from the waters of the ocean; they are
incessantly at action, and many of the reefs rise several feet
in the course of a few years. They do not commence their
labours at great depths, but attach their structures to rocks
from 60 to 100 feet below the surface; and thus the coral
reefs partake of the shape of the submarine ridges on which
they are founded. As their structures approach the surface,
the waves and currents of the ocean detach large pieces, which
are either drifted on the land, and form coral-beaches, or are
piled upon the surface of the growing reef, till it rises above
the sea.
When the animal reaches the surface it ceases its
operations, and the subsequent elevation into islands and dry
land is performed by the waves and tides, and by the elevat-
ing forces described in the preceding section.

86. Coral is almost entirely composed of pure limestone, and
is found in all stages of solidity, from an open porous mass,
with the live animal upon it, to a hard and compact limestone,
with scarcely a trace of its animal origin discernible. There
are many species of the coral animalcule, each variety rearing
its structure after a different form; and from this fact such
names have been given as tree coral, fan coral, organ-pipe
period

our

first organce, heres of • globe!! - Im. ad. BrongBalance. of Organic Nature... quoted by Dumas

in &

coral, brain coral, &c. Whatever be the shape, the substance formed, and their mode of action, is the same. They are found largely over the Southern Pacific, in the Indian Sea, the Red Sea, and other portions of the ocean. As at present, so in former ages of the world; and the student will hereafter find that many of the beds of limestone now deep in the crust of the globe, have been formed by the same kind of organic agency.

87. Shell-fish, like the coral animal, have the power of secreting limy matter from the ocean. In the former case, the secreted matter forms a covering or enclosure for the animal; in the latter, the animal is external, and the structure forms a mere groundwork for its operations, and a wider field for the increase of its kind. There is an immense variety of shellfish, but only a few varieties exist in great numbers, and it is by the agency of these that shell-beds are formed. The oyster, muscle, and cockle, are familiar examples; they live in great shoals or beds, covering from a few acres to many miles of the bottom of our seas and friths. Zoologists have found that most shell-fish live in shallow waters around the shores; and from this habit they are more liable to be covered by the material borne down by floods and rivers. In raised beaches, and in deltas, we actually find such accumulations of shells, sometimes several feet in thickness, and presenting the same appearance as when they lived and multiplied in the waters. If, then, extensive layers of shell-fish now exist, and if they are sometimes found imbedded in the alluvial matter of deltas and lakes, the student will be better enabled to account for the occurrence of thick masses of shells, or limestone wholly composed of shells, among the solid rocky

strata.

88. Although corals and shell-fish are the most important animal agents in adding to the material of the earth's crust, yet the exuviæ of other animals must not be overlooked; for, it is often from the occurrence of these alone, that we are enabled to infer as to the former conditions of the world. Thus, the remains of elephants, lions, and tigers, may be carried down by the waters of the Ganges, and deposited in its delta; those of the rhinoceros, hippopotamus, and ostrich, by the Niger; and those of the buffalo, elk, and reindeer, by the Mississippi; while the rivers of Britain convey no such remains to the ocean. The sand, mud, and gravel of all these deltas are very much alike; and if they should hereafter form rocks, the geologist could tell of the condition of the country in which they were formed only by the kind of fossils which these rocks contained.

89. The formation of coral-reefs and shell-beds is a gradual and ordinary operation; but shoals of other fishes may be entombed during violent storms, or during submarine volcanic eruptions, which are attended by noxious vapours, heat, and a suffocating agitation of the mud of the ocean. The modes in which vegetable and animal life may affect the crust of the globe are extremely complex and varied, but the above are the most obvious and important.

RECAPITULATION.

90. As detailed in the preceding section, the causes chiefly employed in modifying the structure of the globe may be divided into four classes-ATMOSPHERIC, AQUEOUS, IGNEOUS, and ORGANIC. The former two exert a degrading or wasting influence, and if not counterbalanced by other forces, would ultimately wear down the dry land to a level with the ocean; the latter exert an elevating or accumulating influence, and thus maintain that elevation and diversity of dry land essential to animal and vegetable life. To assist the memory, these agents may be briefly arranged as follows:

91. If, then, on the one hand, winds, frosts, rains, rivers, and waves be continually wasting down the solid crust, and depositing the debris in layers along the bottom of lakes and seas; and if, on the other, these layers be consolidated by pressure, by chemical processes, or by heat, and be then elevated into dry land by volcanoes and earthquakes, it must be obvious that the surface of the globe is in a state of perpetual change. These changes may be slow and imperceptible, or sudden and obvious; but in either way the appearances exhibited by the earth's surface must be very different now from what it was many thousand years ago. What was then

covered by the ocean, may now be dry land; and what was dry land, may have since been ocean, and may now be dry land again. These changes will be manifested by the kind of layers or rocks deposited at each successive period in the bottom of the sea; hence the geological history of the

world can only be discovered by the study of these strata. But as these strata were upheaved by volcanic agency, rocky masses of igneous or volcanic origin are frequently mingled with them; hence we find not only stratified rocks from deposition in water, but unstratified, the result of igneous fusion. Again, all strata originally deposited by water will contain more or less the remains of plants and animals which flourished during the period they were deposited; and the consideration of these petrifactions affords the geologist an idea of the kind of life which then peopled the surface of the earth, or inhabited the waters. These unstratified and stratified rocks, with the animal and vegetable remains which they contain, form the solid crust of the globe-the structure, composition, and formation of which it is the province of geology to consider.

EXPLANATORY NOTE.

SECRETION (Lat., secretus, separated or set aside). Both animals and vegetables are said to secrete certain substances. Coral, for example, is an animal secretion composed of lime, which the animalcule has the power of separating from the water of the ocean; resin and gum are vegetable secretions.

EXUVIE (Lat., cast clothes). In Zoology this term is applied to the external integuments of animals which are periodically shed or cast off, such as the skin of the snake, the crustaceous covering of the crab, &c. ; but in geology it is employed to designate fossil animal remains of whatever description.

MINERAL SUBSTANCES COMPOSING THE EARTH'S CRUST.

92. THE MINERAL OR ROCKY SUBSTANCES which compose the crust of the globe are exceedingly numerous and varied. They are commonly known by the name of rocks, minerals, metals, earths, and salts; but, geologically, are all comprehended under the general appellation rock. The individual minerals and elementary substances of which rocks are composed, come more appropriately under the sciences of mineralogy, metallurgy, and chemistry. Passing over the mere surface soil, and proceeding downwards to the greatest known depth, the solid crust may be said to be composed of two great classes of rocks-those arranged in layers, and those occurring in irregular masses; in other words, the STRATIFIED and UNSTRATIFIED. The stratified are those which have been formed from deposition in water; hence they are also known by the terms aqueous and sedimentary. The unstratified are those which have been formed by fire, and are also known by