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that which has been heaved up, an upheaval or hitch: where the strata at the slip lie at different angles, the slip is called a fault, as O. All such displacements of strata are known as dislocations, and they are much more frequent than the regu

lar disposition of rocks on the surface of the earth. When strata are bent in wavelike undulations, they are said to roll, as H; and the hollow or concave portions are termed troughs or basing, and the elevated portions ridges, as I and K.

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2. Unstratified Rocks. rocks are thrown up amidst the stratified, and assume different positions according to the manner of their upheaval. Where they

throw the strata into various angles, the rock upheaved is termed the disrupting mass, as V; at other times, they overlie the other rocks, and are then called overlying, as E; they are also interjected between the other strata, and are said to be interstratified, as F. Sometimes they intersect the other strata by masses like walls, which are called dikes, as G; and sometimes the disrupting mass breaks into branches, which are called veins, as N.

When a broad face of rock is exposed, and the different rocks shewn, as in a cliff on the sea-shore, a railway-cutting, or a quarry, such exhibitions of strata are called sections; and these may be delineated on paper. Sections of the underlying rocks may also be made, by examining the different rocks in a country, though no section be exposed in nature.

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Fig. 80.-Section of the Different Kinds of Strata.

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The Contents of the Rocks.

The contents of the rocks receive the general name of fossils, from the Latin fossus, dug, because they require generally to be dug out of the earth. Fossils may be divided into two great classes, animals and plants.

Fossil Animals. In the rocks, we discover specimens of every class included in the animal kingdom. We find corals of all kinds, and of the most beautiful structure, some branched like some of the corals of the present seas, others standing in masses on the very spots where they lived and died, their remains giving beauty to our finest marbles. We see star-like creatures of all kinds, either spreading abroad their arms or curled up at rest, as they may be seen any day during the ebb of tide. Shells of every form, size, and colour meet us at every step, as distinct as we now find them on the shore; and some formations, of vast thickness and extent, are formed entirely of the habitations of these little creatures. We may also gather crustaceans, such as the crab and the lobster, the minutest parts of their structure being perfectly preserved. We discover fishes of every kind and size, sometimes entire, as they fell to the bottom at death, or crushed and broken in the convulsions to which the rocks have been subjected. We can gather the hard scales, that defended them like armour; can form collections of their teeth, their fins, their jaws, and their eggs; and can construct them again as they swam about in the ancient seas. Insects, too, we can gather of every kind, and can see them as they flew about in the old forests and got entangled in the resin of the great old trees. Birds, too, are found, though not so plentifully as other creatures, as, from their manner of life, they were not so easily carried down by rivers, and deposited in the mud at their mouths. We find reptiles of immense size, crocodiles, and lizards, and flying dragons, with their terrible teeth, sweeping tails, and adamantine hides. We come upon beasts of every size, from little creatures that burrow in the ground, to gigantic deer, elephants, rhinoceroses, and mammoths; and may enter the very dens in which lived beasts of prey, and to which they bore their captured victims.

These creatures differ more or less from those that now inhabit the globe, but they are members of the same classes; and catalogues of them have been formed as of those of the present day. A visit to a museum in which fossils are exhibited would astonish you with the multitude, variety, and beauty of those fossil creatures, and especially with the wonderful preservation of organisms the most delicate and frail.

Fossil Plants.-But the vegetable kingdom is as fully represented in the rocks as the animal. We find trees of the most varied kinds, with their roots, stems, branches, leaves, flowers, and fruit. We can look

with wonder on the exquisite carving on the stems of mighty trunks, hundreds of feet in height, that once formed forests as dense and impenetrable as those of the Amazon. But more, we can behold the trees standing on the very places in which they grew and waved their great branches, and can trace their roots as they penetrate the soil beneath. We can also gather plants of all kinds-reeds, mosses, rushes, seaweeds, and beautiful ferns-preserved entire, and spread out on the rock as delicate and perfect as in the finest herbarium. These fossil plants have, like the fossil animals, been examined and classified by botanists, and we possess elaborate volumes on the botany of the remote ages when these plants grew, similar to those on the existing flora of our globe.

Traces of Natural Operations.-But the rocks bear traces of more than all this. On them, we can see the very dints of the raindrops of these bygone ages, and can calculate the direction and force of the showers that impressed them. We can walk over the rippled sands of the old seas, just as we can do over those we played on in childhood. We can also look on the footprints of primeval birds, as they stalked in the mud of their lake or river homes; or gaze with astonishment on the great footprints, as large as a man's hand, of the huge reptiles that waddled among the reeds by the great old rivers. We can look into the craters of extinct volcanoes, can follow the flow of the destructive lava, and can gather the ashes that once illuminated the darkened heavens. We can trace the sources of ancient rivers, and dig in the mud brought down from their mountain sources; can draw maps of the continents and seas as they existed thousands of ages past; can tell where great ocean-currents flowed, bearing huge icebergs, that grated the sea-bottom, and left their indelible traces on the granite and trap of our present hills; and can shew where mighty glaciers once existed in valleys famed for their beauty, where now the genial sun sheds its warmest rays. In short, every element in nature, whether of air, river, or ocean, has left its deepest traces on the solid crust of our wonderful globe.

Agencies in the Formation of Rocks.

We now proceed to inquire into the manner in which rocks have been formed. Any explanation must account for all the phenomena, equally of composition, structure, arrangement, and contents. We must, for instance, explain how some rocks are stratified, and others not; how some are horizontal, and others inclined; and how plants and animals have come to be embedded in them so far below the surface. Are there, therefore, any agencies engaged in the formation of rocks at the present time that produce effects the same in kind with these older masses? If we find

that such exist, we shall have a key by which to interpret the rock-formations of the past. Let us consider, therefore, the Rock-forming Agents. Volcanic Agents.-The most obvious rock-formers at present in action are volcanoes. From circular openings, called craters1 from their cup-like shape, at the summits of these mountains, there issue forth at certain times great streams of molten lava, boiling water, red-hot fragments of rock, mingled with flames, and smoke, and steam, amidst confused and thundering sounds, and the general convulsion of the surrounding country. These lava-streams, increased by ashes and other substances, are often of great thickness, sufficient to bury cities; as Vesuvius once did Herculaneum and Pompeii, and Etna did Catania at its base, where the river of lava gradually rose round the walls, finally drowning the city in its burning flood, after it had flowed twenty-four miles! Successive accumulations of such outbursts deposit immense masses of rock, in the course of ages, round the centre of eruption ; so great, indeed, that the larger portion of such mountains-and some of those in America are five miles in height-are formed of the successive accumulations of the crater itself. The molten lava assumes various appearances after it has lost its heat: under water, it remains hard and compact; in the open air, it becomes porous and cindery; and in certain cases, it assumes a columnar form. All around, lie light pumice-stone, slaglike masses, fine pulverised dust, and huge calcined blocks. Now, the Unstratified rocks resemble in every feature these volcanic discharges. We meet with the compact lava in our trap and greenstone; with the cinder, in the lighter porous rocks; with the ash, in our trap-tuffs; and with the columnar, in the basalt. In exposed sections, we see the very channel through which these masses burst and overflowed the strata above; and can trace the boundaries of the ancient molten streams in the cliffs and hills that everywhere vary the surface of the country. We can also see hardening and crystallising changes produced on the surrounding strata, wherever the heat of the erupted matter penetrated. We have therefore found the explanation of one great class of the rock-formations, the Unstratified, in the volcanoes scattered over the globe, that are at this moment depositing masses identical in kind with those that issued from the bowels of the earth in bygone ages. Such rocks, therefore, are termed igneous, from being produced by fire; volcanic, from having issued

from volcanoes; and eruptive, from being produced by eruptions.

Aqueous Agents.-Rivers, as they flow over their channels, gather accumulations of mud, sand, gravel, and animal and vegetable remains, according to the size of the stream and the character of the country through which they pass; and these they deposit at their mouths in seas or lakes. Sometimes the amount of débris thus deposited is

1 Greek, a cup.'

2 From Latin ignis, fire.

so great as to form large tracts of land, as at the protruding mouths of the Ganges, Nile, or Mississippi. Even in historic times, the land thus gained is of great extent. For example, at the mouth of the Po, a minor stream, the town Adria, which gave its name to the Adriatic Gulf from its extensive commerce in Roman times, in now nine miles from the sea! The mass of matter held in solution or borne along by the running water, sinks to the bottom when it reaches the sea, in a certain order. First the heavier masses are deposited, such as boulders and gravel; then, the sand; and last, the mud. Mingled with these are various animal and vegetable remains that have been washed into the stream. Thus, every river-mouth presents an ever-growing series of beds of varying thickness and material, superposed the one on the other, and enclosing various remains of animal and vegetable life. These strata would, in the above order, be converted, by pressure, into conglomerate, sandstone, slate, shale, and coal. Thus, again, we have found a beautiful and perfect explanation of the Stratified rocks as they are presented everywhere, by which their composition, stratification, and contents are fully accounted for. Stratified rocks, therefore, obtain the various names of sedimentary, because formed of the sediment of rivers; and aqueous,1 because deposited under water.

Organic Agents.-But animal and vegetable life is also busy in the formation of rocks. Away in the warmer seas of the Pacific, lives the coral insect or zoophyte, the skeletons of which compose the remarkable coral reefs that form the chief part of the numerous isles that stud that greatest of seas. These reefs extend thousands of miles, in broad barriers, over which the wild waves dash, or in detached groups that gradually gather round them material and form new islands. In the rocks, we also find the remains of like corals, standing where they grew, or drifted away, and appearing as extensive formations of limestone.

Again, the bottom of the sea is covered with accumulations of minute shell-fish, of great depth and over extensive areas, as is proved every day by soundings with the lead. Now, the old rocks exhibit strata of identical composition with these microscopic shells; some limestones and chalks, for example, being composed of millions to the square inch of perfect bivalve shells. Again, the sea-bottom contains beds of shell-fish, of different kinds, and of great extent and thickness. Should these die, and then be subjected to sufficient pressure, they would form a rock, exactly like the shell limestones so common in our rock-formations, and so valuable in agriculture and building.

Then we have the remains of ancient forests in our great mosses; and luxuriant growths of swampy plants and impenetrable jungle in the mud islands of the deltas of our great rivers in the tropics. These,

1 From Latin aqua, water.

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