CHAMBERS'S INFORMATION FOR THE PEOPLE. 



that flow therefrom. Thus, while its figure is 

 preserved by the laws of centripetal and centrifugal 

 force, its motions are determined and influenced 

 by the attraction and gravitation of the sun and 

 other planets. From its situation with respect to 

 the sun, it necessarily follows that only one-half 

 of its surface can be exposed at a time to the 

 light and heat diffused from that orb, thereby 

 causing day in the one part, and night in the 

 other. The seasons, again, are caused chiefly by 

 the fact, that in performing its path round the 

 sun, the earth preserves its axis in a slanting or 

 oblique position, as has been more particularly 

 explained in ASTRONOMY and in METEOROLOGY. 



The solar system, however, vast as it seems, is 

 but a unit in space, which is peopled with other 

 systems and orbs circling beyond the bounds of 

 human conception. What we term fixed stars are 

 but suns and centres of revolution ; and the solar 

 system, as a whole, may revolve in space round 

 some vast centre, just as its individual planets 

 have their motions round the sun. From such a 

 revolution may arise cycles of heat or cold, life 

 or death, exuberance of certain living forms, and 

 annihilation of others cycles which meet with a 

 faint analogy in the recurrences of our summers 

 and winters. Even in the known relations of 

 the earth to the sun, astronomers find causes of 

 vicissitudes of temperature sufficient to account 

 for those alternate periods of equatorial heat and 

 glacial cold which geology shews our earth to 

 have already passed through. 



The materials of which the earth is composed 

 present a history not less curious than that of its 

 planetary relations. Superficially speaking, the 

 globe consists of land and water the water occu- 

 pying the extreme depressions of the land, and 

 this land composed of solid or rocky materials. 

 Our direct knowledge of these materials extends 

 only to a small depth a mere rind of the globe ; 

 regarding the interior portions, we can only infer 

 certain things. One of those inferences is that 

 already noticed namely, that the interior, as a 

 whole, is more than twice as dense as the superficial 

 matter. But whether this arises from the materials 

 being different, or from their being more com- 

 pressed, we have no means of knowing. It used to 

 be held that the earth was a molten mass enclosed 

 in a solid shell or cooled crust, through which the 

 agitations of the liquid below produce the pheno- 

 mena of volcanoes and earthquakes. From con- 

 siderations, however, regarding the tides, and other 

 astronomical data, it is inferred with certainty that 

 this is not the case ; and it is most likely that at 

 least half the distance from the surface to the centre 

 is solid and more rigid than the rocks we know. 

 The solid earth to which we have access consists of 

 rocks, differing not only in their appearance and 

 arrangement, but in their mineral and chemical 

 characters ; some being compact and crystalline, 

 as marble, others soft and dull, as chalk ; some 

 lying in layers or strata, others occurring in huge 

 irregular masses ; while, mineralogically and 

 chemically speaking, we have such rocks as 

 granite, quartz, slate, lime, coal, rock-salt, chalk, 

 and clay. But the crust so composed, compact 

 and solid as it may seem, is far from being per- 

 manent and stable ; in other words, the dry land 

 which now appears, with all its irregularities of 

 hill and valley, plain and ravine, lake and river, 

 is not the dry land which existed many thousands 



50 



of years ago. Strictly speaking, indeed, the aspect 

 of the globe is ever changing. Here the sea 

 encroaches on the land, there the debris borne 

 down by rivers silts up bays and estuaries ; here 

 earthquakes sink, and volcanoes elevate the sur- 

 face ; lakes are dried up, and rivers change their 

 course ; and, greater than all of these, vast regions 

 gradually subside, and are covered by the ocean, 

 while others as gradually emerge from the waters, 

 and become dry land. All these changes, past 

 and present, form the subject of geological con- 

 sideration. 



Geology, in its aim to decipher the physical 

 history of our globe, has determined that all the 

 known rocks may be ranked under two great sec- 

 tions the stratified and the unstratified. The 

 former appear in layers or beds, and have evidently 

 been deposited in water, hence said to be aqueous 

 or sedimentary j the latter appear in vast irregular 

 masses, generally disrupting the stratified rocks, 



and have all the appearance of having been formed 

 like the lavas of the present day ; hence they are 

 called igneous or volcanic. Of the sedimentary 

 rocks, sandstone, limestone, slate, and coal may 

 be taken as illustrative examples ; of the igneous, 

 granite, basalt, greenstone, and lava are the most 

 familiar. As at present, so in all time past, the 

 surface of the earth has been subjected to atmos- 

 pheric, aqueous, and other influences, the effects 

 of which are to wear down the exposed material ; 

 and this, borne away by floods and rivers, is depos- 

 ited in the ocean, where, consolidated by pressure, 

 heat, and chemical agency, it forms new strata of 

 rocks, which in time are brought to the surface by 

 volcanic and other elevating forces. Thus, then, 

 one set of agencies degrade, and another recon- 

 struct and elevate ; and in proportion as either of 

 these preponderate, so will any portion of the earth 

 be low and level, or high and precipitous. Such, 

 then, is the origin of the stratified and unstratified 

 rocks the one but the reconsolidated matter of 

 pre-existing rocks, which have been worn and 

 battered down by rains, frosts, waves, and rivers ; 

 the other the cooled and hardened material sent 

 forth from the interior of the earth by volcanic 

 agency. But while rivers and floods bear down 

 mud, sand, and the like, they also carry such 

 vegetable and animal remains as lie in their 

 course; and in this manner plants and animals 

 are entombed in the newly formed layers or strata. 

 As at present, so in former eras, such remains 

 have been enclosed in the stratified rocks, where, 

 subjected to certain chemical agencies, they have 

 become petrified, and are thus preserved as records 

 of the former Flora and Fauna which peopled the 

 globe. Geologists have, accordingly, found that 

 the earth has not always been occupied by the 

 same kinds of plants and animals that now exist ; 

 but that different eras in its onward history have 

 had very different Flora and Fauna, and that not 

 one, perhaps, of the genera at one time in existence 

 now survives. 



By long-continued and widely extended obser- 

 vations in various parts of the globe, based on 



