GEOGRAPHICAL EVOLUTION. 551 



as well as their mode of origin, can be determined. Their component 

 grains are for the most part rounded, and bear evidence of having been 

 rolled about in water. Thus we easily and rapidly reach a first and 

 fundamental conclusion that the substance of the main part of the 

 solid land has been originally laid down and assorted under water. 



The mere extent of the area covered by these water-formed rocks 

 would of itself suggest that they must have been deposited in the sea. 

 We can not imagine rivers or lakes of magnitude sufficient to have 

 spread over the sites of the present continents. The waters of the 

 ocean, however, may easily be conceived to have rolled at different 

 times over all that is now dry land. But the fragmental rocks contain 

 within themselves proof that they were mainly of marine and not of 

 lacustrine or fluviatile origin. They have preserved in abundance the 

 remains of foraminifera, corals, crinoids, mollusks, annelides, crusta-. 

 ceans, fishes, and other organisms of undoubtedly marine habitat, which 

 must have lived and died in the places where their traces remain still 

 visible. 



But not only do these organisms occur scattered through sedimen- 

 tary rocks ; they actually themselves form thick masses of mineral 

 matter. The Carboniferous or Mountain limestone of Central England 

 and Ireland, for example, reaches a thickness of from two thousand to 

 three thousand feet, and covers thousands of square miles of surface. 

 Yet it is almost entirely composed of congregated stems and joints 

 and plates of crinoids, with foraminifera, corals, bryozoans, brachio- 

 pods, lamellibranchs, gasteropods, fish-teeth, and other unequivocally 

 marine organisms. It must have been for ages the bottom of a clear 

 sea, over which generation after generation lived and died, until their 

 accumulated remains had gathered into a compact sheet of rock. From 

 the internal evidence of the stratified formations we thus confidently 

 announce a second conclusion that a great portion of the solid land 

 consists of materials which have been laid down on the floor of the sea. 



From these familiar and obvious conclusions we may proceed fur- 

 ther to inquire under what conditions these marine formations, so wide- 

 ly spread over the land, were formed. According to a popular belief, 

 shared in perhaps by not a few geologists, land and sea have been con- 

 tinually changing places. It is supposed that while, on the one hand, 

 there is no part of a continent over which sea- waves may not have 

 rolled, so, on the other, there is no lonely abyss of the ocean where a 

 wide continent may not have bloomed. That this notion rests upon a 

 mistaken interpretation of the facts may be shown from an examina- 

 tion (1) of the rocks of the land, and (2) of the bottom of the ocean. 



Among the thickest masses of sedimentary rock those of the an- 

 cient palaeozoic systems no features recur more continually than the 

 alternations of different sediments, and the recurrence of surfaces cov- 

 ered with well-preserved ripple-marks, trails and burrows of annelides, 

 polygonal and irregular desiccation-marks, like the cracks at the bot- 



