GEOGRAPHICAL EVOLUTION 343 



pose in view, content ourselves with the broad, useful clas- 

 sification of the materials of the land into two great series 

 Fragmental and Crystalline. 



i. Fragmental. A very cursory examination of rocks 

 in almost any part of the world suffices to show that by 

 far the larger portion of them consists of compacted frag- 

 mentary materials. Shales, sandstones, and conglomerates, 

 in infinite variety of texture and colour, are piled above 

 each other to form the foundation of plains and the structure 

 of mountains. Each of these rocks is composed of distinct 

 particles, worn by air, rain, frost, springs, rivers, glaciers, 

 or the sea, from previously existing rocks. They are thus 

 derivative formations, and their source, 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 cannot imagine rivers or 

 lakes of magnitude sufficient to have spread over the sites 

 of the present continents. The waters of the ocean, how- 

 ever, may easily be conceived to have rolled at different 

 times over all that is now dry land. The fragmental rocks 

 contain, indeed, within themselves proof that they were 

 mainly of marine, and not of lacustrine or fluviatile origin. 

 They have preserved in abundance the remains of foram- 

 inifera, corals, crinoids, mulluscs, annelides, crustaceans, 

 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 

 sedimentary 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 2000 to 3000 feet, and covers 

 thousands of square miles of surface. Yet it is almost en- 

 tirely composed of congregated stems and joints and plates 



