30 THE STORY OF THE EARTH AND MAN. 



morphism took place, and at lengtli the tension 

 becoming too great to be any longer maintained, a 

 second great collapse occurred, crumpling and dis- 

 turbing tbe crust, and throwing up vast masses of 

 the Laurentian itself, probably into lofty mountains 

 — many of which still remain of considerable height, 

 though they have been subjected to erosion through- 

 out all the extent of subsequent geological time. 



The Eozoic age, whose history we have thus shortly 

 sketched, is fertile in material of thought for the 

 geologist and the naturalist. Until the labours of 

 Murchison, Sedgwick, Hall, and Barrande had de- 

 veloped the vast thickness and organic richness of the 

 Silurian and Cambrian rocks, no geologist had any 

 idea of the extent to which life had reached backward 

 in time. But when this new and primitive world of 

 Siluria was unveiled, men felt assured that they had 

 now at last reached to the beginnings of life. The 

 argument on this side of the question was thus put 

 by one of the most thoughtful of English geologists. 

 Professor Phillips : " It is ascertained that in passing 

 downwards through the lower Palaeozoic strata, the 

 forms of life grow fewer and fewer, until in the lowest 

 Cambrian rocks they vanish entirely. In the thick 

 series of these strata in the Longmynd, hardly any 

 traces of life occur, yet these strata are of such a kind 

 as might be expected to yield them. . . . The 

 materials are fine-grained or arenaceous, with or with- 

 out mica, in laminae or beds quite distinct, and of 

 various thicknesses, by no means unlikely to retain 



