GEOGRAPHICAL EVOLUTION 351 



serving how they lie upon each other, the succession of their 

 fossils is at the same time fixed. In this way the sedi- 

 mentary part of the earth's crust has been classified into 

 different formations, each characterised by its distinct 

 assemblage of organic remains. In the most recent forma- 

 tions, most of these remains are identical with still living 

 species of plants and animals; but as we descend in the 

 series and come into progressively older deposits the pro- 

 portion of existing species diminishes until at last all the 

 species of fossils are found to be extinct. Still lower and 

 older rocks reveal types and assemblages of organisms 

 which depart farther and farther from the existing order. 



By noting the fossil contents of a formation, therefore, 

 even in a district where the rocks have been so disturbed 

 that their sequence is otherwise untraceable, the geologist 

 can confidently assign their relative position to each of the 

 fractured masses. He knows, for instance, using for our 

 present purpose the letters of the alphabet to denote the 

 sequence of the formations, that a mass of limestone con- 

 taining fossils typical of the formation B must be younger 

 than another mass of rock containing the fossils of A. A 

 series of strata full of the fossils of H resting immediately 

 on others charged with those of C, must evidently be sepa- 

 rated from these by a great gap, elsewhere filled in by the 

 intervening formations D, E, F, G. Nay, should the rocks 

 in the upper part of a mountain be replete with the fossils 

 proper to D, while those in the lower slopes showed only 

 the fossils of E, F, and G, it could be demonstrated that 

 the materials of the mountain had actually been turned 

 upside down, for, as proved by its organic remains, the 

 oldest and therefore lowest formation had come to lie at 

 the top, and the youngest, and therefore highest, at the 

 bottom. 



Of absolute chronology in such questions science can as 

 c et give no measure. How many millions of years each 

 formation may have required for its production, and how 

 far back in time may be the era of any given group of 

 fossils, are problems to which no answer, other than a mere 

 guess, can be returned. But this is a matter of far less 

 moment than the relative chronology, which can usually be 



