59+ THE POPULAR SCIENCE MONTHLY. 



the very foundation of any attempt to trace the geological history of a 

 country. Did the rocks everywhere lie undisturbed one above another 

 as they were originally laid down, their clear order of succession would 

 carry with it its own evident interpretation. But such have been the 

 changes that have arisen, partly from the operation of forces from 

 below, partly from that of forces acting on the surface, that the true 

 order of a series of rocks is not always so easily determined. By start- 

 ing, however, from where the succession is normal and unbroken, the 

 geologist can advance with confidence into regions where it has been 

 completely interrupted where the rocks have been shattered, crum- 

 pled, and even inverted. 



The clew which guides us through these labyrinths is a very simple 

 one. It is afforded by the remains of once living plants and animals 

 which have been preserved in the rocky framework of the land. Each 

 well-marked series of sedimentary accumulations contains its own char- 

 acteristic plants, corals, crustaceans, shells, fishes, or other organic 

 remains. By these it can be identified and traced from country to 

 country across a whole continent. When, therefore, the true order of 

 superposition of the rocks has been ascertained by observing how they 

 lie upon each other, the succession of their fossils is at the same time 

 fixed. In this way the sedimentary part of the earth's crust has been 

 classified into different formations, each characterized by its distinct 

 assemblage of organic remains. In the most recent formations, 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 proportion of existing species diminishes until at 

 last all the species of fossils are found to be extinct. Still older and 

 lower rocks reveal types and assemblages of organisms which depart 

 further and further 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 rela- 

 tive 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 containing fossils 

 typical of the formation B must be younger than another mass of rock 

 containing the fossils of A. A series of strata fidl of the fossils of H 

 resting immediately on others charged with those of C, must evidently 

 be separated from these by a great gap, elsewhere filled in by the inter- 

 vening 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. 



