350 GEIKIE 



has been strengthened by the welding and crystallisation ol 

 its lowest rocks. It is these rocks which rise along the 

 central crests of mountain chains, where, after the lapse of 

 ages, they have been uncovered and laid bare, to be bleached 

 and shattered by frost and storm. 



ii. The Architecture of the Land. 



Let us now proceed to consider how these materials, sedi- 

 mentary and crystalline, have been put together, so as to 

 constitute the solid land of the globe. 



It requires but a cursory examination to observe that the 

 sedimentary masses have not been huddled together at ran- 

 dom; that, on the contrary, they have been laid down in 

 sheets one over the other. An arrangement of this kind at 

 once betokens a chronological sequence. The rocks cannot 

 all have been formed simultaneously. Those at the bottom 

 must have been laid down before those at the top. A truism 

 of this kind seems hardly to require formal statement. Yet 

 it lies at 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 starting, however, from where the succes- 

 sion is normal and unbroken, the geologist can advance with 

 confidence into regions where it has been completely inter- 

 rupted; where the rocks have been shattered, crumpled, and 

 even inverted. 



The clue 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 sedimen- 

 tary accumulations contains its own characteristic plants, cor- 

 als, 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 ob- 



