THE PALEONTOLOGICAL RECORDS 295 



of more and less generalized anatomical characters, has the same 

 order as the time arrangement of embryonic phases, where the 

 early stages are more generalized than the later ones. Fossil 

 remains can also be arranged in a time-order and the earlier 

 records present more generalized characters than do the later ones. 

 We find, then, that the classificatory, ontogenetic and paleonto- 

 logical series display similar tendencies. These series are 

 as they would be if they were the records of an evolutionary 

 process. 



98. ON THE STRATIGRAPHICAL SERIES OF ROCKS 



The greater part of the body of the earth — up to a depth of 

 two or three hundreds of miles from the surface — consists largely 

 of metallic substances (iron, nickel, etc.). Superficial to this 

 kernel is a layer of heavy basaltic rock and it is believed that this 

 basaltic substratum comes to the surface, over the deep ocean 

 bottoms. Embedded in the substratum are the continental earth- 

 blocks. These consist mainly of relatively light igneous rocks. 

 Covering them in many parts are large areas of sedimentary 

 rocks. 



The latter are conglomerates, breccias, sandstones, mudstones, 

 limestones, etc. The latter consist typically of the skeletons of 

 Molluscs, Echinoderms, Corals, Calcareous Algae, etc., or they 

 may be chemical precipitates. There are also carbonaceous strata 

 (coals, lignites, etc.) and there may be porous sandstones contain- 

 ing petroleum. Such are the main categories of sedimentary 

 rocks. 



Generally they have been deposited, as particles of various sizes, 

 at the bottoms of lakes or shallow seas. The particles have, 

 typically, been carried in suspension in river water or in lakes, 

 or in the sea, and then deposited. But the materials of sedi- 

 mentary rocks may also be air-borne (as in blown sand). No 

 sedimentary rock is known that has been deposited at the bottom 

 of a deep ocean. 



They are arranged as strata, originally laid down in nearly 

 horizontal positions. Groups of strata make formations and 

 groups of formations are systems. The sedimentary rocks can 

 be arranged in a time-series and this is summarized in the following 

 diagram. 



