Related Geological Conditions 25 



higher palaeozoic hill-ranges, and has filled up the de- 

 pressions of ocean beds, the practical absence of a high 

 alpine and of an abyssmal flora and fauna In palaeozoic 

 and even more recent rocks Is partial proof of plastic 

 crustal conditions. 



But more striking still is the frequent alternation, within 

 a few feet or even inches in rock thickness, of numerous 

 layers that may differ markedly in mineralogic composi- 

 tion, or that may yield a typically lacustrine fauna from 

 one bed, a marine fauna from another immediately above 

 or below, and then a series of beds that differ from these 

 as well as from each other, but which may be largely or 

 wholly devoid of organic remains. Such conditions are 

 eminently typical from the Silurian to the Permian or 

 Triassic epoch, but seem to reach a climax from the Upper 

 Devonian to the close of the Carboniferous, and again 

 become marked during the Jurassic epoch. The truth of 

 this is attested in all of the detailed accounts of rock- 

 sections given by geological surveys, in the articles of geo- 

 logical magazines. In technical reports on commercial min- 

 eral strata, and has often impressed the writer — as doubt- 

 less it has Impressed all field workers — when travelling 

 over and examining successive rock sections for their or- 

 ganic contents. 



This relatively plastic and changeable condition was 

 clearly due in part to the thinness, and highly faulted 

 nature, of the early sedimentary crust; in part to the very 

 extensive exhibition of volcanic activity that then prevailed, 

 as compared with Its localized and reduced exhibition at 

 the present time. Thus geologists are agreed (Gelkle,/: 

 861-907 ;Chamberlin-Salisbury, 6* :II :i75-i94, etc.) that the 

 Archaean rocks often represent a thickness of about 21,000 

 yards or 12 miles, as compared with 15,000 to 20,000 

 yards as the thickness of the formations that make up 

 more recent strata. These foundational Archaean rocks 

 are, however, extensively altered, faulted, contorted, and 

 often even overturned on each other. During the Ordo- 

 vlcian and Silurian periods extensive volcanic change also 

 proceeded, and the same is true over wide areas of Dev- 

 onian strata in some continents. 



