12 Causes and Course of Organic Evolution 



Period of crust formation 105 millions of years 



*' " dry or non-oceanic crust 95 



" inorganic evolution IS'^^ 



archsean rock formation 95 



palaeozoic rock formation. ... 75 

 mesozoic rock formation .... 1}4 

 csenozoic rock formation .... SVz 



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This calculation — confessedly vague — would give about 180 

 millions of years for formation of the sedimentary crust. 



The oldest stratified or semi-stratified rocks of the earth's 

 crust have variously been called the metamorphic, archfean, 

 proterozoic, or pre-cambrian, and it is to the early or mid- 

 archsean epoch that all evidence tends, as the starting point 

 for the dawn of organisms on the earth. The rocks then formed 

 are usually quite unconformable with the masses of primitive 

 granite on which they rest, thus indicating that a considerable 

 — even extended — period of time had elapsed between deposit 

 of the two systems. Only rarely do they seem to be continuous, 

 but even then the composition of the older igneous or granitic 

 and newer or archsean rocks differs markedly {IS: 863). Often 

 also the older archsean beds are broken up, distorted, and 

 copiously interbedded with intrusive granitic masses, that 

 indicate active and extensive alterations in the crust. 



But, while the archsean strata or masses consist very largely 

 of schistose and micaceous gneissic materials that have been 

 highly metamorphosed by contact with granitic or other intru- 

 sive igneous flows, in the middle or later beds of the series there 

 occur zones of graphite that may have had a vegetable origin; 

 cherts and iron ores probably also of vegetable origin, as Geikie 

 has suggested (12: 877) and as the writer has further elucidated 

 (13: 257); as well as limestones that may have had an organic, 

 possibly even a vegetable, origin; and all frequently inter- 

 bedded ^ith schists, serpentines, and other rocks. 



The deposits of this age are enormously developed over 

 extensive areas of North America, Europe, Asia, and Austraha, 

 possibly also of Africa. Their vertical thickness, over the 

 regions north of the Great Lakes, has been calculated to be 

 about 65,000 feet or 12 miles. They evidently resulted in 



