CHAPTER IV 

 THE ARCHEOZOIC ERA 



The Oldest Known Geologic Records. — In earth history, as 

 in human history, the recorded events of earliest times are fewest 

 and most obscure, and hence the least intelligible of all. In spite 

 of a certain disadvantage in beginning with the least known part 

 of the history of the earth, the only satisfactory method of present- 

 ing the subject is "to follow the natural order of events. This has 

 the great advantage of bringing out the philosophy of the history — 

 the law of evolution" (J. Le Conte). The earliest known geologic 

 history is recorded in the rocks of the Archean system. While 

 it is true that the most obscure records of any rock system are here, 

 partly because the original structures of these rocks have generally 

 been so profoundly changed (metamorphosed) and partly because 

 of the utter absence of anything like determinable fossils, never- 

 theless, certain very important conclusions regarding the earliest 

 known era of geologic time may be reached through a study of the 

 rocks of the Archean system. The present state of our knowledge 

 does not warrant the subdivision of the Archeozoic into two or 

 more definite periods or systems. 



General Character and Origin of the Archean Rocks. — "Ar- 

 chean Complex," "Basal Complex," "Fundamental Complex," etc., 

 are all terms which have been applied to the rocks of the Archean 

 system which invariably occupy a basal position with reference to 

 all other rock systems. "The Archean system is a crystalline 

 complex beneath the base of the determined sedimentary succes- 

 sion. . . . The United States Geological Survey has restricted 

 the term to a complex of basic and acidic surface and deep-seated 

 igneous rocks, of schists and gneisses in part derived from them and 

 in part of unknown origin, and of shreds and small masses of meta- 

 morphosed sediments, all unconformably below and older than the 

 Algonkian sedimentary rocks, which are the lowest series in which 

 ordinary stratigraphic methods have been applied. Their litho- 



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