196 New. York State Museum 



varies greatly in a short distance. The criteria used 

 for determining geologic sequence in these rocks must 

 be of a physical nature, such as the lithological char- 

 acter of the rock, superposition of the formations, 

 crustal movements and cycles of erosion. Geologists 

 in general admit the extreme length of geologic time 

 and more than half of geologic time has been allotted 

 to the Precambrian eras. Some even believe that 

 the Archeozoic alone may have been longer than all 

 the subsequent eras taken together. The thought is 

 suggested that the appearance of the Cambrian fauna, 

 though many millions of years ago, might be regarded 

 as a comparatively recent event if the degree of life 

 development is taken as a basis by which to measure 

 time. 



These older crystalline rocks so sharply set off from 

 the younger Paleozoic beds have been grouped to- 

 gether into two Precambrian eras, the Archeozoic 

 and Proterozoic. Some authors use for the Archeo- 

 zoic the term Archean (ancient) which once covered 

 all the Precambrian formations. The Proterozoic 

 era includes the Algonkian of authors. Precambrian 

 rocks (Archeozoic and Proterozoic), according to 

 estimations, appear at the surface over one-fifth of the 

 land area ; but, while this means it is not covered by 

 younger rocks; it may be covered with soil or glacial 

 deposits. 



Archeozoic and Proterozoic 



Geology. The Archeozoic era comprises rocks that 

 are greatly altered and has been termed the Age of Larval 

 Life; the Proterozoic era comprises rocks that are not 

 much altered and is known as the Great Iron Age and 



