THE ARCHEOZOIC ERA, 139 



Cambrian systems of rock, composed chiefly of sediments, are here 

 separated from the Archean, and designated Algonkian or Proterozoic. 

 The former term is commonly used by members of the U. S. Geological 

 Survey, and by some others, though it is little employed outside the 

 United States. It has the same terminal form as Cambrian, Silurian, 

 etc., terms applied to groups of strata of a much lower order of im- 

 portance, and hence the term Proterozoic, corresponding to the terms 

 of higher rank, seems a better name for the great group of systems 

 between the Archeozoic below and the Paleozoic above. Since these 

 systems are very thick and separated from one another by great uncon- 

 formities, they represent a vast lapse of time, probably quite comparable 

 in importance to that of the Paleozoic, Mesozoic, or Cenozoic, or perhaps 

 even to all combined. 1 



There are theoretical reasons of some weight for separating from 

 the Archean complex all series of rocks that are made up, in large 

 part, of quartzose sandstones, mudstones, and limestones, or their 

 metamorphic equivalents. These rocks are made up of the products 

 of mature weathering, and therefore imply that the surface from 

 which their materials were derived was long exposed to chemical decom- 

 position without very favorable conditions for ivash; other wise partially 

 decomposed products would have been carried away and deposited 

 before decomposition was complete, and the result would have been 

 arkoses and wackes rather than quartzose sandstones and shales. 

 A covering of vegetation is the chief agency that now prevents this 

 premature wash, and secures mature decomposition. While it may 

 not be wholly safe to conclude that a vegetal covering of the land was a 

 necessary condition for the formation of the quartzose sands and the 

 muds which enter largely into the systems referred to the Proterozoic, 

 it seems best to entertain this presumption to the extent of separating 

 these systems from the more ancient complex of rocks in which the 

 products of such complete decomposition do not seem to prevail. The 

 term Archean is therefore here restricted to those formations, composed 

 chiefly of igneous and meta-igneous rocks, which antedate the oldest 

 known dominantly sedimentary system. Since the Archean system 

 of rocks, as thus defined, is primarily of extrusive-igneous or meta- 

 igneous origin, the era during which they were formed is, under the 

 planetesimal hypothesis, regarded as the time of transition from the 

 1 Van Hise, Bull. 86, U. S. Geol. Surv., p. 491. 



