Age of the Earth. 355 



It will be remembered that most American geologists now 

 subdivide pre- Cambrian rocks as follows : 



I Keweenawan. 



Upper Huroman (Auimikian). 



Algontcian. <J 



Middle 



Archjean. 



Lower „ 



f Laurentian. 

 I- Kewatin. 



Prior to 1904 the Lower and Middle Huronian were 

 together called Lower Huronian. Alternative names for 

 the three divisions of the Huronian are Lower, Middle, and 

 Upper Marquettian. The lin^s represent unconformities. 



A study of the recorded facts shows that the higher 

 estimates of Keweenawan rocks include preponderating 

 amounts of igneous rocks, both effusive and volcanic. The 

 time value of these materials is probably — nay certainly — 

 small. Van Hise cites a case where the accumulation of 

 7000 to 8000 feet of Huronian volcanic materials is parallelled 

 by the collection elsewhere of 700 to 800 feet of ordinary 

 sediments (loc. cit. -p. 146). The estimates which approximate 

 to as much as 45,000 feet include some 30,000 feet of igneous 

 or mixed igneous and sedimentary materials (loc. cit. p. 191). 

 No sedimentary column thicker than 17,000 feet is cited. 



The Huronian, or lower division of the Algonkian, is 

 nowhere, save in an early estimate of WinchelPs, found to 

 embody more than 15,000 feet of sediments. Winchell's 

 estimate (loc. cit. p. 206) is obscured by the nomenclature, and 

 would seem to include Archaean rocks. If his Marquettian, 

 which name he applies to rocks formerly known as Kewatin, 

 includes Lower Huronian only, we have an estimate of 

 27,000 feet for this division. The estimate w r ould be unique. 

 The highest distinct estimate of Lower Huronian which I 

 have found in the Bulletin is "a possible maximum thickness " 

 of 16,000 feet, of which 5000 feet are true sediments (loc. cit. 

 p. 164). _ . . . ' 



The Algonkian generally is variously estimated, but in no 

 case is a thickness greater than 50,000 feet cited. In the 

 Cordilleras the Belt series— 30,000 feet — plus the Cherry 

 Creek series may amount to more. It does not seem likely, 

 however. The former series is characterized by Van Hise 

 and Leith as unique among the pre-Cambrian series of 

 North America for wide extent, thickness, and lack of 



