278 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



fortunately no rocks have been more carefully studied within recent 

 years. The appearance of the monograph of Van Hise and Leith * 

 places the known facts at our disposal along with explanatory 

 remarks of the most helpful character. 



It will be remembered that most American geologists now sub- 

 divide pre-Cambrian rocks as follows : 



Algonkian 



Keweenawan. 



Upper Huronian (Animikian). 



Middle Huronian. 



Lower Huronian. 



. (Laurentian. 



Archaean i Tr ,. 

 IKewatm. 



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 lines 

 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 prob- 

 ably — nay , certainly — small. Van Hise cites a case where the accumu- 

 lation of 7,000 to 8,000 feet of Huronian volcanic materials is paral- 

 leled by the collection elsewhere of 700 to 800 feet of ordinary sedi- 

 ments. 2 The estimates which approximate to as much as 45,000 feet 

 include some 30,000 feet of igneous or mixed igneous and sedimentary 

 materials. 3 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 Winchell's, found to embody more than 15,000 

 feet of sediments. Winchell's estimate 4 is obscured by the nomen- 

 clature, and would seem to include Archaean rocks. If his Marquet- 

 tian, 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 would 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 5,000 feet are 

 true sediments. 5 



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 char- 

 acterized by Van Hise and Leith as unique among the pre-Cambrian 

 series of North America for wide extent, thickness, and lack of defor- 

 mation. There is no apparent unconformity between the Cherry 



i Bulletin 360, U. S. Geol. Survey, 1909. *■ Loc. eit., p. 206. 



2 Loc. cit., p. 146. 6 Loc cit., p. 164. 



3 Loc. cit., p. 191. 



