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: 
Keweenawan. 
Upper Huronian (Animikian). 
Middle Huronian. 
Lower Huronian. 
Laurentian. 
Kewatin. 
Algonkian 
Archean { 
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.? The estimates which approximate to as much as 45,000 feet 
include some 30,000 feet of igneous or mixed igneous and sedimentary 
materials.? 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‘ is obscured by the nomen- 
clature, and would seem to include Archean 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. 
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 
1 Bulletin 360, U.S. Geol. Survey, 1909. 4 Loc. cit., p. 206. 
2 Loc. cit., p. 146. 5 Loc. cit., p. 164. 
3 Loe. cit., p. 191. 
