442 C. K. LEITH 



upward through less altered sediments to the upper members of the 

 Huronian system. 



In the Lake Superior region the Huronian is succeeded upward by 

 the rocks of the Cambrian, represented by the Upper Copper-bearing 

 series, or the Animikie and Nipigon groups ; while in eastern Ontario 

 this portion of the scale is apparently entirely lacking, the formation 

 succeeding the crystalline series being the Potsdam sandstone, which is 

 now held to represent the lowest number of the Cambro-Silurian or 

 Ordovician system. 



Comment. — Adequate discussion of the above scheme of nomen- 

 clature is quite impossible in the space available. It may be said only 

 that the scheme will be dissented from on many important points by 

 many of the United States geologists. 



Crosby 1 describes the Archean-Cambrian contact near Manitou, Col. 

 A sandstone of Cambrian age rests upon an Archean granite complex. 

 The granite floor has very small erosion inequalities. These inequalities 

 are hummocks, not hollows ; erosion remnants, and not channels; clearly 

 marking the end, and not the beginning of a process of base-leveling. 



It is believed that such an even contact plane between the Cam- 

 brian and pre-Cambrian series is widespread and characteristic in 

 North America. It appears to be the case in the valley of the Eagle 

 River and in the canyon of the Grand River above Glenwood, Col.; 

 in the Black Hills of South Dakota, examined by the writer ; in the 

 Grand Canyon of the Colorado, described by Walcott; and in Wiscon- 

 sin, described by Irving. 



In the Manitou, Eagle River, and Black Hills areas, throughout the 

 Rocky Mountains, and eastward to Champlain Valley and beyond, the 

 Cambrian has a non-arkose character; it has been thoroughly sorted 

 and washed by water, a fact which indicates that the incursion of the 

 sea was an extremely slow one. It is believed that the plane surface of 

 the Archean has resulted from the incursion of the sea due to the sub- 

 sidence of the land, and not from the action of subaerial agents, for in 

 the latter case only an approximate plane could have resulted because 

 of differential erosion. 



Emmons 2 describes the Archean rocks of the Ten -mile quadrangle 

 •of Colorado. These rocks outcrop to the east of the great fault — the 



1 Archean-Cambrian Contact near Manitou, Col., by W. O. Crosby: Bull. Geol. 

 Soe. Am., Vol. X, 1899, pp. 141-164. 



2 Ten-mile District Special Folio, Colorado, by S. F. Emmons : Geol. Atlas of the 

 «U. S., No. 48, 1898. 



