368 M. D. Irving — Is there a Huronian Group ? 



(1.) The great basement complex, of crystalline schists, gneiss 

 and granite, as to whose further divisibility or non-divisibility 

 no opinion is now expressed. Above this basement, but 

 separated from it by a great structural hiatus, follows : 



(2). The Huronian Group, mainly of detrital rocks; which 

 is followed in turn, but after a severe structural break, by 



(3). The Keweenaw Group of interleaved detrital and erup- 

 tive beds, which is also entitled to the group rank. Finally, 

 upon the eroded edges of this series follows 



(4). The Potsdam or Upper Cambrian sandstone. 



VIII. Thus far I have directed your attention only to the 

 region between the north shore of Lake Huron and the upper 

 Mississippi River — a region which we may conveniently call 

 the Lake Superior Geological Province. But geologists have 

 often made correlations of the rocks of other and remote 

 regions with the type Huronian, and this not only for various 

 portions of North America, but of other continents as well. 

 Such correlations have been made chiefly, often exclusively, 

 on the basis of a lithological similarity to the original or type 

 series, and should therefore be received with the very gravest 

 doubt; but to make the matter worse the lithological compari- 

 son has been with a standard which does not exist in the type 

 area, and which, where anything like it does appear in the 

 neighboring regions, is plainly pre- Huronian, i. e., is part of 

 the great basement complex. 



Correlations with the type Huronian may be received with 

 more confidence perhaps, if for regions not too remote from the 

 type area, when based on a parallelization of unconformities 

 above and below the type series. Such unconformities are 

 plainly the result of an intervention of mountain-making dis- 

 turbances whose influences must generally have been wide- 

 spread. Thus a succession similar to that of the Lake Superior 

 region, and with similar discordances, obtains beneath the 

 equivalent of the Poisdam sandstone in the Colorado Canon ; 

 and probably also in Central Texas. In Newfoundland again, 

 lying unconformably beneath the lowest known fossiliferous 

 Cambrian horizons, are two mutually discordant series, the 

 upper one of which is detrital, and the lower crystalline and 

 gneissic. The use of such a method of correlation, however, 

 is not advocated as being of value in inter-continental compari- 

 sons, or even in inter-regional ones, when the basins compared 

 are too remote from one another ; though it may be said that 

 it could hardly give much less satisfactory results than have at 

 times been yielded by the paleontological method. 



But while we cannot with confidence correlate the Huronian 

 or Keweenawan of the Lake Superior region with any particu- 



