REPORT ON LAKE SUPERIOR REGION 459 



as old as any rock series that we now know. So here we have a clear 

 case of overlap between the definitions of "Huronian" and "Lauren- 

 tian." Moreover, we do not really know how far up in the Huronian 

 the overlap extends, and this fact weighed much in my mind in favor- 

 ing the final compromise. The Hastings series of the upper Lauren- 

 tian is largely a limestone-dolomite series. So is the Mesnard series 

 (or lower Huronian of the report) from the Gogebic Range through 

 the original Huronian area north of Lake Huron, and it seems to 

 me possible that the Mesnard series may prove to connect with the 

 original upper Laurentian. If this should be so, it should of course 

 be taken into account in drawing the line between Huronian and 

 Laurentian. 



Another factor complicates the situation, I am informed. Just 

 as in going west from New York, through Ohio to Michigan, the 

 only part of the original New York Lower Helderberg remaining 

 was the Waterlime, the rest being absent by unconformity, and so 

 the typical Lower Western Helderberg of Orton and Rominger 

 corresponds to a member that some of the New York geologists are 

 now inclined to prune off from the Helderberg entirely; so it has 

 been with the Huronian. The chloritic schist type, so comparatively 

 insignificant in the original Huronian area, if we have made no 

 mistake, covers great regions and becomes very important elsewhere, 

 and is essentially the formation Lawson called Keewatin, but has 

 often been mapped as Huronian, while granite and light-colored 

 gneisses have been mapped as Laurentian, without knowing or caring 

 (so far as the mapping was concerned) whether they cut the green 

 chloritic schists or not. To call these chloritic schists Laurentian 

 would be to upset the nomenclature very extensively. 



Moreover, we do not really know that the upper Laurentian may 

 not be a formation older and beneath these chloritic schists, though 

 I have not the slightest suspicion that it is. Thus it seemed well 

 to retain for these chloritic green schists, so different in many ways 

 from the Huronian, a name of their own, and among those used for 

 them Lawson's seemed to have clear priority among geographical 

 names. I must confess I am enough of a heretic to yearn personally 

 for the good old descriptive terms like "greenstone schists." 



"Laurentian" is thus left out in the cold as an accurately defined 



