MUSEUM OF COMPAKATIVE ZOOLOGY. 



71 



later observeris to distinguisli the eruptive rocks, even in the " Iluro- 

 nian," detracts from their evidence concerning the " Laurentian." The 

 granite that Mr. Brooks phiccd as Formation XX. of the '* Huronian/' 

 merely because that was the easiest way to dispose of it, unless he 

 wished to acknowledge that his *' Laurentian" granite or gneiss was in- 

 trusive in the '* Iluronian," bids fair to absorb now all the " Laurentiau" 

 region, if we can judge from what Mr. Brooks writes. He says: "Dr. 

 llominger considers certain granitic and gneissoid rocks north and south- 

 west of Marquette, which 1 did not study but regarded as Laurentian, 

 to belong to tliis series. I have but little doubt but that the younger 

 Iluronian rock made out in the Menominee region (the granitic bed 

 XX.) will yet be identified in the Manpiette region, and will be found to 

 bo more or less gucissic. The very rocks mentioned above may pos- 

 sibly be Upper Huronian ; the granites, etc., southwest of Michigannno 

 lake very probably are."* Although this granite is in the region mapped 

 as '^ Laurentian" by Mr. Brooks, he does not tell us how to separate it 

 from tlie *' Laurentian," or where the dividing line is to be found. How 

 does he know that it is not the same as the other "Laurentian" rocks? 

 He has never made any examination to see whether it is or not, and 

 in the above-quoted remark of his he virtually acknowledges tliat he 

 know nothing of the rocks that he mapped as "Laurentian" in the 

 district since examined by Dr. Bominger. 



We tried to find some point where we could trace rock continuously 

 from well marked and mapped "Laurentian" into the "Huronian," but 

 were unable, with the time and opportunity at our disposal, to do more 

 in that direction than we have already pointed out. The evidence is 

 strong, but not so conclusive as we could have wished; yet what would 

 it have availed us if we had found such a locality? We should have 

 been told :. " Oh ! that is Formation XX. that you found ; we knew 

 nothing about the region, so we mapped it as Laurentian." May we 

 suggest that hereafter geological maps of Lake Superior be colored as 

 "Iluronian," and "Formation XX." 1 Let us substitute the last term 

 for the " Laurentian " at once, and have done with it before Formation 

 XXIL is born. At another locality, the granite that Mr. Brooks as- 

 signs to "Formation XX," Professor Irving places under the "Kewee- 



na\van."t 



While at Lake Superior the followers of Dr. Hunt thus place a 



troublesome granite cutting the '* Huronian" as the latest formation 



in it (except one), in Eastern I^Iassachusetts a granite said to cut and 



li 



ii 



* Geol. of Wise, HI. 529, 530. 



t Ibid., HI. 193-195. 



