212 JR. D. Irving — Is there a Huronian Group? 



bonate, the whole appearance of the section differing in no 

 respect from sections I have examined of Palaeozoic limestones. 

 Not that it seems to me necessary to believe in a direct organic 

 origin for the carbonate of lime contained. The whole process 

 may have been a combination of the mechanical and chemical 

 kinds of sedimentation such as we find exemplified in the 

 clayey beds of the Coal Measures carrying carbonate of iron, 

 and at various other geological horizons. The two other lime- 

 bearing members of the series as marked by Logan are spoken 

 of by him as very largely cherty, and as including here and 

 there sandstone layers. In our own examination of these beds, 

 made chiefly in the back country between Echo Lake and Gar- 

 den River, it appeared to us that the cherty material predom- 

 inated very largely over the calcareous. In this region the 

 cherty layers are almost entirely horizontal, the whole appear- 

 ance of the bluff sides of the exposures being that ordinarily pre- 

 sented by the horizontal formations, as, for instance, in the upper 

 Mississippi valley. This horizontality, taken along with the 

 very thin lamination of these rocks, and the presence of more 

 or less distinctly detrital material, as is observable on the larger 

 scale as well as microscopically, serves to place beyond question 

 the sedimentary origin of these layers. Often these cherty 

 beds are quite ferruginous, and at the same time brecciated, in 

 which case they parallel very closely occurrences in the Mar- 

 quette and Penokee iron regions on the south side of Lake 

 Superior, and are not unlike occurrences among some of the 

 later fossiliferous formations. Whatever may be true, then, of 

 the origin of those great masses of coarsely crystalline lime- 

 stone which are associated with the ancient gneisses, no doubt 

 can be entertained of the sedimentary origin of these lime- 

 stones of the original Huronian. Moreover, they have under- 

 gone since their, first deposition changes no greater than are 

 attributable to the action of infiltrating waters, or than those 

 often found among the Palaeozoic limestones. 



Of the four principal kinds of rocks which constitute the 

 series according to the estimate I have given above, there re- 

 main to be mentioned only those crystalline rocks which I 

 have spoken of collectively under the convenient term of 

 greenstone. Of these I need now say only that they are mainly 

 diabases in no important respect different from eruptive dia- 

 bases at various other horizons, that they are mainly non-schist- 

 ose, and that they present no sign whatever of gradation into 

 the adjoining sedimentary strata. 



Thus the first part of our inquiry, with regard to the original 

 or type Huronian is answered in the affirmative. Intrinsically, 

 it is fully entitled to the group rank. Its detrital or clastic 

 origin; its largeness of volume; its inclusion of a number of 



