46 TWELFTH REPORT. 



Temagauii. Cobalt Lake, ou Avliich the town is situated, lies almost 

 entirely in Huron ian conglomerate. The conglomerate in turn lies in 

 a deeper valley formed by rocks of the Keewatin group. It might be 

 expected therefore that the coarser material in the conglomerates would 

 be largely detritus from those old greenstones and cherts; but such 

 is by no means the case. The basal portion is made up very largely of ma- 

 terial similar to that Avhicli enclosed the old valley ; but the greater 

 Ix>rtion of both boulders and matrix is quite ditferent. There are not 

 now exposed any near-by hills from which these materials might have 

 been brought down, nor is there good reason to believe that such hills 

 existed in Huronian times. The nearest outcrops of Laurentian rocks 

 from which many of the boulders nmy have been derived are some miles 

 distant. Many of the large boulders are quite unlike any rocks which 

 have been found in place in the district. Evidently ice was the most 

 comjietent agent to bring such materials to their present ix»sitiou, and 

 to deposit them in such a heterogeneous manner. 



At Temagami the conglomerate lies on and at its base contains numer- 

 ous fragments of Keewatin schists. The conglomerate also contains 

 numerous rounded and subangular bouldei's of rocks not found close 

 by. The underlying rock presents a fairly fresh but not a smooth sur- 

 face. The matrix of the basal conglomerate contains numerous well 

 formed rhombohedra of siderite and similar crystals are abundant in 

 shale and (piartzite beds in the conglomerate. They are evidently de- 

 rived by Aveathering from the adjacent iron formation, and the crystals 

 were growing freely contemporaneous with the mechanical deposition of 

 clay, sand, and boulders. It is probable therefore that the conglomer- 

 ate was formed under water and that there was carbonate in solution. 

 If the larger eri-atics were brought by ice it was probably not land ice. 



Summary — In ai»i>earauce the conglomerate-quartzite-shale series of 

 the Huronian represents nothing so closely as compacted glacial and 

 glacio-flmial debris. The finding of striated and soled pebbles confirms 

 the supposition of such an origin. The character of the contacts thus 

 far found do not disprove that glaciers placed the basal conglomerate, 

 though they suggest that such was not the case. 



There are some sudden transitions from shale to coarse conglomerate 

 which suggest that the earlier deposits may have been overriden by 

 land ice. It seems probable however that part of the material was de- 

 posited under water and that floating ice contributed its load of glacier 

 derived material. 



There is no reason for supposing that the thick boulder free beds of 

 shale and greywacke are not ordinary water lain sediments, though 

 they may contain glacial flour. 



Michigan College of Mines, Houghton, Mich., April, 1910. 



