214 It. D. Irving — Is there a Huronian Group ? 



masses of the different kinds of eruptives could be truncated or 

 yield fragments to detrital deposits. In case the basement 

 rocks are a combination of plutonic eruptives with crystalline 

 schists, additional proof may be found in the fact that the erup- 

 tives intersect the schists, while they are truncated by the over- 

 lying fragmental rocks, to which also they have yielded 

 fragments. 



Now all of this evidence is forthcoming in the original 

 Huronian region, and in the clearest and most unequivocal 

 manner. The Huronian series is detrital and non-schistose ; 

 the underlying rocks are crystalline schists (mainly mica- 

 schist), gneiss and granite ; the mica-schists being without 

 remaining fragmental texture, though at times banded in such 

 a manner as to render an original fragmental condition very 

 probable. The upper rocks are not intersected by the granite 

 where the two come in contact; the lower schists and gneiss 

 are deeply invaded by it in great bosses, in irregular masses of 

 smaller size, and in an intricate net-work of veins ; as may be 

 seen most beautifully eastward from Algoma Mills, on the 

 Lake Huron coast, and along the line of that branch of the 

 Canadian Pacific Railway which extends from Algoma Mills to 

 Sudbury on the main line. 



Fragments of the older rocks occur in the detrital series at 

 various horizons, but in most especial beauty at the contacts 

 of the two series. These latter conglomerates are true basal 

 conglomerates, whose asserted absence certain geologists* have 

 urged with much emphasis as an argument against the exist- 

 ence of any separate Huronian series ; i. e., separate from the 

 remainder of the great Archsean complex. I may therefore not 

 improperly enlarge somewhat on their actual occurrence. Prob- 

 ably the handsomest instance is that at the contact of Logan's 

 basal member of the series, on the Lake Huron coast, about three 

 miles east of the mouth of Thessalon River. From this point 



1.. UIIBOS. 



S.YriUr QUARTBITR. BASAIi COXGt 



Fig. 2. Section about three miles east of the mouth, of Thessalon River, Lake 

 Huron, showing conglomerate at the base of the Huronian. Scale, 400 feet to 

 the inch- 

 eastward the older gneiss, granite, etc., spread along the coast 

 to Algoma Mills, and beyond. The contact is best seen in 

 a group of small islands lying close to the mainland. These 

 islands are moutonneed to a very high degree, and are largely 

 bare, so that the rocks are finely displayed. The figure is a 

 section through three of these islands, the middle one of 

 which shows the granite and gneiss at one end, and, resting 



* J. D. "Whitney and M. E. Wadsworth, in the " Azoic System," pp. 556, 557. 



