344 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



forms a nucleus from ^vliich emanates a complexity of dykes, proceeding to 

 considerable distances. As dykes of a similar character are met with inter- 

 secting the rocks of the Huronian series, the nucleus in qiiestion is supposed 

 to be of the Huronian age, as "vvell as the greenstone dykes which are inter- 

 sected by it." (/. c, p. 58.) 



Mr. Logan seems in this report to have abandoned the idea that the 

 coppei'-bearing rocks of Lake Superior were of the same age as those 

 north of Lake Huron, except in some minor districts classed as Huro- 

 nian ; for he calls the Huronian formation the Lower Copper-bearing 

 rocks, and places the others as the Upper Copper-bearing rocks. These 

 latter rocks he divides into two groups, and writes of their age as fol- 

 lows : — 



*' The precise age of the upper copper-bearing rocks of Lake Superior is a 

 question attended with some difficulty. Mr. Whitney appears disposed to 

 regard the whole series from the sununit of the sandstones of Sault Ste. Marie 

 to the base of the Kaministiquia slates as one group equivalent to the Potsdam 

 formation ; but the suspicion of a want of conformity between the Sault Ste. 

 Marie sandstones and the trappean rocks beneath, would induce us to separate 



the two The affinities of the red sandstone of Sault Ste. Marie would 



thus appear to bring it into the position of the Chazy rather than the Potsdam 

 formation ; and if this were established, the copper-bearing portion of the Lake 

 Superior rocks might reasonably be considered to belong to the Calciferous and 

 the Potsdam formations." (l. c, pp. 84-86.) 



This is an abandonment of the attempt to determine the age of the 

 Lake Superior copper-bearing rocks by lithological characters only, and, 

 so far as Mr. Logan is concerned, a removal of those rocks from the 

 Azoic series. 



In the supplement to this report the labradorite rocks occurring in 

 the Laurentian series are thought to unconformably overlie the lower 

 portion of the Laurentian formation, cutting out some limestone bands. 

 The contacts were not seen ; but, as the labradorite rocks were assumed 

 to be sedimentary, this replacement of the limestone was held to prove 

 their unconformability with the Laurentian, (I. c, pp. 837-839.) 



The same year Dr. Hunt remarked : — 



" The so-called granites of the Laurentian and Lower Silurian appear to be 

 in every case indigenous rocks ; that is to say, strata altered in situ, and still 

 retaining evidences of stratification. The same thing is true with regard to 

 the ophiolites and the anorthosites of both series ; in all of which the general 

 absence of great masses of unstratified rock is especially noticeable." (Am. 

 Jour. Sci., 1863, (2 ) XXXVI. 226.) 



