FORMAL PROPOSAL OF '' LAURENTIAN " AND " HURONIAN." 97 



"Consisting of silicious slates and slate conglomerates, overlain by pale green, or 

 slightly greenish-white sandstone, with quartzose conglomerates. The slate con- 

 glomerates are described as holding pebbles and bowlders (sometimes a foot in diam- 

 eter) derived from the subjacent gneiss, the bowlders displaying red feldspar, trans- 

 lucent quartz, green hornblende and black mica, arranged in parallel layers which 

 present directions according with the attitude in which the bowlders were accidentally 

 inclosed. From this it is evident that the slate conglomerate was not deposited until 

 the subjacent formations had been converted into gneiss, and very probably greatly 

 disturbed; for while the dip of the gneiss, up to the immediate vicinity of the slate 

 conglomerate, was usually at high angles, that of the latter did not exceed nine de- 

 grees, and the sandstone above it was nearly horizontal." 



We interrupt this quotation for the purpose of remarking that such jux- 

 taposition of unconformable dips is evidence that Director Logan is here 

 speaking of the " upper slate conglomerate," for it is notorious that the 

 " lower slate conglomerate " passes, through the interposition of pure slates, 

 with perfect conformity, to the crystalline schists, and thence to the gneissic 

 rocks.* This is further evinced by the low dip ascribed to it, because, while 

 this is common with the " upper slate conglomerate " and the overlying 

 quartzites, it is almost never the case that the " lower slate conglomerate " 

 has a dip so low as 60° or 70°; and when thus low the occurrence is in the 

 immediate vicinity of a local disturbance. The mean attitude of the lower 

 slate conglomerate is vertical. It is evident, therefore, if slates of low dip 

 lie upon usually vertical or high-dipping gneisses, they belong to the upper 

 portion of the series here called Huronian ; while the strata of the lower 

 portion are wanting — a state of things actually observed by the writer and 

 previously described by the Canadian geologists — along the north shore of 

 Lake Huron. Sir William Logan therefore deliberately embraced in one 

 system an extensive body of strata, generally of very low dip, and an equally 

 extensive body of strata, generally of very high dip, equally contrasted also, 

 in lithological characters. 



The communication continues : 



" In the report transmitted to the Canadian government, in 1848, on the north 

 shore of Lake Huron, similar rocks are described as constituting the group which is 

 rendered of such economic importance from its association with copper lodes. This 

 group consists of the same silicious slates and slate conglomerates, holding pebbles of 

 syenite instead of gneiss, similar sandstones [quartzites?] sometimes showing ripple- 

 marks, some of the sandstones [quartzites?] pale red-green, and similar quartzose 

 conglomerates in which blood-red jasper pebbles become largely mingled with those 

 of white quartz and in great mountain masses predominate over them. But the 

 series is here much intersected and interstratified with greenstone trap, which was not 

 observed on Lake Temiscaming. These rocks are traced along the north shore of 

 Lake Huron, from the vicinity of the Sault Ste. Marie, for 120 miles east." 



Sir William Logan indicates the further extension of this group toward 



*See the author's memoir, Bull. Geol. See. Amer., Vol. I, 1890, pp. 357-394, and all recent writers 

 on the structural relation of the semi-crystalline schists to the crystalline schists and gneisses. 



