94 THE ROYAL SOCIETY OF CANADA 



siliceous slates and slate conglomerates, overlaid by pale sea-green or 

 slightly greenish-white sandstone, with quartzose conglomeiates. 

 The slate conglomerates are described as holding pebbles, sometimes a 

 foot in diameter, derived from the subjacent gneiss, the boulders dis- 

 playing red feldspar, translucent quartz, green hornblende, and black 

 mica, arranged in parallel layers, which present directions according 

 with the attitude in which the boulders were accidentally enclosed. 

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

 until the subjacent formation 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 degrees, and the sandstone above 

 it was nearly horizontal." 



"The group on Lake Huron, we have computed to be about 10,000 

 feet thick; and from its volume, its distinct lithological character, its 

 clearly marked date posterior to the gneiss, and its economic importance 

 as a copper-bearing formation, it appears to me to require a distinct 

 appellation, and a separate color on the map. Indeed, the inves- 

 tigation of Canadian Geology could not be conveniently carried on 

 without it. We have, in consequence, given to the series the title 

 of Huronian. 



"A distinctive name being given to this portion of the Azoic rocks, 

 renders it necessary to apply one to the remaining portion. The only 

 local one that would be appropriate in Canada is that derived from the 

 Laurentide range of mountains, which are composed of it, from Lake 

 Huron to Labrador. We have therefore designated it as the Lauren- 

 tian series." 



The Huronian conglomerates, etc., here mentioned as occurring 

 on Lake Timiskaming are undoubtedly those since described by Dr. 

 Miller as the Cobalt Series, while the Huronian sediments north of 

 Lake Huron are described by Collins, where they are stated by him 

 to have a thickness of 14,290-16,565 feet. 



As far as my observations have gone Logan's subdivision has a 

 general application throughout the Pre-Cambrian rocks of north cen- 

 tral Canada. Mistakes were undoubtedly often made by Logan him- 

 self and his successors in placing various groups of rocks incorrectly 

 in one or other of his great subdivisions, but those mistakes do not 

 vitiate the correctness of the general idea. The law of priority of no- 

 menclature should be strictly adhered to. Where other and later 

 names have been applied they are only valid as long as the groups for 

 which they have been used are not known to be identical with groups 

 previously named, but as soon as identity with formations of known 

 age has been established the earlier names should prevail and the 



