HA X ^r 



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340 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



the same age as the Huronlan rocks of Lake Huron, were opposed the 

 same year by one of the present writers in the May number of th( 

 American Journal of Science (1857, (2) XXIIL 305-314). He als: 

 pointed out Logan's violation of the law of prioiity in appropriating the 

 term Laurontian from Desor.* It was there shown that the reason Logan 

 had for separating the Huronian from the other Azoic rocks was his 

 (Logan's) belief that the copper-bearing rocks (Potsdam) of Lake Supe- 

 rior were the same as the Azoic schists north of Lake Huron : hencC; as 

 the copper-bearing rocks of Lake Superior rested unconformably on 

 Azoic granites, etc., therefore the Lake Huron rocks must. Later, Logan 

 abandoned his premises, but did not, however, give up his conclusion. 



(See Hunt, Azoic Rocks, p. 80.) 



In the Proceedings of the American Association for the Advancement 

 of Science (August, 1857, pp. 44-47), Mr. Logan made some statements 

 regarding *' the division of the Azoic Rocks of Canada into Huroniau 

 and Laurentian," the chief of which we reproduce here : 



'* The sub-Silurian Azt)ic rocks of Canada occupy an area of nearly a quarter 

 01 a million of square miles. Independent of their stratification, the parallel- 

 ism that caji be shown to exist, between their lithological character and that of 

 nietamorphic rocks of a later age, leaves no doubt on my mind that they are a 



series of ancient sedimentary deposits, in an altered condition So. 



early as the year 1845, as will be found by reference to my report on the 

 Ottawa district (presented to the Canadian government the subsequent year), 

 a division was drawn between that portion which consists of gneiss and its 

 subordinate masses, and that portion consisting of gneiss interstrutified wit,h 

 important bands of crystalline limestone. I was then disposed to place the 

 lime-bearing aeries above the uncalcarcous, and although no reason has since 

 been found to contradict this arrangement, nothing has been discovered espe- 

 cially to confirm it. ... . In the same report is mentioned, among the Azoic 

 rocks, a formation occurring on Lake Temiscaming, and cpnsisthig of silicious 

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

 white sandstone, with quartzose conglomerates. The slate conglomerates are 

 described as holding pebbles and boulders (sometimes a foot in diameter) de- 

 rived from the subjacent gneiss, the boulders displaying red feldspar, translu- 

 cent quartz, green hornblende, and black mica, arranged in paralhd layers, 

 which present directions according with the attitude in which the boulders 

 were accidentally inclosed. From this it is evident that the elate conglomer- 

 ate was not deposited until the subjacent formation had been converted into 



* Tliis paper of Prof. Whitney's, in common with some others, was accidentally 

 omitted in Mr. WadswortVs "List of Papers " appended to the " Kotes on the 

 Geology of the Iron and Copper Disfiicts of Lake Superior," although reference was 

 made to it in the text. (See also Canadian Journal, 1857, (2 ) IL 302.) 



