158 abstracts: chemistry 



The Laurentian series is represented by great masses of granite, grani- 

 toid gneiss, and syenite. Intermediate and basic rocks are subordinate. 

 The Laurentian intrudes the Keewatin series in masses ranging from 

 great batholiths many miles in diameter to dikes and minute injections. 

 It appears that the Archean was a period of regional igneous activity, 

 igneous rocks making up more than 90 per cent of the whole. This 

 activity, both plutonic and volcanic was on a tremendous scale. 



The Algonkian system on the whole contrasts with the Archean in 

 being dominantly sedimentary, in being less metamorphosed, in having 

 a distinctly recognizable stratigraphic sequence, and in topography. 

 A subaerial deposition is regarded probable for the Keweenawan and the 

 lower Huronian of the North Shore. The upper Huronian, the greatest 

 carrier of iron ore, is regarded as a delta deposit. The iron-bearing for- 

 mations are regarded as having been derived partly from submarine 

 volcanic rocks, in magmatic solutions, or by the reaction of hot volcanic 

 material with sea water, or by weathering, or by some combination of 

 these methods. The Algonkian system comprises in its fullest develop- 

 ment four unconformable divisions — lower Huronian, middle Huronian, 

 upper Huronian, and Keweenawan. 



For the most part the formation which overlies the Proterozoic rocks 

 is a nearly horizontal sandstone, generally recognized as Cambrian. 

 In general, the relations between this sandstone and the Proterozoic 

 rocks are those of most profound unconformity, except where Keween- 

 awan sandstones are the underlying beds. 



Of the several periods of deformation, three stand out conspicuously 

 ■ — that at the close of the Archean, throughout the region; that at the 

 cose of the lower-middle Huronian, mainly on the north shore; and that 

 at the close of the Keweenawan on the south shore. As a result of these 

 successive deformations the Lake Superior region is essentially an asym- 

 metric synclinorium with nearly east-west axis. The copper ores of the 

 Keweenawan are regarded as directly related to contemporaneous 

 volcanism. 



Iron ores occur in a large number of the pre-Cambrian stratigraphic 

 subdivisions. The iron-bearing formations consist essentially of inter- 

 banded layers of iron oxide, silica, and combinations of the two, variously 

 called jasper or jaspilite (where anhydrous and crystalline) and ferrugin- 

 ous chert, taconite, or ferruginous slate (where softer and more or less 

 hydrous). These rocks become ore by local enrichment, largely by the 

 leaching out of silica and to a less extent by the introduction of iron oxide. 

 There are accordingly complete gradations between them and the iron 



