PRE-CAMBRIAN NORTH AMERICAN LITERA TURE 753 



shown by boring to be at least 1500 feet thick, and to consist of red 

 sandstone with several streaks of marl. The likeness of this sand- 

 stone to the upper Keweenawan sandstone, the faulting along or near 

 the contact line of the two formations, and the thinning of the traps 

 and amygdaloids in passing toward the Eastern sandstone, seem to 

 strongly favor the theory that the two formations are of the same age. 



COMMENTS. 



The question of the relations of the Eastern sandstone to the 

 Keweenawan is too difficult a one to discuss here, but it may be said 

 that it is the reviewer's opinion that the evidence presented is far too 

 slight for so important a conclusion. For a comprehensive discussion 

 of the question the reader is referred to Bulletin 23 of the United 

 States Geological Survey. 



Winchell,' discusses the origin of the Archean greenstones. The 

 great bulk of them are pyroclastic. They were distributed and some- 

 what stratified by the waters of the ocean into which the material fell. 

 As evidence of their arrangement by water is their very general strati- 

 form structure, which can only be explained by the action of water. 

 This structure stands vertical or nearly so. These greenstones consti- 

 tute a distinct terrane, forming the latest portion of the Keewatin, at 

 the top of the Fundamental Complex of the Lake Superior region. 

 Below the greenstones are found chloritic slates and schists, chloritic 

 schists, clay-slates, graywackes, conglomerates, quartzites, novaculites, 

 and jaspilites. The thickness of the greenstones in Minnesota exceeds 

 that of any other Archean terrane. The Keewatin passes gradually 

 down into crystalline mica-schists or hornblende-schists, and finally 

 into acid gneiss. 



COMMENTS. 



In certain parts of the Lake Superior region the greenstones 

 are predominantly pyroclastic, and in other parts are predomi- 

 nately intrusive or extrusive lavas. Not only is this so, but within the 

 same series in the same district the basic igneous rocks in one part are 

 mainly tuffs, and in other parts are almost wholly massive. 



I would altogether dissent from the conclusion that the banding of 

 the igneous rocks alone is evidence of their arrangement by water. In 



' The Origin of the Archean Greenstones, by N. H. Winchell. 23d Ann. Rep. 

 Geol. & Nat. Hist. Sur. of Minn., for 1894, Part II, 1895, pp. 4-35. 



