318 



SCIENCE. 



[Vol. II., No. 31. 



means for discrimination between the clays, but ad- 

 mitted that there were instances where tlie different 

 types seem to blend insensibly into each other. 



West of the Scioto valley, the border of the drift- 

 bearing area is not marked by what is regarded as a 

 moraine. There is, however, an extension of what 

 Professor Wright has characterized as the 'glacial 

 fringe,' consisting of bowlders. In Dakota county, 

 Minn., this fringe is very wide. At Crystal lake 

 there is a well-marked moraine, and possibly there is 

 another a little to the westward. Farther to the 

 west, there is no accumulated morainic drift. West 

 of the Missouri, there is no evidence of glacial 

 ploughing. 



A line of drift-hills known as the Potash Kettle 

 range, in eastern Wisconsin, had been regarded as an 

 old beach-line. Dr. Chamberliu has ascertained that 

 the range is a glacial moraine. He described it as 

 an interlobate moraine, formed jointly by glacial lobes 

 occupying the valleys of Lalve Michigan and Green 

 Bay, respectively. This was correlated with mo- 

 raines to the westward in Wisconsin. 



Furthermore, there was a system of moraines, — 

 a belt or group, including the glacier lobes at Lake 

 Michigan, in the Chippewa valley, at the western ex- 

 tension of Lake Superior, and in the valley of the 

 Minnesota River, and Red River of the North. These 

 moraines were more pronounced, with a few excep- 

 tions, than those on the outer edge. Investigations 

 were being carried eastward with a view of showing 

 their correlation witli other moraines in that direc- 

 tion. The hypothesis of their exact correlation, of 

 course, would imply that they were contemporane- 

 ous; but there are doubts upon that point. 



The author claimed, that there were evidences that 

 the lake-basins were caused in part by depressions 

 during the ice age, caused by the exceptional accu- 

 mulation of ice in the basins. He deprecated the no- 

 tion that subsidence must always take as long a period 

 as elevation, or that the reverse is true. He applied 

 this to the case which he alleged of the depression 

 during the presence of ice in the lake-basins, and the 

 elevation since. 



In discussing this paper. Professor Lesley said it 

 was time to cry halt as to this theory of depression 

 by weight of ice. It was made to do duty for a 

 great variety of emergencies. In point of fact, ice 

 was much lighter, very much lighter, than any rock. 

 Professor Lesley pointed out instances wliere this 

 theory had been advanced to account for depressions 

 which now contained a greater weight than the ice 

 could have made with any reasonable hypotliesis of 

 its thickness. 



Professor Cbamberlin explained the theory further, 

 and claimed that in instances which he cited the 

 depression was greatest at the weakest part of the 

 strata. 



Prof. E. S. Morse referred to some English ex- 

 periments to determine the question whether the 

 moon's attraction deformed the earth's outline. It 

 was found (according to newspaper report), that the 

 weight of the incoming tide deformed the surface to 

 sUch an extent that the effect of the moon's attrac- 



tion could not be separately calculated. Major 

 Powell called attention to the theory, that, if the 

 earth were divided into conical sections radiating 

 from its centre, there would be found iin equal press- 

 ure in each. Every sediment, every erosion of the 

 surface, must be balanced by corresponding depres- 

 sion or rise elsewhere. Finally, the case of Lake 

 Saltonstall was cited by Mr. Hovey. It is evidently 

 situated in a valley that was ploughed out by the foot 

 of a glacier; certainly not in a hollow caused by 

 pressure. Professor Cox clung to another theory 

 entirely, as to the great lakes. He believed them to 

 be prolongations of a sea-coast which had at one 

 time extended to them through the valley of the St. 

 Lawrence. 



The Minnesota valley in the ice age. 



BY WARREN UPHASI OF MINNEAPOLIS, MINN. 



The paper was based upon the author's observa- 

 tions for three years as assistant on the geological 

 and natural-history survey of Minnesota, under the 

 direction of Professor Winchell. To the question: 

 During what ages was the glacial, rock-walled chan- 

 nel of the valley of the Minnesota River formed? — 

 the paper offered an answer. Deposits of cretaceous 

 clay were found in water-worn hollows at several 

 enumerated localities; and, in other places, cretaceous 

 sandstone and shale occasionally containing lignite. 

 It thus appears, that, before the cretaceous age, a 

 deep channel had been cut by some river in the lower 

 magnesian sandstone, and the Potsdam formation. 

 The slopes, the drainage, perhaps even the channel, 

 of that river, were not widely different from those of 

 the present; but that channel was probably eroded 

 during the later paleozoic and earlier mesozoic ages, 

 before the cretaceous subsidence. 



In the first epoch of glaciation, when the ice cov- 

 ered its greatest area, a thick drift-sheet, mostly un- 

 modified, probably covered all this region, including 

 the preglacial valley, witli an unbroken, though un- 

 dulating, expanse of till. During the ensuing iu- 

 terglaeial epoch, the drainage cut a channel, whose 

 position was largely determined by the slopes of the 

 erosion which had preceded the glacial epoch. The 

 jjreglacial, and also the interglacial river, lay far be- 

 low the present stream. The till of the later epoch 

 blocked the course of the river only in part of its ex- 

 tent, and the obstacle was soon channelled anew. 



During the recession of tire last ice-sheet, the val- 

 ley was filled with modified drift. After the ice was 

 melted in the Minnesota basin, this avenue of drain- 

 age was, for a long period, the outlet of Lake Agassiz. 

 The volume of water that it carried was very large, 

 being supplied by tlie melting ice-fields of North- 

 western Minnesota, and from the region of Lake 

 Winnipeg and tlie Saskatchewan. While streams 

 poured into this river from the melting ice-sheet, its 

 modified drift continually increased in depth; but, 

 when tlie great glacier liad sufficiently retreated, the 

 water from Lake Agassiz not only ceased to contain 

 drift, hut became a powerful eroding agent. The de- 

 posited drift was mostly swept away, and the channel 



