DEFORMATION" OF SHORE LINES. 509 



their action in mere vastness of area or of matter involved, the transfer of tremendous volumes 

 of fluid magma through great distances beneath the earth's crust is not of this class. The 

 difficulty of explaining such a transfer is enormously increased when a mass of magma from 

 1,000 to 4,000 feet thick and underlying about one-third of the continent of North America 

 is involved. The displaced magma could go nowhere without producing effects that would be 

 readily recognized. With the amounts and the distances so great time would be of great 

 importance, especially if the material was more or less viscous. To make readjustment easy 

 would require the whole interior body to be perfectly fluid and responsive to pressure, like 

 water in a water-filled rubber ball when dented. It seems incredible that the interior of the 

 earth can be in any such condition, and if it is not in such condition the difficulties of explaining 

 resilience in this way are incalculably greater. The very vastness of the region affected aj^ears 

 to exclude this cause. 



It is also a question whether both modes of depression can be postulated as occurring at 

 once with the values stated, for Woodward's calculation of the effect of elastic compression 

 is based on the assumption that the earth is solid to the center and highly rigid throughout — ■ 

 an idea that is hardly compatible with the postulate of a widespread fluid magma beneath the 

 crust. If the interior is fluid, the elastic compression found by Woodward must reside solely 

 in the rigid cmst above the fluid interior. But, assuming for the moment that the two modes 

 of depression can occur together with the values assumed, it becomes possible, on the basis of 

 definite assumptions as to the thickness of the ice, to calculate roughly the amount of resili- 

 ence which would arise from each of these causes on the removal of the ice weight. Thus, 

 emplojnng the values used by Lane, if the ice be supposed to be 1,250 feet thick, the amount 

 of elastic resilience on its removal would be one-fifth, or about 250 feet. This, presumably, 

 would be instantaneous; that is to say, it would follow the decrease of ice weight at the same 

 rate, but the hydrostatic readjustment of the fluid magma would be much slower, depending 

 upon the degree of viscosity. On this assumption of ice thickness, according to Lane, the 

 hydrostatic relief would give an additional uplift amounting to one-third of the ice thickness, 

 or about 400 feet. This would give a total uplift of about 650 feet. On account of its slow- 

 ness, however, the hydrostatic relief may be still in progress and not yet fully satisfied. 



THEORETICAL AND ACTUAL UPLIFT. 

 LAKE SUPERIOR REGIOX. 



Lane applies his theory to the region of Lake Superior, where he supposes the Huron 

 Mountains on the south side of the lake to have been barely overtopped by the ice at its greatest 

 thickness. If it be granted for the moment that Lane's assumption on this point is true, the 

 thickness of ice measured from the present water surface of Lake Superior is approximately 

 in accord with the assumed thickness of 1,250 feet in the Lake Superior basin close north of 

 these mountains. The amount of uplift on the Keweenaw Peninsula has been on the least 

 estimate 500 or 600 feet and may have been more. On the assumption made, therefore, the 

 results seem to accord fairly well with expectation. 



But although it is true that the Huron Mountains were not covered by a great depth of ice, 

 even at the maximum of the Wisconsin ice sheet, Lane's assumption that they were barely 

 overtopped is at variance with evidence recently found by Mr. Leverett showing clearly that their 

 highest parts were heavily scored by the ice sheet that passed over them. On the most con- 

 servative estimate the ice could hardly have been less than 1,500 feet thick and was in all prob- 

 ability 2,000 to 2,500 feet thick on the northern part of the high ground north and northwest 

 of Champion, for at the maximum the ice moved nearly 150 miles southwest over this high 

 region, nearly all of it more than 1,500 feet above sea level (900 feet above Lake Superior). 

 Allowing for an average slope of only 10 feet to the mile for the surface of the ice back from the 

 extreme front, the depth 150 miles back would be 1,500 feet, as stated above. But in all 

 probability the average slope was nearer 20 feet to the mile, which would give a depth of 3,000 

 feet. The Keweenaw and Chippewa ice lobes merged on this highland and covered it completely, 



