of Shells in the Till near Boston. 371 



many localities, rooted in the ground where they grew, and by 

 tracts of marsh and peat-swamps covered by the sea.* During 

 part of the time of lower level of the sea, its temperature was 

 apparently warmer, as indicated by the range of Venus mer- 

 cenaries with other southern species northward to the Gulf of 

 St. Lawrence, though now it is wanting along most of the 

 shore of Maine, the Bay of Fundy, and Nova Scotia. 



Proceeding from Boston toward the north and northwest, 

 the elevation of fossiliferous marine beds lying on the glacial 

 drift increases to about 225 feet in Maine, about 520 feet in 

 the St. Lawrence valley at Montreal, and 440 feet at a dis- 

 tance of 130 miles west-southwest of Montreal ; but eastward 

 along the St. Lawrence it decreases to 375 feet opposite the 

 Saguenay, and does not exceed 200 feet in the basin of the Bay 

 of Chaleurs, while these marine deposits are wanting in Nova 

 Scotia and Cape Breton island, f The changed condition in 

 the relative heights of land and sea at the time of the recession 

 of the ice-sheet thus caused the land to be submerged in in- 

 creasing amount northwestward from a line drawn through 

 Nova Scotia, Boston, and New York. This condition, due 

 probably in part to depression of the land and in part to up- 

 lifting of the sea level by gravitation, seems to have been 

 caused by the ice- sheet, which had its greatest thickness, esti- 

 mated by Dana to be not less than two miles, on the highlands 

 between the St. Lawrence and Hudson bay, where its influ- 

 ence to produce such changes of level would be greatest. The 

 submergence seems to have been more than can be wholly at- 

 tributed to gravitation of the sea toward the ice-sheet ;£ but it 

 is much less than would be expected for agreement with the 

 views advanced by Jamieson§ and Shaler,| that the ice-sheet 

 must depress the earth's crust to a vertical extent approximately 

 measured by a thickness of rock equal to the ice in weight. 



* Outlines of the Min. and Geol. of Boston, p. 95 ; Memoirs B. S. N. H., vol. 

 i, p. 324 ; Quart. Journ. Geol. Soc, vol. xvii, 1861, pp. 381-8; Geol. of N. H., 

 vol. iii, p. 173 ; J. W. Dawson's Acadian Geology, third ed., 1878, pp. 28-32, and 

 Supplement of do., pp. 13-17. 



•f A. S. Packard, jr., in Memoirs B. S. N. K, vol. i, pp. 231-262. J. W. Daw- 

 son in Notes on the Post-pliocene Geology of Canada; and Am. Jour. Sci., Ill, 

 vol. xxv, 1883, pp. 200-202. C. H. Hitchcock in Proa, Amer. Assoc, for Adv. of 

 Sci., Portland, 1873, vol. xxii, pp. 169-175; Geol. of N. H., vol. iii, pp. 279-282; 

 and Geol. Mag., II, vol. vi, 1879, pp. 248-250. R. Chalmers in Transactions of 

 the Royal Society of Canada, sec. iv, 1886, pp. 139-145. 



% Sixth Annual Report U. S. Geol. Survey, 1885, pp. 291-300. 



§ Quart. Journ. Geol. Soc, vol. xxi, 1865, p. 178. Geol. Mag., II, vol. ix, Sept. 

 and Oct., 1882; and III, vol. iv, Aug., 1887. Aleo, see Fisher's Physics of the 

 Earth's Crust, and Geol. Mag., II, ix, p. 526. 



|| Proceedings B. S. N. H., vol. xii, 1868, pp. 128-136 ; and xxiii, 1884, pp. 

 36-44. Memoirs B. S. N. H., vol. i, 1874, pp. 320-340. Am. Jour. Sci., Ill, vol. 

 xxiii, 1887, pp. 210-221. Lowell Lectures, Nov. and Dec, 1888. 



Am. Jour. Sci.— Third Series, Vol. XXXVII, No. 221.— May, 1889. 

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