W. TJpham — Quaternary Changes of Levels. 493 



the rate of erosion of the shores of Lake Michigan and the resulting 

 accumulation of beach sand blown into dunes at the south end of 

 the lake ; l by Prof. G. F. Wright, from the rate of filling of small 

 peat bogs in hollows surrounded by kames and osars at Andover, 

 Mass., and from the erosion of streams tributary to Lake Erie; 2 

 by Mr. G. K. Gilbert, from the recession of Niagara Falls ; 3 and by 

 Prof. B. K. Emerson, from the rate of deposition of modified drift 

 in the Connecticut valley at Northampton, Mass. 4 These measure- 

 ments and estimates agree in showing that only 6,000 to 10,000 

 years have passed since the ice-sheet of the last Glacial epoch was 

 melted away from the northern part of the United States. It is 

 therefore impossible to refer that glaciation to an epoch of increased 

 eccentricity which ended 80,000 years ago. An equally small 

 estimate is also indicated by the studies of Gilbert 6 and Russell 6 

 for the time since the last great rise of Lakes Bonneville and 

 Lahontan, which appear to have fluctuated contemporaneously with 

 the growth and departure of the North American ice-sheets. 



In Wales and Yorkshire the amount of denudation of limestone 

 rocks on which boulders lie has been regarded by Mr. D. Mackintosh 

 as proof that a period of not more than 6000 years has elapsed since 

 the boulders were left in their present positions. 7 The vertical 

 extent of this denudation, averaging about six inches, is nearly the 

 same with that observed in the south-west part of the Province of 

 Quebec by Sir William Logan and Dr. Robert Bell, where veins 

 of quartz marked with glacial striae stand out to various heights not 

 exceeding one foot above the weathered surface of the enclosing 

 limestone. 8 



Another indication that the final melting of the ice-sheet upon 

 British America at the close of the last Glacial epoch was separated 

 by only a very short interval, geologically speaking, from the pre- 

 sent time, is seen in the wonderfully perfect preservation of the 

 glacial striation and polishing on the surfaces of the more enduring 

 rocks. Of their character in one noteworthy district, Dr. Bell writes 

 as follows : — " On Portland promontory on the east coast of Hudson's 

 Bay, in latitude 58°, and southward, the high rocky hills are com- 

 pletely glaciated and bare. The striae are as fresh-looking as if the 

 ice had left them only yesterday. When the sun bursts upon these 



1 Transactions of the Chicago Academy of Sciences, vol. ii. James C. Southall's 

 " Epoch of the Mammoth and the Apparition of Man upon the Earth," 1878, 

 chapters xxii. and xxiii. 



2 Am. Journ. Sci. in. vol. xxi. pp. 120-3, Feb. 1881 ; "The Ice Age in North 

 America," 1889, chapter xx. p. 466. 



3 Nature, vol. xxxiv. p. 560; Proc. Am. Assoc, for Adv. of Science, vol. xxxv. 

 for 1886, p. 222; "The History of the Niagara Eiver," Sixth Ann. Eep. of 

 Commissioners of the State Eeservation at Niagara, for 1889, pp. 61-84. 



4 Am. Journ. Sci. in. vol. xxxiv. pp. 404-5, Nov. 1887. 



5 U.S. Geol. Survey, Second Annual Report, p. 188. 



6 U.S. Geol. Survey, Monograph xi. "Geological History of Lake Lahontan," 

 p. 273. 



7 Quart. Journ. Geol. Soc. vol. xxxix. 1883, in Proceedings, pp. 67-69. Compare 

 id. vol. xlii. 1886, pp. 527-539. 



8 Bulletin of the Geological Society of America, vol. i. 1889, p. 306. 



