W. Upham — Estimates of Geologic Time. 215 



sides of Lake Michigan and the consequent accumulation of 

 sand around the south end of the lake, Dr. E. Andrews esti- 

 mates that the land there became uncovered from its ice-sheet 

 not more than 7,500 years ago. Prof. G. Frederick Wright 

 obtains a similar result from the rate of filling of kettle-holes 

 among the gravel knolls and ridges called kames and eskers, 

 and likewise from the erosion of valleys by streams tributary 

 to Lake Erie ; and Prof. B. K. Emerson, from the rate of de- 

 position of modified drift in the Connecticut valley at North- 

 ampton, Mass., thinks that the time since the Glacial period 

 cannot exceed 10,000 years. An equally small estimate is also 

 indicated by the studies of Gilbert and Russell for the time 

 since the last great rise of the Quaternary lakes Bonneville 

 and Lahontan, lying within the arid Great Basin of interior 

 drainage, which are believed to have been contemporaneous 

 with the great extension of ice- sheets upon the northern part 

 of our continent. 



Prof. James Geikie maintains that the use of palaeolithic 

 implements had ceased, and that early man in Europe made 

 neolithic (polished) implements, before the recession of the 

 ice-sheet from Scotland, Denmark, and the Scandinavian 

 peninsula ; and Prestwich suggests that the dawn of civiliza- 

 tion in Egypt, China, and India, may have been coeval with 

 the glaciation of northwestern Europe. In Wales and York- 

 shire the amount of denudation of limestone rocks on which 

 bowlders lie has been regarded by Mr. D. Mackintosh as proof 

 that a period of not more than 6,000 years has elapsed since 

 the bowlders were left in their positions. The vertical extent 

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

 same with that observed in the southwest 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. 



Another indication that the final melting of the ice-sheet 

 upon British America was separated by only a very short 

 interval, geologically speaking, from the present 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 Hud- 

 son's bay, in latitude 58°, and southward the high rocky hills are 

 completely glaciated and bare. The striae are as fresh-looking 

 as if the ice had left them only yesterday. When the sun bursts 

 upon these hills after they have been wet by the rain, they 

 glitter and shine like the tinned roofs of the city of Montreal."* 



* BulletiD, Geol. Society of America, vol. i, p. 308. 



