210 WAREEN UPHAM, ESQ., ON CAUSES OF THE ICE AGE. 



Yorksliire the amount of denudation of limestone rocks on 

 which drift bouhlers he has been regarded by Mr. D. Mackin- 

 tosh as proof that a period of not more than (3,000 years has 

 elapsed since the boulders were left in their positions. The 

 vertical extent of this denudation, averaging about six 

 inches, is nearJy the same with that observed in the south- 

 west part of the Province of Quebec by Sir Wilham 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 lime- 

 stone.* 



18 From tliis Avide range of concurrent but independent 

 testimonies, we may accept it as practically demonstrated that 

 the ice-sheets disappeared only G.OOO to 10,000 years ago. 

 It is therefore manifestly impossible to ascribe their exis- 

 tence to astronomic causes which ceasied 80,000 years ago. 

 as is done by Croll's theory. Instead, I now beheve, with 

 Prestwich,t that the whole duration of the Ice age, probably 

 20,000 to 30,000 years, more or less, was not only terminated 

 but begun after the end of the last period of maximum 

 eccentricity of the earth's revolution around the sun. 



19 Another astronomic theory, which assigns a date and 

 duration of the Glacial period from about 24,000 to 6,000 

 years ago, agreeing nearly with the estimate by Prestwich, 

 has been brought forward by JMajor-General A. W. Drayson, 

 who first published it in the Quarterly Journal of the Geological 

 Society for 1871, and later in successive books, of which the 

 earliest is On the Cause, Date, and Duration of the Last Glacial 

 Epoch of Geology, and the probable Antiquity of Man, with an 

 J nvestigation and Description of a New Movenientof the Earth 

 (187:^), and the latest, Untrodden Ground in Astronomy and 

 Geology (1890j.$ This theory asserts that the earth's axis 



* For more ample statements of many of these evitlences of the recency 

 of the Glacial period, see the followinar papers in this Journal of the 

 Transactiuiis of the Victoria Institute : The Lapse of Time since the Glacial 

 Epoch determined by the date <f the Polished ,Stone Age, by J. C. Southall, 

 vol. xiii, 1880, pp. 109-132 ; and On the Recency of the Close of the Glacial 

 Period in England and Wales, as shown by the limited depth of Postglacial 

 tStream Channels, the small Extent of Denudation of Limestone Rocks, and 

 the fresh aspect of Moraines, by D. Mackintosh, vol. xix, 1885, pp. 73-92. 



+ Geology, vol. ii, 1888, p. 534. 



\ In the Journal of the Transactions of the Victoria Institute, vol. xxvi, 

 1893, pp. 259, 260, Major-General Drayson concisely states his theory in 

 a .ftter commenting on Prof. James Geikie's paper. 



