42 ZPpham — Review of the Quaternary Era, 



that this return of glacial conditions affected also the Macken- 

 zie basin and the country bordering Hudson Bay, as seems to 

 be indicated by the observations of Dr. Robert Bell.* In 

 New England and the Saint Lawrence region, and in Green- 

 land, Iceland, northwestern Europe, and even Spitzbergen, this 

 refrigeration, following an interval of somewhat warmer cli- 

 mate than that which prevails at the present time, has caused 

 the isolation of colonies of southern mollusks whose geographic 

 range must have been formerly continuous from southern lati- 

 tudes; and corresponding changes in the forests and other 

 flora of the land are discovered in European peat-bogs, f Very 

 probably these recent climatic changes, both marine and ter- 

 restrial, in the North Atlantic region, have been due in large 

 measure to variations in the volume of the Gulf Stream ;J but 

 in the Cordilleran area, from the Sierra Nevada and the con- 

 tiguous great Quaternary lakes north to Alaska, the increased 

 precipitation of rain and snow and the extension of glaciers, 

 constituting for that area a third Glacial epoch, were probably 

 caused by elevation of that side of the continent. Evidences 

 of very late uplifting of our Pacific coast, followed now by 

 subsidence, in harmony with this explanation of the late glacia- 

 tion there, have been observed by Dr. G. M. Dawson, but the 

 extent of this uplift may have greatly exceeded his estimate.§ 

 What has been the length of the Quaternary era, we cannot 

 yet determine with very close approximation ; but from the 

 extent of river erosion during the principal interglacial epoch, 

 and from many other correlative observations, McGee con- 

 cludes, as it seems to me reliably, that the interval between the 

 first and second Glacial epochs was several times longer than 

 the interval between the close of the second Glacial epoch, 

 when the latest ice-sheet of the northeastern United States was 

 melted away, and the present day. || In proportion with the 

 estimates of 7,000 to 10,000 years for the time since that glacial 

 recession, it appears probable that the whole Quaternary era, 

 including the stages of glaciation and the warm intervals of 

 retreat or complete departure of the ice-sheets, that is, all the 

 period from the end of the Pliocene till now, may be 100,000 

 or 200,000 years. Though the earliest glaciation is thus appar- 

 ently referable to some part of the last stage of maximum 

 eccentricity of the earth's orbit, the astronomic theory so ably 

 advocated by Croll seems untenable, not only from the mete- 



* Bulletin, G. S. A., vol. i. p. 308. 



fThis Journal, III, vol. vii, pp. 134-8, Feb., 1874. James Geikie, Prehistoric 

 Europe, chapters xx and xxi. 



\ American Geologist, vol. vi, pp. 336-7, Dec, 1890. 



§ Canadian Naturalist, new series, vol. viii, pp. 241-8, April, 1877. 



I This Journal, III, vol. xxxv, pp. 465-6, June, 1888. U. S. Geol. Survey, 

 Seventh Annual Report, pp. 637-9. 



