488 THE ICE AGE IN FORTH AMERICA. 



glacial action, we are justified in believing that this deposit 

 is a product of ice-action, though the waste has evidently 

 been worked over by water since its production. " The 

 thickness of the deposits he estimates to be in some places 

 nearly twenty thousand feet. These deposits correspond in 

 age to the Roxbury conglomerates in Boston, which are 

 about five hundred feet in thickness, and " are composed of 

 materials derived from various points in eastern Massachusetts 

 and southern New Hampshire. The pebbles are rarely over 

 a foot in diameter." But Professor Shaler thinks " their 

 frequently subangular forms and the wide range of sub- 

 stances associated together make it pretty clear that they 

 have a glacial origin." 



Upon this the same remark is applicable which was made 

 in a preceding section, namely, that along this whole Appala- 

 chian border there were formerly Archaean highlands of in- 

 definite height, of which the stumps are all that now remain 

 in the present hills and mountains. The erosion of these 

 mountains on their western flanks has furnished the material 

 of the vast sedimentary deposits of the eastern part of the 

 Mississippi basin. For all we know, the material spread out 

 over this area of sedimentary rocks was all within reach of 

 rivers coming down from Archaean heights, and so there is 

 no necessity of supposing extensive glacial transportation 

 from more northern water-sheds such as w T e are compelled to 

 suppose in the glacial age of recent date. The same remark 

 may be extended to all the evidence adduced in the preced- 

 ing sections concerning a succession of glacial periods.* In 

 all cases they are of such limited character that local glaciers 

 coming down from isolated mountain-masses, such as now 

 come down from the mountains of Alaska, Patagonia, and at 

 no very distant date from those of New Zealand, are sufficient 

 to account for the facts. 



Returning to the point under discussion, it is proper to 

 remark that the conclusions here presented with reference to 



* See Lyell, " Principles of Geology," vol. i, pp. 203-210. 



