GEOLOGY. 273 



of a sheet of ice, they would have been placed as they are; for 

 when in course of time the ice began to wane, it would lessen in 

 thickness nearest the prominent points underneath, and would 

 gradually melt away from them, and drop tlie bouldei's on their 

 summit, and in time leave them firmly stationed, away from its 

 action. He concluded, then, from these facts that, at one time, in 

 that now troijical region, there was an immense sheet of ice mov- 

 ing over the valleys and mountain peaks, and that gradually it had 

 melted away, leaving its marks and tracks behind. 



On the supposition that the valley of the Amazon was once filled 

 by an immense glacier, the horizontality of the strata would be 

 well explained. At first the ice melted slowly, with still water 

 underneath, with a deposit of the finest materials ; as the ice melted 

 more rapidly, the under-current became stronger, with a more 

 distui'bed deposition of coai'ser materials. A moraine was also 

 found on the southern side. These deposits could not have been 

 made by the sea, nor in a lar'ge lake, as they contain no marine 

 nor fresh-water fossils. 



From the facts developed, the conclusion bad been reached, 

 that there was a time when not only the northern and southern 

 hemispheres, and the temperate zones, were covered with fields of 

 ice, but when the phenomena extended over the trojjical regions. 

 It might be said that one proof of the phenomena was wanting, 

 for nowhere had he been able to trace the polished rocks. But 

 then, nowhere had he seen rocks which had not been more or less 

 decomposed, owing to the action of moisture and heat; so he 

 could not say that he had in any case seen the natural surface of 

 the rocks, and therefore it could not be wondered at that the evi- 

 dence of attrition was wanting. The other collateral evidence is 

 full, and as extensive as in the northern and more temperate 

 regions. 



