1867.] Ice Marks in North Wales. 41 



It was about half-a-inile from an isolated granite mountain in 

 lat. 0° 30' N., long. 68° 50' W.* 



This observation becomes of considerable importance now that 

 Professor Agassiz tells us that he has found plain traces of glacial 

 action in the valley of the Amazon. That glaciers have ever 

 descended from the Andes to the Atlantic ocean, a distance of more 

 than 2,000 miles, will hardly be credited except on such overwhelming 

 evidence as even Professor Agassiz does not pretend exists. There 

 are not, however, the same difficulties in the way of the supposition 

 that icebergs once floated over what is now the great Amazonian 

 plain. A depression of 1,000 feet would sink the whole of that 

 plain deep under the ocean, and that such a depression has occurred 

 is rendered probable by the great extent and almost perfect level of 

 its alluvial deposits. Neither is it unreasonable to suppose that 

 during the glacial epoch of Europe and North America the tempera- 

 ture of South x\merica was so much lowered as to bring the line of 

 perpetual snow down to 12,000 or 13,000 feet. This would cause 

 a wide extent of the plateaux in South Peru and Bolivia to become 

 the feeders of glaciers, which might have been as much larger than 

 those of the Alps, as the comparative height and extent of the two 

 mountain systems would lead us to expect. Such glaciers descend- 

 ing the highly inclined Andean valleys would move with propor- 

 tionate rapidity, and might not improbably reach down into an 

 almost tropical climate and send off rock-laden icebergs into the 

 warm inland sea that then washed the base of the Andes. This, 

 however, is quite a digression from our present subject. 



On the very summit of Cader Idris there are several detached 

 eminences formed of large square and polygonal blocks, which in 

 some places stream down the slopes of the undulating surface of 

 the mountain top. Were they lower down we should at once pro- 

 nounce them to be moraines, but in their present position they are 

 somewhat difficult to account for. I think, however, there can be 

 little doubt bu L j that they are due to the action of the snow and 

 frost during the last portion of the glacial period. As soon as the 

 perpetual snow line reached the top of the mountain, and the per- 

 manent glaciers below had all melted away, there must have been a 

 long period during which the rocks on the summit were subjected 

 to the alternate action of ice, snow, and water. During the winter 

 they would be buried under many feet of snow, which would be ■ 

 forced into every crevice in the form of compact ice. During the 

 short summer the snow would melt from the surface, but the water 

 in the fissures would be probably frozen every night, leading to the 

 further fracture and displacement of the rocks. The pressure of 

 the snow and ice in the succeeding winter would force these always 



* See Wallace's ' Tavels in the Amazon and Rio Negro,' p. 219. 



