602 



Scientific Proceedings, Royal Dublin Society. 



abundant deep canadones and river-valleys, the sides of which ai-e steep 

 and crowned with sheer cliffs of basalt. In fact, in every place there 

 is evidence of powerful erosion having taken place during comparatively 

 recent times, so far as geological history is concerned. Now, on going down 

 these rivers and cafiiadones one very pregnant fact is noticed, that is, that no 

 basaltic boulders of any considerable size are found to have travelled far from 

 the parent tables. In fact, when one goes more than two or three miles, one 

 never comes across a piece of basalt larger than can be lifted in one hand. 

 This fact would lead one to believe that whatever action cut down these 

 caiiadones, &c., it was not a large glacial one. If there was any glacial action, 

 as I hope presently to show, it was of a mild kind, which, although it may 

 have had some cutting power, had no very great transporting influence. The 

 accompanying figure represents the Eiver Gallegos a little further down. 



?'i';:A^< 



Denudation of volcanic masses in the Rio Gallegos Valley. 

 \_Prom a photograph by the author.'] 



Point A is the remains of a volcanic cone which has been cut down in the 

 middle, and falls sheer into the river. B is the river-valley, and point C is a 

 volcanic table consisting of basaltic rock, poured out in the first instance 

 probably by A, with which it was originally continuous. The gap between A 

 and C is about half a mile, and the summit of A would be about 800 feet above 

 the river. The volcanic table C is more or less elliptical in shape ; looking at 

 it from above, it is cut clean down sharply on both sides, and is bounded on the 



