Geological Society. 503 



will at once meet with the general concurrence of geologists ; and 

 he admits that the study of the phenomena of glaciers in different 

 latitudes, as well as at different altitudes, together with the exami- 

 nation of their different effects where in contact with the sea, will 

 introduce many modifications in the consideration of analogous 

 phenomena in countries where glaciers have disappeared ; but he is 

 prepared to discuss his theory within the limits of observed facts, 

 conscious of having searched for truth solely to advance the interests 

 of science. 



To avoid useless discussion, he states, that in attributing to the 

 action of glaciers a considerable portion of the results hitherto 

 ascribed exclusively to that of water, he does not wish to maintain 

 that everything hitherto assigned to the agency of water has been 

 produced by glaciers ; he only wishes that a distinction may be made 

 in each locality between the effects of the different agents ; and he 

 adds, that long- continued practice has taught him to distinguish 

 easily, in most cases, the effects produced by ice from those produced 

 by water. 



Proceeding to the consideration of facts, he says the distribution 

 of blocks and gravel, as well as the polished and striated surfaces of 

 rocks in situ, do not indicate the action of a mighty current flowing 

 from north-west to south-east, as the blocks and masses of gravel 

 everywhere diverge from the central chains of the country, following 

 the course of the valleys. Thus in the valleys of Loch Lomond and 

 Loch Long, they range from north to south ; in those of Loch Fine 

 and Loch Awe from north-west to south-east ; of Loch Etinc and 

 Loch Leven from east to west ; and in the valley of the Forth from 

 north-west to south-east, radiating from the great mountain masses 

 between Ben Nevis and Ben Lomond. Ben Nevis, in the north of 

 Scotland, and the Grampians in the south, are considered by the 

 author to constitute the great centres of dispersion in that kingdom ; 

 and the mountains of Northumberland, of Westmoreland, Cumber- 

 land, and Wales ; the hills of Ayrshire, Antrim, the west of Ireland, 

 and Wicklow, to be other points from which blocks and gravel have 

 been dispersed, each district having its peculiar debris, traceable in 

 many instances to the parent rock, at the head of the valleys. 

 Hence, observes M. Agassiz, it is plain the cause of the transport 

 must be sought for in the centre of the mountain ranges, and not 

 from a point without the district. The Swedish blocks on the coast 

 of England do not, he conceives, contradict this position, as he 

 adopts the opinion that they may have been transported on floating 

 ice. 



In describing the phenomena presented by erratic blocks and 

 gravel, M. Agassiz first insists upon the necessity of distinguishing 

 between stratified gravel and mud containing fossils, which could 

 not have been accumulated by true glaciers, although the materials 

 may have often been derived from them, and unstratified masses, 

 composed of blocks, pebbles, and clay. These stratified deposits 

 he considers to be of posterior origin to the glacier epoch. The till 

 of Scotland, or the great unstratified accumulation of mud and 



