428 Correspondence — Mr. J. A. Birds. 



erosive power? I have shown (Quart. Journ. Geol. Soc. xxvii. 312 ; 

 xxix. 382 ; xxx. 479) that in several districts of the Alps there is 

 evidence that the glaciers have descended important valleys, filling 

 them almost down to the level of the present torrents, yet have been 

 incompetent to modify their principal features, which are most 

 characteristically those of fluviatile erosion. This argument, I venture 

 to assert, has never been met. Every year that I travel gives me 

 fresh instances, and during the present summer I have met with one 

 or two other curious facts bearing on the subject of glacier erosion, 

 which I hope to be permitted to lay before the readers of this 

 Magazine in a month or two. 



I have thus ventured to indicate some of the reasons why Mr. 

 Goodchild's arguments fail to convince me. If they seem rather 

 curtly stated, I must ask him to believe it is because I am trying to 

 discuss in a letter a subject which requires a lengthy article. 



T. G. Bonnet. 



St. John's College, Cambridge, Aug. 9th, 1875. 



THE POST-PLIOCENE FOKMATIONS OP THE ISLE OF MAN. 

 Sik, — Will you kindly allow me space for a brief rejoinder to the 

 articles by Mr. Home and Mr. Kinahan in the July Number of the 

 Geological Magazine ? 



1. Allowing, as Mr. Home says, that intercalated beds of sand, 

 gravel, etc., are of common occurrence in the Lower Boulder-clay, 

 still I cannot see that this entirely destroys the force of the argument 

 a priori, that they would probably be of more frequent occurrence 

 in a deposit like the Upper Boulder-clay, which was formed when 

 the cold was less severe, and warm seasons oftener to be expected ; 

 and therefore that the highest beds in the Isle of Man, which Mr. 

 Home considers Lower, are, so far, more likely to be Upper Boulder- 

 clay. 



2. Although it may be true that the glaciers of the post-sub- 

 mergence period were confined mainly to the upland valleys, and 

 therefore that moraines might be all the memorials to be expected of 

 them, still the sea, both before and after the second continental period, 

 must have contained ice in sufficient quantity to produce a thick de- 

 posit of clay, such as in Lancashire, for example, is found extending 

 from an elevation of above 1000 feet to the cliffs on the sea-coast 

 (see Geol. Survey Map 91) ; and it was principally to marine coast- 

 ice, and not to glaciers, that I attributed the Upper Boulder-clay in 

 the Isle of Man. 



3. No doubt Mr. Home is right as to the general characteristics 

 of Lower Boulder-clay in South Scotland, viz. that it is a tough clay 

 with an abundance of ice-marked stones, without stratification, and 

 without shells. But even this true Lower Boulder-clay, or Till, 

 varies according to the nature of the rocks from which it is derived, 

 and it was with the Lower Boulder-clay as it appears in the cliffs at 

 Blackpool, and not in Scotland, that I compared the deposits which 

 I have taken to be such around the point of Ayre. If, however, they 

 should prove not to be Lower Boulder-clay properly so called, nor 



