290 DR. A. GILLIGAX OX THE PETROGRAPHY OP [vol. IXXV, 



they consist of the same minerals as the coarser beds, only in a 

 finer state of subdivision, then mechanical agencies have been 

 dominant in causing the disintegration of the original mass. I 

 have applied this test to the shales of the Millstone Grit, and 

 find that they are composed in a large part of mica with which are 

 associated finely-divided quartz and felspar and an abundance of 

 heavy minerals, such as zircons and rutile, in beds that do not 

 carry marine fossils. These latter are quite different from the 

 normal shales, so that in an exposed section it is possible for any- 

 one acquainted with these marine shales to identify them at once, 

 on account of their exceedingly fine texture, quite different 

 from the normal shales, and on analysis they yield a greater 

 amount of water, pointing to their composition being much more 

 like that of kaolin than the normal shales. Was it glaciation or 

 insolation which was more active in Millstone Grit times ? There 

 is no evidence of the former, while everything points to the latter 

 as being extremely probable, and to this opinion I strongly incline. 

 It may be. as previously pointed out, that the height of the old 

 continental land was such as to nourish an icefield in the same 

 way as at present; Ave have such conditions in the Himalayas. If, 

 however, the high mountains were bordering the sea during the 

 earlier part of the period, as would appear to have been the case, 

 in the area under review at least, then their flanks facing south 

 would have been kept free of ice by the proximity of the warm 

 water laving the shore. As the deltas grew seaward the distance 

 between the ocean and the foothills (which were continually decreas- 

 ing in height by the intense denudation that the} r were undergoing) 

 would be increasing, and so the climate of the same tract of 

 country would be passing from one approximating to an insular 

 climate to the continental type. At the same time, with the decrease 

 in height of the hills, the area above the snow-line would decrease, 

 especially near the shore-line, where the effects of denudation 

 would be felt most severely. It seems to follow, therefore, that 

 the part played by ice was a quite subordinate one, and that it was 

 confined to the work of an icefield or several isolated icefields, the 

 descending glaciers from which never reached very far down the 

 mountain-valleys. 



In conclusion, I wish to express my sincere thanks to those who 

 have so kindly assisted me in many ways. To Prof. T. G-. Bonney, 

 Prof. H. L. Bowman, Prof. W. C. Brogger, Dr. Herbert H. 

 Thomas, and Mr. George Barrow, for help in the identification of 

 roeks and minerals; also to Dr. J. Home, Mr. J. Holmes, and 

 Mr. G. F. Pickering, who have generously supplied me with 

 specimens of rock and of pebbles. To Prof. P. F. Kendall I 

 express my thanks for his interest and helpful advice on many 

 occasions in the long period during which the work has been in 

 progress. 



