376 Correspondence — Mr. W. T. Aveline. 



Geologists will probably look to the author to show that, as he has 

 proved the inadequacy of the contractional theory, as usually under- 

 stood, so he will establish by some calculation, based on probable 

 values of the thickness of rocks affected, and of the changes of 

 density due to hydrothermal action, that the inequalities of the 

 earth's surface are not of greater magnitude than his theory would 

 account for. 



COI^I^IEJSIPOZsriDIBISrOiH]. 



THE SILUEIAN ROCKS OF THE LAKE DISTRICT. 



Sir, — My time at present is too much occupied with my official 

 duties to answer Mr. Hicks's letter in the last Geological Mag. at 

 any length. Besides, the results of my survey of the Silurian Eocks of 

 the Lake District must be recorded on the maps and in the memoirs 

 of the Government Geological Survey. But I should like to know 

 from Mr. Hicks if it is from the fossil evidence or from a careful 

 stratigraphical survey of the rocks of the Lake District that he speaks 

 so confidently of their position. If from the former, there is the 

 danger of arguing in a circle. Certain beds were supposed to be 

 Lower Silurian, therefore the fossils collected from these beds were 

 considered Lower Silurian fossils ; and so now these beds are called 

 Lower Silurian, because they contain Lower Silurian fossils. But 

 if a careful stratigraphical survey went to show that these beds were 

 connected with the Upper and not the Lower Silurian, the fossils in 

 these beds would no longer be characteristic of Lower Silurian. 



I feel confident that the Stockdale shales of the Lake District are 

 the equivalents of the Tarannon shales of Wales, and the Coniston 

 grits and flags equivalent to the Denbighshire grits and flags, and 

 that the Tarannon shales of Wales overlie the Llandovery rocks. 

 H.M. Geol. Survey, Kendal, W. Talbot AvELlNE. 



July 8, 1876. 



THE EROSION OF LAKE-BASINS BY GLACIERS. 



Sir, — Some of Mr. Fisher's interesting remarks on the excavatory 

 action of glaciers seem to call for a few words of reply. It is 

 quite true that the " possibility of the erosion of a great lake by a 

 glacier has not been disproved," and that observations made on the 

 Swiss glaciers now cannot do it ; but these observations (and that is 

 all I have ever contended) may render the theory probable or 

 improbable. I doubt whether, in the sense in which we both should 

 use the word, 'proof would ever in any case be possible. Mr. Fisher, 

 however, seems not to have apprehended the point on which I have 

 always laid most stress in my reasoning (which did not fall within 

 the scope of my last communication to you), viz. that the great 

 majority of Alpine valleys show no tendency to lake-basins, in 

 places near, but above, the present lakes ; where the conditions of 

 glacial action must have been as nearly as possible the same as over 

 the area of the basin, that is to say, that if you ascend a valley from 

 the head of a lake, you often find it throughout a true valley of 



