336 Correspondence — Mr. Henry Hicks. 



Nicholson in the same Number of the Magazine, p. 246 : — ''It is 

 highly satisfactory to me to know that Dr. Linnarsson's investigations 

 fully corroborate the view always maintained b}^ Prof. Harkness and 

 myself on this matter — namely, that the Coniston Mudstones are in- 

 dubitably inferior to the true Upper Silurian rocks." In the same 

 pajjer he mentions several facts in support of this view, and I 

 certainly think the evidence at present tends strongly to show that 

 these mudstones are considerably lower in the succession than Mr. 

 Aveline would have us believe. The Coniston mudstones, flags, and 

 grits collectively appear to represent the Upper and Lower Llando- 

 very rocks of part of Wales, and jDrobably also the Tarannon shales 

 and Denbighshire grits. These latter are not, in my opinion, to be 

 placed altogether above the Llandovery rocks, but to a great extent 

 to be considered their equivalents. That difficulties will continually'' 

 arise in any attempt that is made to correlate these rocks in dif- 

 ferent districts there can be no doubt, and the reason is obvious 

 when we recollect how great the changes were which took place at 

 the time these were deposited. In the May Number of the Geol. 

 Mag., pages 215 and 216, I attempted to explain what the result 

 of necessity would be in those areas which were undergoing these 

 great physical changes, and the effect on the deposits, and the order 

 of succession. It is tolerabl}' clear that the uplifting of the sea- 

 bottom in Cumberland, and in North Wales, occurred after the 

 Up23er Bala and Coniston Limestone had been deposited, for the suc- 

 cession is perfectly clear in both areas up to that point. The up- 

 lifting which now occurred raised the sea-bottom above water-level 

 in parts of those areas only, and to unequal heights. Hence portions 

 remained submerged, some became shore-lines and shallows, and 

 others were raised sufficiently to become dry land. It is clear, 

 therefore, that as these parts were again gradually submerged, sedi- 

 ments were laid dow^n more or less unconformably upon the uplifted 

 portions, and that the character of the sediments and the thicknesses 

 would vary greatly even within very narrow limits. The denuda- 

 tion of these uplifted portions would also furnish abundant material, 

 and so the sediments would be heaped up rapidly in the surrounding 

 sea. In the Welsh area the Lower and Upper Llandovery rocks of 

 South Wales were heaped up at this time, and as this part continued 

 under water during the whole period, the sediments there show 

 perfect conformability throughout. But as we ' advance nearer 

 towards the uplifted portions, the series gradually thin out, and an 

 unconformity may show itself at almost every point in the succession 

 up to near the close of the Upper Silurian. The Tarannon shales of 

 the extreme North of Denbighshire and the Denbighshire grits, 

 being entirely the result of the denudation of these islands, were 

 also heaped up chiefly at the time the Llandovery rocks were de- 

 posited in South Wales; consequently, for the most part they may be 

 correlated with them. The same order in the deposits appears to be 

 shown in Cumberland, and I entertain no doubt but that at some 

 part of that area the same general succession may be recognized. 

 Heeiot House, Hendon. Henky HiCKS. 



