Stcgh Miller — Formation of Rock- Basins. 453 



The tendency to proceed witli a rock-basin once fairly begun 

 explains a difficulty stated before by Mr. Bonney, and now reiterated.^ 

 ''The great majority of Alpine valleys," he says, "show no tendency 

 to lake-basins, in places near, but above, the present lakes. . . . 

 Surely the glacier would have ' tried its prentice hand ' now and 

 then before, for example, excavating Como." Prof. Eamsay long 

 ago represented a vale-confined glacier as lacking opportunity to 

 scoop imtil it reached the plains. Of that I cannot judge; but by 

 the vi^w now set forth, when once fairly prenticed, the glacier 

 would be bound to respect its indentures, and make the best of the 

 groove it had got into. 



On rocky plateaux, such as the Norwegian "FJeld," especially 

 among gneisso-schistose rocks, tarns of a size and depth probably 

 admitted to be within the compass of the glacial, are often found 

 linked together by shallow straits (generally constricted by the 

 harder rock) into straggling sheets of water that might deserve the 

 name of lake. These furnish an argument relevant to the present 

 question, and not, to my knowledge, before used. Ko valid objec- 

 tion can be urged against the admission of such combinations of 

 tarns to the glacial list that does not equally apply to the tarns 

 singly. But this admission can hardly come alone. The harder 

 ridges separating the tarns were, so to speak, accidents in the strata, 

 breaking up a body of scooping force into detachments, and, if 

 absent, a lake of one basin would exist, instead of the straggling 

 loch compounded of several. The former are found to graduate into 

 the latter, and cannot, I hold, be excluded without proof from the 

 same category. I am far from disputing, however, that to include 

 all lochs seemingly of these characters would be to commit mistakes. 



On the grounds I have adduced, and upon others, it appears to me 

 that no halting-place can logically be found by those who, with Sir 

 Charles Lyell, allow only "some mountain tarns" ^ to Prof. Ramsay's 

 demand for lakes : that either a glacier is inadequate to hollow 

 even a tarn, or that it can enlarge it to any reasonable proportions. 

 But it is almost unnecessary to remark that, though area cannot (within 

 moderate limits) be argued against the theory, proportion between 

 depth and area might. A lake 50 miles long and 2000 feet deep 

 may be proportional in its gradients to a tarn half a mile long and 

 20 feet deep ; and a glacier forced through the latter would — other 

 things being proportional — have one hundred times as much extra 

 inducement to scoop as in the former ; but could such a monstrosity 

 exist as a half-mile tarn 2000 feet deep, it might be proved that the 

 ice, once in, must lie imprisoned and powerless. 



Having thus imperfectly expressed some views on the point 

 which Mr. Bonney accuses me of "missing" in my letter, I may 

 pass to the second part of his criticism. I can perhaps best rectify 

 the injustice I did myself in naming " sharp synclinals," even in 

 connexion with a generalization, by illustrating my meaning further. 



^ I am not versed in the proprieties of controversy, but it may be right to apologize 

 to Mr. Fisher for taking up this point, as it belongs more directly to him to answer. 

 2 Student's Manual, 1871, p. 164. 



