Correspondence — Rev. T. O. Bonney. 377 



river erosion, only very superficially modified by glacial action. 

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

 then before, for example, excavating Como. 



I demur to Mr. Fisher's statement that if a glacier would be 

 competent to deepen a lake basin, it could no doubt originate it. 

 This may be only to say, "if a thing can be done under very 

 favourable conditions, it can be done under all conditions," which, 

 as it seems to me, is not a safe conclusion. Besides, if a basin exists 

 into which the glacier descends, What made the basin f I may 

 grant that a whetstone sharpens a razor, but doubt whether it is 

 usually the tool with which razors are made. 



With regard to the latter part of Mr. Fisher's letter, the hypotheses 

 which he advances are such as it is almost impossible to disprove ; 

 for it is very difficult to understand what would be the procedure of 

 the subglacial water in a lake-basin. I do not, however, think that 

 under the circumstances there would be much abrading action 

 exercised by the water (below the level of the rim of the basin) 

 which is passing between the rock and the ice. A subglacial stream 

 usually either drills out a tunnel through the ice or furrows a 

 channel in the rock below, so that its erosive action is limited to a 

 small area ; what it would do in the case of a lake-basin I can hardly 

 say. Possibly it might continue to act in the same way, but if it did 

 not, and a layer of water were introduced between the rock and the 

 ice, throughout the basin, I imagine there would be little motion in 

 this, and it would rather be unfavourable to denudation and 

 favourable to accumulation of sediment. 



With regard to Mr. Hugh Miller's letter, I may remark that he 

 misses the point that I, and I think that I may venture to say my 

 friend Mr. Judd, have always maintained — viz. that because glacial 

 erosion may seem the simplest explanation of certain tarns, therefore 

 it is to be applied to certain lalces. Further I may remark that the 

 infrequency of sharp synclinals does not militate against the sub- 

 sidence theory of lakes. Those who uphold this theory do not re- 

 quire sharp synclinals, as Mr. Miller will find if he will draw the 

 lakes on a true scale. He has forgotten an argument often used by 

 his friends. T. G. Bonney. 



St. John's College, Cambridge, June 12, 1876. 



TRUE AND APPARENT DIP. 



Sir, — Mr. Hill is a decided improvement on Mr. Penning, and I 

 think I can improve a little on Mr. Hill. There are many excellent 

 geologists to whom Trigonometry is a sealed book, and who not un- 

 naturally look upon sines and cosines, tangents and cotangents, with 

 a mixture of suspicion and dislike. But all geologists know what is 

 meant when the dip of a bed is said to be 1 in 6 ; and it will remove 

 the alarm which trigonometrical symbols are apt to raise in the 

 minds of non-trigonometrical geologists, to be told, that, if a bed 

 dips 1 in 6, the cotangent of the angle of dip is 6. 



Bearing this in mind, Mr. Hill's construction may be thus simply 

 expressed : — 



