Correspondence — 0. W. Lcunplugh. 571 



In regard to the line {cf) in ray sketch (the basement bed, 

 according to Mr. Brydone, of the grey chalk) and my missing it on 

 the opposite side, I can only plead the difficulty of seeing what one 

 believes to be non-existent, but must confess that I did not 

 understand him to mean that the 0. lunata chalk had such 

 a curiously irregular surface as he assigns to it in his last letter ; 

 that, however, in my opinion, only increases the difficulties in his 

 hypothesis of an intra-Cretaceous unconformity. As this hypothesis 

 appeared to me (as it still does) a fundamental one, and the 

 other evidence insufficient to overcome its inherent improbability, 

 I considered myself justified in limiting my criticism to the questions 

 which lay within my more special field of work, and am now content 

 to await further developments as the sea continues its inroads. 



T. G. Bonnet. 



9, ScROOPE Terrace, Cambridge. 



THICKNESS OF LAND-ICE. 



SiE, — T have just sufficient acquaintance with your reviewer of 

 Charaberlin & Salisbury's Text-book, vol. i, to be able to discuss 

 what was in his mind in penning the sentence to which Professor 

 Schwarz takes exception in your November number, though I shall 

 not venture to defend his gratuitous interjection of a reference to 

 Professor Schwarz's views on the occasion in question. 



Professor Schwarz claims that certain physicists have proved by 

 calculation that ice cannot attain a greater thickness on the earth's 

 surface than 1,400 to 1,600 feet, and with implicit faith in this 

 calculation he seeks to reconcile the result with the geological 

 evidence. The reviewer, however, probably lacking sufficient 

 knowledge of physics to criticize the calculation, and being also 

 doubtful whether the result is one on which all physicists are 

 agreed, has fallen back upon the available geographical and geological 

 evidence, and on this evidence alone has felt no hesitation in 

 rejecting the postulated limits. He has, no doubt, considered that 

 the Greenland ice-sheet, as described by Peary, must at its maximum 

 far exceed the thickness allowed by these physicists; and he 

 probably also still believes that the Antarctic ice in the valleys 

 of the interior surpasses this limit, in spite of the ingenious 

 argument of Professor Schwarz as to the progressive deepening of 

 such valleys. 



Then, as regards bygone glaciation, the reviewer perhaps 

 remembered the glacial phenomena in British Columbia, where 

 there is every indication that ice-sheets have filled pre-existing 

 valleys to a much greater depth than 2,000 feet; and he may have 

 recalled the conditions in the north-eastern portion of the United 

 States, where the uplift of boulders in the Adirondacks, if due, as 

 usually believed, to land-ice, must imply a thickness of ice on the 

 Canadian lowland far exceeding the supposed limit. 



Or without going so far afield, he may have had in mind the 



