526 Correspondence — Prof. Ernest H. L. Sclucarz. 



Chalk in the cliff hereabouts, and the cliff itself is much fissured 

 and crevassed. The beds dip towards the north-east (approximately). 



" • ^Sand anxC'Gravel 



floor of Quarri/ 



The elevation of the church at Trimminghara is marked in the old 

 map as 195 feet, and the top of the cliff here is from 40 to 50 feet 

 lower. W. H. Hudleston. 



Note. — The mass of Chalk forming the arch (see PI. XXVII) was only entirely 

 isolated and detached from the adioining cliff early in this year. (See Mr. Brydone's 

 Plates, Geol. Mag., 1906, PI. II, Figs. 2, 3 ; PI. IV, Fig. 7 ; PI. V, Fig. 12 ; 

 PI. VIII, Fig. 13. See also description by Prof. Bonney in Sept. No., pp. 400-403, 

 Fig. 1, A, and Fig. 2.)— Edit. Geol. Mag. 



THE THICKNESS OF THE CIRCUM-POLAR ICE. 



Sir, — Your reviewer of the first volume of Professors Chamberlia 

 and Salisbury's Geology has the following sentence on p. 376 : 

 "We note that the thickness of the Greenland ice-dome at its centre 

 is estimated at 5,000 feet or more, and we recommend the statement 

 to the attention of the writer in this Magazine (March, 1906, p. 120) 

 who has recently, on hypothetical grounds, revived the idea that ice 

 cannot attain a greater thickness than 1,600 feet." May I again point 

 out that the 5,000 feet ice-sheet is a pure assumption, w^hereas the 1,600 

 feet limit rests on physical experiment and direct field observation? 

 Professors Chamberlin and Salisbury's statement, which your reviewer 

 refers to with such satisfaction, is as follows: "The height of the 

 land surface beneath [the ice-cap] is unknown, but it is unlikely 

 that it averages half this amount [9,000 feet], and hence the ice is 

 probably 5,000 feet or more thick in the centre. There is reason to 

 think it is much thicker in Antarctica." This is simply an appeal to 

 ignorance ; and as regards the Antarctic, Captain Scott's observation 



