334 Correspondence— J. J. H. Tea//— Dr. J. W. Evans. 



Similar contortions may often be seen in glacial gravels and sands 

 and beneath boulder-clays. Near Spendon, Derbyshire, a mass of 

 Boulder-clay was contorted into the Keuper Marl and masses of marl 

 were enclosed in the Boulder-clay ; there were also striated boulders 

 of .Mountain Limestone deeply embedded in the red marl. 



6. Many of the disturbances in the Chalky Clay seem to have 

 been formed by the direct pressure of the ice front, others as at 

 Cromer by the overriding of the Boulder-clay by the ice. 



I agree with the Rev. 0. Fisher that the subject is worth 

 investigation, and feel sure that the question of how the disturbances 

 and folds were produced can be ascertained by studying the details of 

 the phenomena. One generally finds the subject dismissed with the 

 remark 'surface creep'. 



R. M. Deelet. 

 Inglewood, Lingcroft Avenue, 

 Harpenden. 



May 23, 1911. 



GEOLOGY OF PADSTOW AND CAMELFOED. 

 Sir, — Bef erring to the letter signed "Reviewer" which appeared 

 in the April number of the Geological Magazine concerning the 

 memoir on the Geology of Padstow and Camelford, I should be obliged 

 if you would allow me to say that I entirely agree with the statements 

 contained therein as to Mr. Ussher's priority in representing on 

 a map the three main divisions of the Devonian Rocks in the area 

 in question, and also to express my regret that no reference was made 

 to this fact in the memoir. 



J. J. H. Teall. 

 Geological Survey Office, 



28 Jermyn Street, S.W. 

 May 26, 1911. 



DEEIKANTEE, 



Sir, — With reference to the discussion in your columns on the use of 

 the word Dreikante, I should like to point out, as I have already done 

 elsewhere, that the term is more appropriately employed for the 

 comparatively common form with three long, nearly parallel edges, 

 than for the rarer type which is roughly tetrahedral and has typically 

 six instead of three edges. 



If a stone lies on a sandy tract, the wind may, by means of the sand 

 that it carries with it, bevel the upper portion of the side turned 

 towards it, and at the same time gradually remove the sand beneath 

 till the stone falls, turning over towards the wind on to its abraded 

 surface. A new plane of abrasion will then be formed on the stone, 

 making an angle of about 60° (a crystallographer would call it 120°) 

 with the first, and, under favourable circumstances, by the repetition of 

 the same movement a trigonal prismatic form with three parallel sides 

 and edges will be more and more distinctly developed. 



The tetrahedral or ' tripyramidal ' form and other more irregular 

 shapes would appear to be due to the stone falling over obliquely 



