192 Correspondence — Mr. A. Harker — Rer. A. Irving. 



SPACE BETWEEN GRAINS OF SAND. 



Sir, — In his article on Miniature Domes in Sand, Mr. Mellard Eeade 

 compares (p. 22) the interstitial air-space between grains of sand with 

 that between small round shot of uniform size. The former he finds 

 by an experiment to be about five-twelfths or •4167 of the whole 

 volume, the sand therefore occupying seven-twelfths or '5833. The 

 proportion of space occupied by the round shot he supposes to be the 

 ratio of a sphere to its circumscribing cube, that is, •5236. This 

 assumes that the shot arrange themselves in " squai-e order," that is, 

 adjacent shot have their centres at the corners of a cube and each 

 shot touches six others ; but in reality they would be more closely 

 packed, falling into what may be called " pyramidal order," in which 

 the centres of adjacent shot are at the corners of a regular triangular 

 pyramid and each sphere is in contact with twelve others. The shot 

 will then occupy -7405 of the whole volume, leaving only "2595 for 

 air. Sand is therefore much less compact than small round shot ; 

 this is probably due partly to the inequality in the sizes of grains of 

 sand, but mainly to their irregular shape. Perhaps the degree 

 of compactness of any particular kind of sand, determined as in 

 Mr. Eeade's experiment, might serve as an index of how far the 

 grains have been rounded by attrition. A. Harker, 



St. John's College, Cambmdge. 



THE PEEMIAN-TRIAS QUESTION. 



Sir, — Will you oblige me with space for one or two brief remarks 

 by way of a rider to M. Marcou's paper on the '•' Permian-Trias 

 Question," in the March Number of the Geol. Mag.? The letter of 

 mine in the January Number, to which M. Marcou refers, was in- 

 tended merely to point out that although the name "Permian" might 

 possibly continue to be of value as a local name for the rocks of 

 that age in the Eussian area, it was not only undesirable, but even 

 misleading, as a term for Europe in general. Of course, if, by general 

 consent, the great cuprifei'ous series of sandstones and marls, which 

 overlie the true Dyassic strata in European Eussia, be assigned to 

 the Trias, the name "Permian" ceases to have any accurate meaning 

 even for the Eussian series. The only question to my mind is as to 

 the true Triassic age of those sandstones and marls. I suppose that 

 Ludwig, d'Eichwald, and others to whom M. Marcou refers, have 

 seen their way to the elimination of the difficulty presented by the 

 Palaeozoic facies of the few plant-remains that are found in the 

 cupriferous sandstones and marls (= Murchison's 'Upper Permian'); 

 but until this difficulty is removed, it seems safer to regard these 

 strata (which have no equivalents in Britain or Central Europe) as 

 a transition-series between the Dyas and Trias. This idea, to say 

 the least, ought not to be overlooked in any future mapping of the 

 Eussian area. A. Irving. 



"Wellington College, March 9th, 1884. 



