AI^NIVERSAEY ADDEESS OF THE PRESIDENT. 6 1 



Yorkshire. In the Upper Coal-measures, and in the Lower New 

 Eed, the wearing is greater, and approaches that of the New Red. 



As an example of extreme wearing, I w^ould refer to the blown 

 sand of the Egyptian desert, in which almost all the grains are 

 rounded, evidently on account of the material having been drifted 

 about by the wind for a long period. 



I very much regret that I have not been able to treat this ques- 

 tion of the wearing of sand in a more satisfactory manner, but yet 

 hope that what I have said may, at all events, serve to prove that 

 we may thus learn whether sand is of recent and comparatively 

 local origin, or very ancient and transported far from its original 

 source by drifting along the bottom. 



Fractured Grains. 



We now come to another kind of mechanical change, viz. not 

 wearing, but fracture. That this does occur is clearly shown by 

 the occurrence of well-worn rounded grains which have been 

 broken across, and by the occasionally large number of true 

 splinters and chips. Still, on the whole, the amount of change 

 due to fracture does not appear to be great, and it is only in a 

 few instances that it becomes an important feature of grains y-J-Q 

 of an inch in diameter. The most striking example I have seen is 

 a specimen from the Greensand near Aylesford, which appears to 

 indicate some unusually violent local action. 



Corroded Sands. 



As I have already said, the quartz grains in decomposed granite 

 appear as if corroded. Many specimens of sand also show what 

 is probably only this original corrosion ; but sometimes it appears 

 to have taken place after the grains had been worn and deposited. 

 In typical examples the surface has a peculiar texture, differing 

 from that due to mechanical wearing or to crystallization. I am by 

 no means sure that occasionally corrosion has not taken place in 

 one part and deposition of quartz in another, even in the case of 

 individual grains. The extent of the corrosion, however, is usually 

 very small, and the irregular solution of y-oijo part of an inch from 

 the surface would explain all the appearances. That well-rounded 

 grains in porous sandstones may retain perfectly all the original 

 characters, though water could, and probably has, largely passed 

 through the rock, is clearly proved by some specimens of the New 

 Eed. Considering that we could easily detect the irregular re- 

 moval of j-o^xFo' o^ ^^ moh, the solubility of quartz in water must 

 therefore be so small that it may well be said to be insoluble. 

 Perhaps, when corrosion has occurred, it is evidence of the former 

 presence of water rendered alkaline by the decomposition of asso- 

 ciated felspar. 



The best examples of corrosion that I have seen are from the 

 saud below the Maguesian Limestone at Conisbrough, from 



