R. D. Old/mm — Essays in Theoretical Geology. 71 



If this bypothesis is true, the sections of deep borings near either 

 limit of the alluvium should show diverse results as to the relative 

 j)roportion of fine to coarse-grained deposits near the surface and 

 further down. On the north, the southward march of the edge 

 of the bills would lead to coarse-grained deposits extending over 

 finer, while near the southern margin the increasing distance from 

 the edge of the rock area should lead to fine-grained deposits being- 

 laid down over coarser. 



The deep borings, wbich have been put down in the Gangetic 

 alluvium, are few in number, and, except two, are not well situated 

 for testing the hypothesis. 



The first of these is the boring made in Fort William, Calcutta, 

 in 1836-38. In discussing this and the other borings I shall adopt 

 the method of classifying the beds passed through as "sand" and 

 "clay." A more perfect classification is impossible, owing to the 

 vagaries of nomenclature indulged in by the men, never trained 

 geologists, to whom the sinking of the borings must necessarily 

 be entrusted. By classifying "sand," "coarse sand," "clayey 

 sand," etc., as sand; and "clay," "silt," "sandy silt," "limey 

 silt," etc., as clays, we shall get a very fair general idea of the 

 relative coarseness of grain of the beds passed through in different 

 parts of the same boring. 



Adopting this system of classification, we may make an abstract 

 of the Fort William boring ^ thus : — 



sand clay 



100 



11 89 



95 5 



98 2 



81 



The increase in coarseness of grain of the beds passed through is 

 conspicuous enough in this abstract, but the reality is even more 

 striking, for, in the sand from 180 feet downwards, some beds of 

 gravel and pebbly sand are included, and the boring was finally 

 brouo-ht to a standstill in a bed of gravel which it was not found 

 possible to penetrate. 



The second boring of importance is that made at Umballa.* 

 Adopting the same broad classification of clay and sand, we get the 

 following: result : — 



Here we have, as the hypothesis requires, a very distinct increase 



1 For detailed section see Eec. Geol. Surv. Ind. vol. xiv. p. 221 (1881) ; Calc. 

 Journ. Nat. Hist. vol. i. p. 324 (1841). 

 ^ Kec. Geol. Surv. Ind. vol. xiv. p. 233. 



