34° DR. J. A. SMVTHE ON 



of the blocks, about £-inch thick (I.), of the rust-coloured 

 layer i inch thick below this (II.), and of the bluish cores 

 (III.), were analysed with the following results : 



Table E. Upper exposure of Hartley Dyke. Determinations of carbon 



dioxide and iron (soluble in dilute acid) in the outer scale (I.), the layer 



beneath (II.) and the central cores (III.) of the weathered whin-blocks. 



I. II. III. 



C0 2 none none 10-15 



Fe (ferrous) none 0^36 5 - i2 



Fe (ferric) 2 , i6 3*32 i - 6o 



The rock at the cores (III.) is thus quite comparable with 

 the rock of the lower exposure, containing carbon dioxide 

 equivalent to 23 per cent, of calcium carbonate, and acid- 

 soluble ferrous and ferric iron in the proportion of 3 to 1. In 

 passing outwards, the calcium carbonate and ferrous iron are 

 quickly abstracted, until at the outer scale only the ferric 

 compounds remain. This phase of weathering is evidently 

 super-imposed on the one described above, and clearly corres- 

 ponds to the action of water containing carbon dioxide and 

 oxygen in solution. It is the one at present in operation. 



The first type of weathering described, namely that assumed 

 to be due to the action of calcium bicarbonate solution, is 

 evidently, so far as the few observations available enable one 

 to judge, the one to which many of the local dykes conform. 

 It is, however, not the only one, and, in fact, most of the 

 well investigated cases are of a very different character, and 

 hardly permit of safe inference as to the agents operative and 

 the mechanism of the processes involved. A few of the best 

 known cases may be mentioned in passing, in order to 

 draw attention to a subject well worthy of study but greatly 

 neglected. 



The sill connected with the Hett Dykes has been investi- 

 gated, though not from the point of view of weathering, with 

 great wealth of chemical detail by Sir Lowthian Bell.* The 



* "On some supposed changes Basaltic Veins have suffered during 

 their passage through and contact with Stratified Rocks, and on the 

 manner in which these rocks have been affected by the heated Basalt." 

 I. Lowthian Bell, Proc. Roy. Soc, 23 (1875), pp. 543-553. 





