TWO NEWLY-DISCOVERED WHIN- DYKES 339 



calcite and conserving the lime-felspars, especially the purer 

 ones. The differential attack of the bicarbonate solution is 

 clearly not determined by the amount of lime in the minerals 

 presented to its action, for the augite is certainly as rich in 

 lime as the felspar (see calculations, op. cit. p. 7): it is rather 

 to be ascribed to those other bases, ferrous iron and mag- 

 nesium, which, in the form of metasilicates, bulk so largely in 

 the composition of the augite and, like calcium, are capable of 

 forming bicarbonates readily soluble in water. In the case of 

 the lime-felspars, such determining factors of reaction are 

 wanting — hence their immunity from attack by solution of 

 calcium bicarbonate. The carbonic acid set free during the 

 conversion of calcium bicarbonate into calcium carbonate 

 would help to weather the rock by attacking the calcium 

 silicate present (1) originally in the augite and (2) formed by 

 double decomposition of calcium bicarbonate with the silicates 

 of magnesium and ferrous iron. 



There is thus considerable probability that both the Crook- 

 dene and the Hartley dyke have been similarly weathered by 

 percolating solutions of the same reagent, calcium bicar- 

 bonate, and it may be added that though about Crookdene, 

 limestone, from which such solutions could come, makes up 

 much of the country rock, the reverse is the case in the neigh- 

 bourhood of the Hartley Dyke ; and yet not only is the dyke 

 highly calcareous, but the fissures and joints for some distance 

 round it are usually filled with calcite. The supplies of 

 calcium carbonate then are either far-travelled, or possibly 

 they date from some past age when the overlying beds, now 

 removed by denudation, were rich in this mineral. The 

 occurrence of an outlier of Permian Yellow Sands, near to the 

 dyke, suggests that the infiltrated calcium carbonate is a relic 

 of Permian limestones long swept away. 



Evidence of a later phase of weathering of the dyke is 

 observable at its upper exposure, just below the top of the 

 cliff. Here the rock assumes a reddish hue and disintegrates 

 into roundish blocks having the exfoliating form characteristic 

 of many whin outcrops. Specimens of the shell-like coating 



