22 MR W. F. P. M'LINTOCK ON THE ZEOLITES AND ASSOCIATED MINERALS 



augite. It is difficult to determine whether the albite of the laths is primary or 

 secondary in the sense that it is the albitised representative of a plagioclase originally 

 more basic. There can be little doubt, however, that the clear compact patches were 

 most probably deposited originally as albite, and, as they contain crystals of augite, 

 no long period can have elapsed between the formation of the two minerals. The 

 presence of this albite points to a transition from igneous crystallisation to a hydro- 

 thermal stage which would favour the formation of alkali-felspar* and also of the 

 chlorite present in the pegmatitic layer. 



During this hydrothermal stage the infilling of the vesicles was completed, the 

 constituent minerals being derived partly from the residual material of the magma 

 and partly from the breaking down of the igneous minerals which had already con- 

 solidated. In some of the chlorite and epidote we see the hydrothermal representa- 

 tives of augite, a conclusion supported by the occasional occurrence, as noted above, 

 of augite crystals in the cavities partly replaced by epidote, whilst the labradorite 

 finds its counterpart in some of the albite, scolecite, and prehnite. It is evident, 

 however, that whilst hydrothermal crystallisation of the residual magma accounts 

 for the formation of a portion of the contents of the vesicles it probably does not 

 account for them all. Some of the vesicles, especially the smaller ones, are packed 

 with chlorite containing a kernel of epidote, or albite, or both ; but the great 

 majority contain a proportion of scolecite much larger than that of any other con- 

 stituent. This fact can be accounted for by the albitisation of the felspars and the 

 partial conversion of the augite of the rock to chlorite and hornblende, each of 

 which changes liberates lime. 



Albitisation is now known to be a fairly common occurrence in the igneous rocks of 

 this country,"}" but the nature of the process seems to be imperfectly understood. 

 In the cases cited by Bailey and Grabham, it is a pneumatolytic phenomenon ascribed 

 by them to the presence of carbon dioxide (or some other unknown constituent) 

 in the original magma, which thus retained a large amount of soda in solution. The 

 liquor then began to act on the felspars and albitised them. They also record the 

 association with the albite of chlorite and epidote, the latter mineral representing 

 some of the lime derived from the felspar ; in most cases, however, the lime has 

 been carried off in solution. Dewey and Flett also ascribe the albitisation of the 

 felspar of the spilites to pneumatolytic action. J 



At Maol nan Damh various considerations point to the albitisation having been 

 a pneumatolytic change which took place during the cooling of the lava and marked 

 the first step in the filling of the vesicles. This view gains support from the rarity 

 of calcite and other oxidation products, which excludes the assumption that the 

 change may be due to weathering ; there is also the additional fact that rocks of 



* Cf. F. W. Clarke, Data of Geochemistry, 2nd edition, 1911, p. 348. 



t Cf. E. B. Bailey and G. W. Grabiiam, "Albitisation of Basic Plagioclase Felspars," Geol. Mag., vol. vi, 1909, 

 pp. 250-CG ; II. Dewey and J. S. Flett, "On Some British Pillow-Lavas," ibid., vol. viii, 1911, p. 246. 

 1 hoc. cit , p. 204 



