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



third there is a small vesicle filled with chlorite, epidote, and clear albite with corroded 

 grains of garnet enclosed in the albite and epidote [66]. 



A difficulty arises in the interpretation of these occurrences, for, as noted above, 

 garnet is typically developed in the vesicles which have undergone metamorphism 

 due to the intrusion of the granophyre. As will be shown later, however, the 

 development of garnet in these circumstances is accompanied by other changes in the 

 vesicles, none of which is shown in the slides described above. It may also be noted 

 that the garnet in the Maol nan Damh slides always occurs practically at the walls 

 of the amygdale, and shows obvious signs of corrosion and replacement. On the 

 whole, the evidence points to the garnet and diopside being original vesicle-minerals, 

 probably amongst the first to be deposited, which became unstable and experienced 

 re-solution and replacement as the conditions changed and new minerals were de- 

 posited. Fenner records the similar occurrence of garnet in the Watchung basalt, 

 where it also shows signs of corrosion and replacement by later-formed minerals.* 



Thomsonite has not been detected in the vesicles on Maol nan Damh, but it occurs 

 on the neighbouring hill of Coire Bheinn, which lies about two miles to the west. A 

 section through one of the vesicles shows a thick layer of albite, which is only slightly 

 turbid, succeeded by yellow epidote (in places), then prehnite, and finally thomsonite. 

 The albite shows slight traces of corrosion against the epidote, but its boundaries 

 with that mineral, though often rounded, are sharp and well defined. The epidote 

 ends against the prehnite in a series of tufted aggregates and, when a plate of that 

 mineral adjoins both albite and prehnite, the contrast between the two junctions 

 is most marked. Albite shows corroded junctions with prehnite, which, in turn, has 

 irregular outlines with thomsonite. The sequence is obviously (1) albite, (2) epidote, 

 (3) prehnite, (4) thomsonite, and the earlier-formed minerals show signs of having 

 been unstable towards the conditions under which the later ones were deposited. 



III. Sequence of Events in the Vesicles. 



In his summary of the characters of the occurrence under consideration, Mr Currie 

 dwells upon the peculiar facies of lime-bearing minerals — a facies not recorded from 

 any other locality in Scotland nor, as far as I am aware, from any locality hitherto 

 described — and ascribes the origin of the vesicle-minerals to pneumatolytic action.f 

 He did not examine the petrographical characters of the lava, but inferred from the 

 minerals in the amygdales that it must have been originally a basalt containing a 

 basic plagioclase, a pyroxene (either rhombic or monoclinic), and magnetite or olivine. 

 Seasoning from the abundance of green earth, he also suggested the original presence 

 of biotite. 



It is clear from an examination of numerous sections that the rock is a typical, 

 ophitic olivine basalt, now, of course, in an altered condition and, neglecting mean- 

 * C. X. Fenner, luc cit., p. 138. t Loc cit., pp. 227-28. 



