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



scolecite, epidote, heulandite, stilbite, prehnite, etc., from Maol nan Damb, a spur of 

 Ben More running down to Loch Scridain. Mr Currie remarks upon the peculiar 

 assemblage of lime-beariuQ- minerals in the cavities of the lavas and the absence of 

 soda-bearing ones ; but, as I shall show later, albite is quite a common associated 

 mineral at this locality and at many others in the vicinity. 



Apart from the points of interest already indicated, there is the further one that 

 the lavas have been pierced by intrusive masses, and, as noted by Sir Archibald 

 Geikie, have suffered in consequence considerable alteration — an alteration which is 

 particularly prominent in the somewhat unstable minerals of the vesicles. In making 

 a traverse of the country around Ben More one is forcibly impressed by the fact that 

 for considerable distances around the plutonic centre the lavas differ entirely in 

 character from those of the plateau-country far removed from the zone of intrusive 

 rocks.* One of the most striking differences lies in the abundance of epidote in the 

 cavities and veins of the central lavas, and its absence or extreme rarity in the 

 basalts of the plateau. The central lavas, again, never show the spheroidal 

 weathering to a brown loam so characteristic of the plateau-basalts, whilst there is 

 the further difference, referred to by Mr Currie,! that the mineral association in 

 the cavities is entirely different from that found in the normal lavas of the plateau. 

 Different interpretations have been placed on these facts by Professor Judd and 

 Sir Archibald Geikie. 



In the memoir already cited, the former observer groups the central lavas under 

 the name of propylites, and he regards them as andesites of various types altered by 

 solfataric action which "accompanied the intrusion of the acid masses." J This 

 action was widespread, and he distinguishes it from the contact metamorphism 

 locally induced in the rocks lying near the margins of the intrusions. As regards 

 the date of the solfataric changes in the lavas, it is stated that in many places they 

 have preceded the action of contact metamorphism, and in others the opposite may 

 have taken place. § 



Sir Archibald Geikie, on the other hand, sees in the propylites merely the 

 representatives of the plateau-basalts altered by contact metamorphism, || and he 

 states that he was unable to find any trace of the solfataric action described by Judd. 

 The point is of considerable geological siguificance, for upon it rest questions of 

 interpretation of a complicated series of igneous rocks. ^[ 



It occurred to me that an examination of the minerals in the cavities of the 

 lavas might throw some light on the question of the cause and date of the alteration 

 of the rocks, and in the present communication I shall deal with a very well-defined 

 zone of zeolite-bearing lavas which can be traced from areas free from contact 

 metamorphism almost up to the margin of one of the large acid intrusions. This 



* Sir Archibald Geikie, loc. cit., p. 388. t James Currie, loc. cit, p. 226. 



I J. W. JUDD, loc. cit., p. 382. § J. W. Judd, loc. cit., p. 367. 



|| Sir A. Geikie, loc. cit., p. 185 (footnote), p. 388 (footnote). 1 J. W. Judd, loc. cit., p. 353. 



