FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 25 



described by Brogger * in his account of the syenite-pegmatite veins of Norway, 

 during the last of which the zeolites were deposited. 



The Application of the Term " Propylite." 



It is clear from the descriptions of Professor Judd already cited (ante, p. 2) that 

 the vesicular basalt of Maol nan Damh, An Gearna, and Ben Fhada belongs to his 

 group of propylites. The term, however, has been reserved by Eosenbusch f for a 

 peculiar type of altered andesite, and was used by Judd in that sense. The propylitic 

 alteration is regarded by him as due to solfataric action connected with the presence 

 of the acid intrusions, but quite distinct from the effects of contact metamorphism 

 (ante, p. 2), to which cause Sir Archibald Geikie ascribed the peculiar condition 

 of these rocks (ante, p. 2). 



The lava of Maol nan Damh is a typical olivine basalt, and the evidence goes to 

 show that its present characters and its peculiar vesicle-minerals arose during the 

 formation of the rock itself. A propylite, according to Posenbusch,! is an andesite 

 altered by solfataric action, and it is clear that in this restricted sense the term 

 cannot be applied to the basalt under discussion. But the pneumatolytic changes 

 which it has undergone bear a strong resemblance to the alteration produced by 

 the conversion of an andesite to a propylite, and, as basalts with these peculiar 

 characters have a wide distribution in Mull, it might be advisable to extend the 

 term propylite to them also. 



It is natural to inquire into the cause of this peculiar condition of the 

 basalts around the plutonic centres, a condition not observed so far in the 

 plateau-ground ; but the question can only be answered when the volcanic 

 history of the district is made out. Meanwhile it is safe to say that the basalt of 

 Maol nan Damh and some of the surrounding spurs must have been rich in 

 vapours at the time of its eruption and was probably kept at a high tempera- 

 ture for a prolonged time. This suggests that the lava now occupies a site 

 near the focus from which it was erupted. 



The Metamorphism of the Vesicle-Minerals. 

 I. Introduction. 



The subject of the metamorphism of the vesicle-minerals of lavas by intrusive 

 masses of rock has been dealt with by Drs Harker and Marr,§ who note that 

 the minerals of the amygdales are the first to show the effects of metamorphism. || 



* W. C. Brogger, loc. cit., pp. 160-81. 



t H. Rosenbusch, Mikroskopische Physiographic der Massigen Gesteinc, 4th edition, 1908, pp. 1102-05. 

 | Loc. cit., p. 1105. 



§ A. Harker and J. E. Marr, "The Shap Granite and the Associated Igneous and Metaraorphic Rocks," 

 Quart. Journ. Geol. Soc, 1891, vol. xlvii, p. 292. 

 || Loc. cit., p. 296. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART I (NO. 1). 4 



