FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 13 



to understand correctly the course of the metamorphism, it is necessary to find out 

 what were the original minerals in the vesicles, and, if possible, the date of their 

 formation. Interesting light is thrown on these questions by a study of tbe micro- 

 scopic characters of the amygdales of Maol nan Damh, which also shows the relation- 

 ships of the minerals to each other and to the lava in which they occur. 



The Origin and Relationships of the Vesicle-Minerals. 



I. Introduction. 



The question of the origin of the zeolites so frequently found filling the vesicles 

 of lavas is one which has excited very great diversity of opinion. In modern works 

 of reference on mineralogy they are usually classed as secondary minerals which owe 

 their origin to the decomposition of the minerals of the lava at a date subsequent to 

 its consolidation.* This view was strongly supported by the late Mr J. G. Goodchild, 

 who expressed the opinion f that they were formed by the action of percolating 

 surface waters upon the original minerals of the rock. He seems to have had doubts, 

 however, about this explanation holding good for the occurrences under description, 

 for he refers specially to the association in the centre of Mull as owing its origin 

 perhaps to solfataric action.! In this he no doubt was following Professor Judd, who 

 expresses that view in the memoir already cited. 



Dr A. Harker, in his description of the Tertiary igneous rocks of Skye, regards 

 the zeolites as the products of the final phase of consolidation of the lavas,§ and states 

 that the subsequent changes produced by the action of percolating meteoric waters 

 are of a different order and readily distinguishable from the process of zeolite- 

 formation. This view has been strongly advocated by Mr James Strachan, || who 

 considers that zeolites, agates, green earths, etc., were formed during the last period 

 of cooling of the lavas in which they occur. Similar explanations for certain 

 occurrences of zeolites have been offered by Blumrich*[I and Pelikan,** whilst Dr 

 J. S. FLETT,ff in his account of the teschenites of the Edinburgh district, states that 

 some of the analcite in these rocks may be primary. 



Directly bearing upon the question of the origin of zeolites are the occurrences 

 in the syenite pegmatite veins of the Norwegian syenites ascribed by Brogger XX to 



* Of. Hintze, Handbuch der Mineralogie, vol. ii, p. 1658. 



t J. G. Goodchild, "On the Genesis of Some Scottish Minerals," Proc. Roy. Phys. Soc. Edin., 1899, vol. xiv, p. 190. 



\ hoc. cit., p. 211. 



§ A. Harker, Mem. Geol. Surv., "The Tertiary Igneous Rocks of Skye," 1904, p. 45. 



|| James Strachan, " The Carnmoney Chalcedony, its Occurrence and Origin (with a General Note on the 

 Formation of Secondary Siliceous Minerals in Volcanic Lavas)," Proc. Belfast Nat. Field Club, 1906, vol, ii, appendices 

 vii and viii, p. 336. 



IT J. Blumrich, Tschermak's Min. Pet. Mittheil, 1892, vol. xiii, p. 482. 

 ** A. Pelikan, ibid., 1906, vol. xxv, p. 113, and Sitz-Ber. Wiener Akad., 1901, iii, p. 341. 

 tt Mem, Geol, Surv., "The Neighbourhood of Edinburgh," 1910, p. 296. 

 XI "W. C. Brugger, Zeitschrift fiir Krist. u. Min., 1890, vol. xvi, p. 168. 



