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



The changes induced in the surrounding andesites by the Cheviot granite have been 

 described by Mr H. Kynaston,* whilst Dr. J. J. H. Teall has described the meta- 

 morphic effects produced in the Arenig lavas by the Galloway granites.f 



More closely related to the subject under consideration are the Tertiary 

 basic lavas of Skye which, as described by Dr Harker,J have been considerably 

 altered by the large intrusions of granite and gabbro. He notes that the 

 first minerals to be affected are the unstable contents of the vesicles, and 

 records the interesting . conversion of lime-soda zeolites to lime-soda felspars ; 

 epidote is also formed, and hornblende and biotite are developed at the expense 

 of chlorite. 



The metamorphosed amygdales of An Gearna and Beinn Fhada afford most 

 instructive material for study because, on account of their large size, they usually 

 contain portions of the material originally filling them, and it is only in close 

 proximity to the granophyre that all traces of their original contents are obliterated. 

 It is, therefore, possible to follow with confidence the course and nature of the 

 metamorphism. How far the metamorphism induced in the lavas is due to the 

 granophyre and how far to the minor intrusions so common in the district, it is 

 impossible to say. It is, however, a fact that, despite local variations, the intensity 

 of the changes increases as the margin of the granophyre is approached. 



II. The Petrography of the Metamorphosed Amygdales. 



It has been remarked already that one of the differences, obvious in hand- 

 specimen, between the amygdales of An Gearna and those from Maol nan Damn 

 consists in the somewhat paler appearance of the outermost layer of specimens from 

 the former locality. Sections through this layer show that it varies considerably in 

 character. 



In some cases, especially when scolecite is abundant, it consists of long, bladed 

 crystals of a pale green, pleochroic hornblende sometimes partially enclosed in large 

 plates of albite full of inclusions. Associated with these are rod-like growths of 

 black oxide of iron showing partial conversion to sphene. The hornblende shows 

 good cleavage and frequently contains grains of magnetite and kernels of epidote. 

 The albite encloses grains of epidote and crystals of hornblende. An occasional 

 rounded mass of chlorite showing partial conversion to hornblende is also present. 

 This layer is overlain by epidote, succeeded by scolecite, both of which enclose 

 numerous fibres of hornblende. It is probable that the layer of hornblende and 

 albite represents the pegmatitic zone of common occurrence in the vesicles of Maol 

 nan Damh. The conversion of auo-ite into hornblende liberates lime which has 



o 



* II Kvna.ston, "Note on Contact Metamorphism round the Cheviot Granite," Trans. Ed. Geol. Soc, 1899, 

 vol. viii, pp. 18-26. 



+ Mem. Geol. Surv., "The Silurian Rocks of Scotland," 1899, p. 647. 



\ A. Harker, Mem. Geol. Bur v., "The Tertiary Igneous Rocks of Skye," 1904, p. 50. 



