FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 27 



gone to form epidote and also to convert the chlorite to hornblende and the 

 titaniferous oxide of iron to sphene [112]. 



In other amygdales, however, in which the chief contents are albite, prehnite, and 

 epidote, the underlying layer consists of albite and augite which shows conversion 

 to epidote and not to hornblende. Under the microscope the augite crystals are paler 

 than those in the unaltered vesicles, and they contain kernels of epidote which, in 

 some cases, replaces entirely the purple augite. A dull, turbid chloritic substance 

 sometimes surrounds the augite crystals. It is not clear whether this change of 

 augite to epidote is to be ascribed to metamorphism or to pneumatolysis during the 

 original filling of the vesicle, since, as noted above, a partial conversion of augite to 

 epidote has been observed in the unaltered vesicles [111]. 



Other instances occur in which the vesicle-minerals have been completely 

 metamorphosed and the pegmatitic augite at the junction is practically unaltered, 

 the only change being a slight marginal corrosion and alteration to hornblende. In 

 such cases, however, the augite is enclosed by albite and was not originally in 

 intimate association with an unstable zeolite which would give rise to steam with 

 increase of temperature [26]. 



Some of the most interesting points connected with the metamorphism are 

 shown by the zeolites, scolecite and thomsonite. 



In the case of scolecite the first sign of alteration consists in the production of 

 a slight turbidity, and the subsequent changes seem to depend to some extent on 

 the minerals with which the zeolite was in intimate association. Alteration to 

 epidote and prehnite is exceedingly common. 



The secondary epidote occurs sometimes as a network of interlocking crystals 

 .with the spaces filled up with altered scolecite, and it can be distinguished in several 

 ways from the primary epidote of the amygdales. Thus, it is usually colourless and 

 shows no pleochroism, and it sometimes occurs as colourless rims surrounding a 

 yellow kernel which often shows the original corroded outline and the inclusions of 

 chlorite so typical of the primary epidote ; the rim occasionally possesses a zonal 

 arrangement parallel to the boundary of the core, and it shows weaker bire- 

 fringence [15, 17]. 



The prehnite produced by metamorphism can be readily distinguished both by 

 its appearance and mode of occurrence from the original prehnite of the vesicles. 

 It forms confused aggregates of small crystals which ramify into the zeolite and 

 sometimes replace it fibre by fibre [100, 50, 81]. The crystals are often large 

 enough to permit of accurate determination under the microscope, but sometimes the 

 fibres building the secondary aggregate replacing the zeolite are too small for that 

 purpose, and, in such cases, the material was always crushed and the refractive index 

 tested in a suitable oil. The secondary prehnite resembles in some respects the 

 corroded rim which, as noted before, often separates the original prehnite from the 

 scolecite ; but it can be distinguished by its much greater abundance in the 



