16 MR W. F. P. M'LTNTOCK ON THE ZEOLITES AND ASSOCIATED MINERALS 



(a) This type of vesicle tends to be common in the more finely grained portions of 

 the lava, and, with exceptions, the larger specimens conform to it. In the simplest 

 cases the amygdale is lined with a layer of chlorite containing an occasional lath 

 of albite and succeeded by scolecite which is crowded by inclusions of chlorite 

 for some distance from its highly irregular junction with that mineral [73]. # 



In the more complex types spherules of prehnite enclosing chlorite and, 

 occasionally, minute greenish fibres, which may be augite, occur sporadically on 

 the walls of the amygdale. They are succeeded by a zone of chlorite, enclosing 

 crystals of turbid albite and an occasional irregular grain of yellow epidote, and this, 

 in turn, gives way to a layer of cloudy albite ; the rest of the amygdale is packed 

 with scolecite into which some of the albite occasionally wanders [74]. 



The spherules of prehnite are roughly hemispherical in shape, and under crossed 

 nicols the black bar travels round the section when it is rotated. Not infrequently 

 the extinction direction is continued into the rock so as roughly to complete the 

 circle, and close examination shows that the albite laths of the rock in the 

 complementary hemispherical area are replaced by prehnite which is optically 

 continuous with the prehnite of the amygdale. Under high powers minute crystals 

 of purple augite can be seen projecting into the spherules of the vesicles and converted 

 at their tips to a green mineral which may be pyroxene or amphibole. The replace- 

 ment of albite by prehnite is also shown by the fact that occasionally the spherules 

 contain small rectangular turbid areas exactly similar in appearance to the albite 

 laths, but which now consist of prehnite [113]. 



The same relationship of scolecite to albite is also exhibited. The junction 

 between the two minerals is highly irregular, and the scolecite frequently contains 

 corroded crystals of albite which it is clearly replacing. The further the. albite 

 is removed from the junction the more it is corroded, until finally it is entirely 

 replaced by scolecite, the only trace left of it being a turbid patch corresponding 

 in outline to the original lath. 



When epidote occurs it is always greenish-yellow in colour and shows the usual 

 pleochroism. It is found chiefly in the outer zone of the amygdale, and is frequently 

 studded with inclusions of chlorite. Occasionally it encloses sharp, idiomorphic 

 crystals of albite somewhat clearer than usual [71], and its junctions with that 

 mineral are often quite sharp and well defined, although sometimes the albite 

 exhibits outlines suggestive of corrosion [63]. 



The relationships of prehnite and scolecite to epidote offer some interesting 

 features. In the case of prehnite one finds that the junctions between it and the 

 epidote are not infrequently sharp, but often there are traces of corrosion of the 

 epidote crystal. Particularly is this the case when the epidote is enclosed in 

 the centre of the prehnite spherules, where it often occurs as a patch showing irregular 

 and corroded junctions with numerous minute fibres projecting into the prehnite. 



* The numbers refer to a collection of slides in the Royal Scottish Museum. 



