FROM THE TERTIARY LAVAS AROUND BEN MORE, MULL. 17 



When the epidote is bounded by scolecite, this peculiarity is even better shown. 

 Large crystals of epidote end against the zeolite in a series of fibrous tufts, whilst 

 the outline of the crystal is frequently interrupted by little rectangular patches 

 filled with clear scolecite ; occasionally a large crystal has been dissected into 

 a number of separate fragments which extinguish simultaneously under crossed 

 nicols [113]. 



The junction of the prehnite with the scolecite is never a sharp one. The 

 spherules of prehnite are separated from the zeolite by a band of fibrous, crystalline 

 prehnite, which terminates against the scolecite in a highly irregular manner. 

 Irregular patches of prehnite are also found well within the scolecite layer. 



The minerals in these vesicles were thus deposited in the following order : — 

 (1) albite, (2) chlorite, (3) epidote, (4) prehnite, (5) scolecite, and there is distinct 

 evidence that with changing; conditions some of the material formed in the first 

 stages has been partly replaced by later- formed minerals. 



A marked feature in some of the slides is that the rock for some distance around 

 the vesicle is very much altered and shows a large development of chlorite and 

 black oxide of iron. This altered zone is sometimes vesicular, the vesicles being 

 filled with epidote or chlorite, or a mixture of the two [67], and it grades off into 

 the normal basalt. 



(6) This type of vesicle is best developed in the coarser basalt, and is usually 

 characterised by a considerable amount of prehnite, although there is a good deal 

 of variety in the way in which the minerals occur. In the simplest cases small 

 cavities are filled with prehnite, which encloses prismatic crystals of purple augite, 

 fresh and unweathered and similar in all respects, except habit, to the ophitic augite 

 of the surrounding rock. Frequently they are clearly attached to the walls of the 

 amygdale, but occasionally they are surrounded on all sides- by prehnite and occur 

 well in the centre of the vesicle. The mode of occurrence of these crystals and their 

 difference in habit from the augite of the rock show clearly that the vesicles were 

 the seat of igneous crystallisation after they had been formed [107]. 



Some of the larger vesicles show similar crystals round their junctions with the 

 rock. When chlorite forms the first zone of the amygdale, the long prismatic augites 

 project into it and are sometimes completely surrounded by it. Their boundaries are 

 clear and sharp, although occasionally there is evidence of corrosion and alteration 

 to hornblende and chlorite ; but, generally, their fresh and unaltered condition 

 contrasts rather forcibly with the decomposed state of much of the augite of the 

 rock. The chlorite, which sometimes forms a layer of considerable thickness, may 

 be succeeded by a confused, zone of chlorite, albite, and yellowish-green epidote with 

 a few patches of muddy scolecite. Upon this is seated a layer of turbid albite, and 

 scolecite fills the central portion of the vesicle. Prehnite may be present as small 

 spherules occurring here and there around the walls ; it usually contains a fair 

 amount of chlorite, and is frequently pierced by the large crystals of augite [65]. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART I (NO. 1). 3 



