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



In another type of vesicle the wall is lined with a more or less well-developed 

 layer consisting of large acicular purple augites with idiomorphic laths of albite 

 which sometimes pierce the augite. This layer may be succeeded immediately by 

 scolecitc intermingled with chlorite, which grades into a well-defined zone of chlorite 

 upon which is seated scolecite, enclosing crystals of ycllowisb-green epidote. The 

 purple augite usually projects from the augite-albite layer, but occasionally crystals 

 are found surrounded by scolecite and at some distance from the walls of the vesicle ; 

 a large projecting crystal has been noted in which a transverse fracture has been 

 filled with scolecite. This augite is often quite fresh and unaltered, but sometimes 

 it has been partially converted to green fibrous hornblende or yellow epidote similar 

 to that found in the amygdales ; crystals have been noted of which the one half is 

 purple augite, and the other, yellow epidote. The laths of albite are full of inclusions, 

 and they, too, wander into the overlying scolecite [70]. 



The augite-albite layer is usually thin and patchy, but specimens occur in which 

 it reaches a thickness of 1 cm. When it is well developed, chlorite is abundant in it 

 and fills up the spaces between the large augite crystals which it frequently encloses. 

 Sections across this layer show it to consist of crystals of purple augite, long skeletal 

 crystals of magnetite, with albite, chlorite, and apatite. The augite is idiomorphic 

 and has sharp junctions with the chlorite, although here and there it shows signs of 

 corrosion and marginal alteration to chlorite and hornblende. When the lono- 

 prismatic crystals are attached to the walls they project radially into the vesicle, but 

 when they occur well within the layer they lie often with their long axes parallel to 

 the walls. This layer may be succeeded by a zone of chlorite and epidote, the latter 

 mineral showing the usual irregular junctions and full of chlorite inclusions ; a layer 

 of turbid albite is the next deposit, then a layer of prehnite, and finally scolecite [23]. 

 The prehnite in some cases is obviously replacing the turbid albite, for not only does 

 it contain phantom crystals of that mineral but occasionally it occurs as veins cutting 

 across the felspar, crystals of which are optically continuous on opposite sides of the 

 vein. Some of the smaller amygdales show this phenomenon even better. The first 

 layer is a well-defined zone of pegmatitic augite, albite, and chlorite, the augite 

 showing the usual features of corrosion and, occasionally, occurring as detached 

 crystals well towards the centre of the vesicle. This layer is succeeded by prehnite, 

 more or less turbid and enclosing thoroughly corroded patches and phantom crystals of 

 albite as well as a little chlorite. Scolecite comes next ; it, too, contains corroded albite 

 and also patches of prehnite with little phantom albites near their centres [111]. 



Besides being highly vesicular, the lavas are sometimes traversed by white veins 

 showing at their junction with the rock a very well-developed pegmatitic layer of 

 augite, albite, and chlorite. Under the microscope this layer shows the features 

 already described : namely, the long prismatic augites, often roughly parallel to the 

 walls, the turbid albite, the magnetite, and the chlorite. Upon it is deposited a layer 

 of turbid albite with some prehnite and small nests of scolecite. The latter mineral 



