56 MICROSCOPIC STEUCTURE 



■the cavities, crystallises into definite forms, and then shows 

 clusters of chabazite, phillipsite, with beautiful patches of 

 mesolite interspersed. It is extremely brittle, and thus 

 easily reduced to fragments. 



Microscopical Characters. 



This is the glassy form of basalt, a true vitreous basic lava, 

 with pheno-crysts of olivine sparsely scattered in a structure- 

 less glass of a pale yellow tint, occasionally deepening into 

 gamboge. Apparently, it occurs massive, and does not form 

 a mere tachylytic selvage. It, consequently, falls into 

 Rosenbusch's division of basaltvitrophyres. It is a volcanic 

 product, which is typically represented by the Kilanea lavas 

 in the Sandwich Islands, and its structure is strikingly re- 

 peated in slides of modern lava from Hawaii. The olivine 

 crystals are nearly as fresh-looking, and have the same! i5Q- 

 roads of the corrosive magma. 



Like the same form in the Sandwich Islands, the glass is 

 wonderfully clear, a marked contrast to the opaque nature 

 of so many European tachylytes. It carries small colourless 

 or yellowish globulites, some with opaque margins, but the 

 bulk of the iron, instead of separating out into magnetite, 

 would seem to have been used up by the olivine. There 

 are no complete displays of perlitic structure, but it is in- 

 cipient, and some of the porphyritic crystals are surrounded 

 by a perlitic ring associated with globulites. The strain 

 phenomena are instructive. Several olivines have tufted 

 fissures proceeding from their borders into the surrounding 

 glass, arranged like cilia, evidently thei result of the strain 

 of crystallisation, upon the glass. These fissures some- 

 times connect two fissures, and spring, too, from 

 larger cracks, which traverse the glass in various 

 directions. The same crystals under partly-crossed nicols 

 show a reaction rim, which in plain light is seen to be 

 a granulated border. Wherever the smaller fissures are 

 numerous, they are associated with granulation, yellow 

 translucent globulites. The crystals of olivine often en- 

 close the glassy base in ovoid and circular forms, some of 

 Ivhich are prolongations of the base, being connected with 

 the outside magma by a narrow neck. I could not detect 

 more than a crystal or two of augite and triclinic felspar. 

 In the darker portions of the glass zeolitic cavities occur 

 with spherulites round their margins. Elsewhere a spheru- 

 lite exists with an approach to an axiolitic nature, being 

 elliptical in form, with an elongated medium axis. 



