FENNER, THE WATCHING BASALT 127 



large areas, and the onl}'- trace of it left is a hazy or muddy appearance of 

 the prehnite. In many places, the patches of turbid material within the 

 prehnite still show in their outlines the appearance of albite groups. 



In slide 65, a similar process of replacement has produced the results 

 shown in fig. 6. As in slide 62, veinlets of albite and prehnite are found 

 cutting a crystalline basalt. The albite lies next to the walls and gives 

 way rather abruptly to prehnite, but a few outlying crv'stals of albite, 

 greatly corroded, appear to have survived within the prehnite at some 

 distance from the walls. Still farther out, phantom forms of former 

 albite crystals are outlined by a slight turbidity in the prehnite. They 

 are completely replaced by prehnite and can be connected with albite 

 only by the appearance of the terminal crs'stal faces. These, however, 

 are characteristic. The radiating and fanlike groups of prehnite crystals 



J^re/inite 



A/ ate 



Phan toms 



Aa;tc 



Fig. G. Replacement of albite by preliuite. Phantom forms of former albite 

 crystals are dimly perceptible. X 35. Slide 65. 



are superimposed upon the former structure of the albite, but in places 

 the albite has had an influence upon the crystallization of the prehnite. 

 In slide 63, minute veins similar to those described are filled with 

 albite and datolite. The albite crystals occur in interlocking aggregates 

 of varying crystallographic orientation. In places, they occupy the veins 

 to the exclusion of all other material, or they may project at various 

 angles from the sides of the vein into masses of datolite. Crystals up to 

 0.5 mm. in length are not uncommon, and the general average of size is 

 noticeably larger than the plagioclase rods in the walls. In most places, 

 there is no evidence of corrosion of albite at contact with datolite, but in 

 some areas, phenomena of replacement similar to those which have been 

 described in association with prehnite appear. 



