FENNER, THE WATCHUNG BASALT 



155 



The relations of prehnite to albite, quartz, garnet and datolite have 

 been described. With regard to pectolite, the slides appear to present 

 evidence that in its growth, needles of the mineral have been advanced 

 through previously deposited prehnite, as if the latter offered no resist- 

 ance. It will appear later that natrolite also, whose habit of growth is 

 in similar slender needles, possesses the same property of advancing 

 through previously existing crystals. In 87, finely felted pectolite lies in 

 contact with a spherical mass of radiating prehnite. The terminations 

 of the prehnite crystals have lost their characteristic appearance and 

 sharpness of outline, and the borders appear muddy. In places, the felt 

 of pectolite is plainly advancing across the prehnite. 



In 76, masses of prehnite are thoroughly impregnated with pectolite 

 needles. These cross the prelinite areas singly or in groups of three or 



Fig. 21. Remnants of prehuite groups (Pr) iu stilbite (St). X 35. Slide 74. 



four or lie in bundles or diverging rays. The perfectly straight and 

 sharply defined needles pierce numerous grains of prehnite without diver- 

 sion. Similar effects, though not developed so extensively, appear in 50a 

 and are illustrated in Plate XI, fig. 6. In this illusrtation, the straight 

 needles near the center represent pectolite. 



In 93, the hand specimen shows grains of prehnite, the size of buckshot 

 or smaller, which lie isolated in the midst of masses of pectolite. In the 

 thin section, the prehnite appears turbid and between crossed nicols has a 

 mottled look. Along the borders, it is difficult to decide what is pectolite 

 and what is prehnite, though normally the two have a very different look. 

 In 67, badly corroded prisms of prehnite are being replaced by chabazite. 



Fig. 21 (slide 74) illustrates the manner in which originally radiate 



