FENNER, THE WATCHUNG BASALT 157 



plagioclase have been almost wholly replaced by prelmite, whose brilliant 

 tints and radial structure when seen with crossed nicols give a quite un- 

 expected appearance to the former diopside and plagioclase areas. 



Although without doubt the effects of alteration and replacement are 

 generally most pronounced immediately adjacent to cracks and crevices 

 of various kinds, or where the solutions acted upon the glassy phase of 

 the rock, nevertheless it appears that in places the solutions were able to 

 penetrate by capillary action to considerable distances within unbroken 

 rock of normally crystalline character and effect almost complete miner- 

 alogic rearrangement. At the same time, the texture was left almost 

 undisturbed. Prehnite seems to l)e especially characteristic of this 

 process, and the effects are visible in the slides mentioned. Under such 

 conditions, large diopside phenocrysts or nuclei of resorbed olivine would 

 partake of the transformation and would appear as prehnite nodules. A 

 nearly circular form of radial prehnite, probably of the latter derivation, 

 appears in slide 65. 



Prehnite is determined to be later in the sequence than albite, quartz 

 and garnet, and earlier than datolite, pectolite, chabazite, stilbite, natro- 

 lite, apophyllite and calcite. 



, Pectolite 



Pectolite occurs in the usual groups of radiating fibres, some of which 

 reach a length of two or three inches, but usually they are smaller. It 

 commonly occurs in hemispherical masses. It is very frequently asso- 

 ciated with prehnite, but it is found also with a great variety of other 

 minerals. It is not specially well represented in the slides, but the posi- 

 tion in the series can be fairly well determined. In descriptions of 

 albite, quartz and prehnite which have preceded, it was found to replace 

 these minerals. It appears under the microscope as masses of finely 

 radiating fil>res of rather high birefringence, elongated parallel to Z. It 

 strongly resembles natrolite, but is distinguished therefrom by the higher 

 interference colors. 



In 143, the hand specimen shows an association of pectolite and natro- 

 lite in which the appearance of pectolite suggests decomposition. This 

 inference is confirmed in the slide. The radiating needles of pectolite 

 are buried in a mass of natrolite, the sharpness of outline has been lost 

 and at the terminations, the crystals appear to fade out gradually, as it 

 were, instead of terminating abruptly with sharp definition. 



In 130, the hand specimen shows small clumps of pectolite fibres which 

 are quite evidently portions of originally larger masses, buried in chaba- 

 zite. The thin section was taken at the contact and shows the increasing 



