156 



ANNALS NEW YORK ACADEMY OF SCIENCES 



groups have been broken up by stilbite into isolated individuals or small 

 scattered clusters. In 101, a prehnite group is cut by veins of stilbite. 



In 127 (fig. 3), which was described under albite, it is found that the 

 prehnite clusters which had been deposited upon albite are in turn giving 

 way to natrolite. In 68, also, prehnite is being replaced by natrolite, and 

 portions of groups have been cut off by the later mineral. 



The relations of prehnite and apophyllite are shown in 138. Small 

 groups and fragments of prehnite are isolated by the apophyllite in 

 numerous instances. The manner in which a vein of apophyllite cuts off 

 a portion of a radiate group of prehnite crystals is illustrated in fig. 22. 



Calcite effects striking results. Crystals appear in the midst of preh- 

 nite and develop in euhedral forms, which cut out areas of the earlier 

 mineral as sharply as by a knife. This appears in slides 53, 62 and 76. 



^/.o/j/ly/Z/'te 



Pj-cA, 



Fig. 22. Prehnite cut by vein of apophyllite. X 35. Slide 138. 



By a continuation of the process, fragments of prehnite of extreme 

 irregularity are left, whose outer form is determined by calcite, but 

 through all portions of which the same fan structure prevails. 



Occasionally in specimens of basalt which appear to the naked eye 

 perfectly normal and unaltered, small, shotlike nodules of prehnite are 

 seen. The rock seems entirely impervious, and the relations are such as 

 to make it appear that the prehnite had developed in the fused magma. 

 This is so improbable that another explanation is naturally looked for^ 

 and the effects seem to be very well explained by certain phenomena 

 which appear in several of the slides, for example, 50a, 53, 65 and 118. 

 In 50a, crusts of prehnite border areas of basalt in which the texture 

 appears normal at first sight, but it is found that the diopside and 



