154 A^'NALS JEW YORK ACADEMY OF SCIENCES 



Fig. 20 shows the appearance of swarms of corroded datolite crystals in 

 stilbite in 83. Many of them have the rounded and etched appearance 

 which is seen when crystals of a salt slowly dissolve in water. 



Other examples might be given of the corrosion of datolite in contact 

 with heulandite, chabazite, stilbite and calcite. In a great number of 

 slides, there are at least a few remnants of datolite, and it may have been 

 present originally in great quantity. The general relations and mode of 

 replacement have been brought out in the examples given. These show 

 that albite, quartz and prehnite were replaced by datolite, which in turn 

 yielded to chabazite, heulandite, stilbite, natrolite and calcite. This order 

 has been found to be true in so many cases that there is little doubt that 

 it is general. Nevertheless, one abnormal occurrence has been observed, 

 in which crystals of heulandite and calcite are encrusted with datolite, 

 implying deposition at a much later period. The formation of datolite 

 was probably conditioned chiefly by the concentration of boric acid in the 

 solutions. In the ordinary course of events, the solutions appear to have 

 gained access to the available supplies of this material at an early period 

 and to have efliected the reactions by which datolite was formed, while at 

 a later period the process appears to have been one of gradual removal; 

 but it is not difficult to conceive that under somewhat irregular condi- 

 tions of circulation the boric acid might have been left until a much later 

 period before it was taken up in the general circulation. 



Prehnite 



Prehnite is found in the familiar form of groups of tabular crystals, 

 more rarely in small, distinct prismatic forms. Both varieties appear in 

 the thin sections. The structural characteristics and optic properties are 

 normal. The microscopic twinning lamination, which produces a plaid 

 effect, is well shown in a number of the slides, and when seen, differen- 

 tiates it immediately from all other minerals present. Equally charac- 

 teristic is the deep azure-blue, low-order interference color. It is observed 

 that when sections give the normal gray tone as the minimum interfer- 

 ence, the dispersion may be either p > v or p < v, but in those in which 

 the minimum is the abnormal Berlin blue, the dispersion for red is much 

 less than that for violet. Iddings gives p >■ v, in some cases p <i v. 

 Leather-brown tints are also distinctive. The interference figures rarely 

 show distortion, except when it is evident that several individuals or 

 groups of twinned crystals are concerned in the effect. Commonly the 

 habit is radiating or plumose, but in two or three slides (for example, 74 

 and 67), it is found in small distinct prisms. The feathery forms and 

 brilliant polarization colors often suggest bright-colored plumage. 



