68 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. GG, 



The comparison of the above analyses indicates that the prehnit© 

 from this locahty is ordinary in composition except in being rather 

 higher than usual in ferric-iron, a little higher, in fact, than the prehn- 

 ite from Admiralty Inlet described by Johnston ^^ to which the vari- 

 etal name " ferriprehnite" has been applied. That the iron is pres- 

 ent as ferric oxide replacing alumina rather than as ferrous oxide re- 

 placing lime was definitely determined. 



The optical properties of the analyzed powder were found very 

 difficult of determination. The confused optical structures observed 

 in the relatively simple crystals described above are greatly multi- 

 plied in the massive material. Although composed of pure prehnit e 

 the sample gave variable refractive indices and is clearly somewhat 

 zoned with some variable constituent, probably ferric oxide, affecting 

 the indices of the different zones. The average indices, which are 

 the best that could be obtained, are «= 1.635, i8= 1.640, 7= 1.655. 

 The mineral is biaxial positive and the value for 2V varies from 

 nearly 0° to about 30° or more with an average of 15°. The dis- 

 persion, r<v, varies from weak to extreme, most intense in the 

 grains of smallest axial angle. Confused interference figures give 

 extreme crossed dispersion. 



Paragenetically the prehnite is early, definitely earlier than stilbite^ 

 laumontite, apophyllite, and calcite. In most specimens it precedes 

 datolite but in other specimens crusts of datolite crystals are over- 

 lain by clearly later crusts of "dumb-bell" prehnite. Evidence 

 definitely proving whether this means two generations of prehnite or 

 two of datolite was not found. From the existence of two types of 

 crystallization of the datolite, however, it is assumed that there are 

 two generations of the datolite and only one of prehnite. 



Datolite occurs in a large number of specimens and has been 

 assumed to be of two generations because in many specimens it is 

 underlain by a thick crust of prehnite while in others the datolite 

 crystals are more or less covered by a later crust of prehnite, although 

 both generations of datolite were nowhere found in the same specimen. 



The first datolite found in the quarry, in October, 1922, was only 

 yellowish transparent granular material on diabase, not associated 

 with any other mineral, although laumontite occurred on the opposite 

 sides of the same specimens. This was identified by its character- 

 istic optical properties which are: Biaxial, negative ( — ), 2V large, 

 r>v weak, |8=1.653±. 002. 



•'R. A. A. Johnston, Canada Oeol. Surv. Victoria Memorial Museum, Bulletin 1, p. 95, 1913 



