Fairbanks.] 



A iici /cite Diabase. 



the analcite is largely replaced by natrolite in radial aggregates. 

 It also occurs in veins traversing the rock. 



Secondary Minerals in the Analcite. — The polyhedral or wedge- 

 shaped areas of analcite contain two secondary minerals in a fresh 

 condition. One, having the character of prehnite, occurs in clear 

 grains and idiomorphic crystals projecting into the analcite from the 

 borders or wholly inclosed in it. The other is probably feldspar, 

 bordering the analcite and apparently replacing that substance. 



The prehnite crystals are in some cases more than a millimeter 

 in diameter, but generally smaller, and are found in many of the 

 analcite areas of all the slides prepared from the large dike. (Plate 

 16, Figs. I and 2.) It is as clear and colorless as the analcite, but 

 in ordinary light is distinguished from the latter by the high index 

 of refraction, producing an apparently rough surface and high relief. 

 The polarization colors are very brilliant. The optical anomalies 

 often seen in prehnite do not appear. The form of the idiomorphic 

 individuals is commonly hexagonal or octagonal. No trace of 

 cleavage is to be seen. Two sections were obtained cut perpendic- 

 ular to a bisectrix. Each of these showed a bi-axial interference 

 figure, in which the dispersion was apparently symmetrical with 

 reference both to the plane of the bisectrices and that perpendicular 

 to it. When the crystallographic boundaries are present the extinc- 

 tion is parallel to one edge, indicating an orthorhombic mineral. 

 One of the sections referred to gave a negative, the other a posi- 

 tive sign. The dispersion is that given for prehnite, red being less 

 than blue. Several treatments with hot hydrochloric and sulphuric 

 acids failed to decompose it; the only effect produced seemed to be 

 a roughening of the surface and the appearance of irregular cracks 

 extending inward from portions of the surface. 



Owing to the minute size of the crystals, and to the fact that the 

 optical properties of this mineral are identical with those of olivine, 

 much difficulty was experienced in making a positive determination. 

 The greater resistance of the mineral under discussion to acids and 

 the unlikelihood of olivine occurring as a secondary product, made 

 the determination in favor of prehnite probable. To make this cer- 

 tain a few grains were obtained perfectly free from the other com- 

 ponents and treated with hydrofluosilicic acid. This developed 



