54 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 6G, 



been described as occurring in crystals in the miarolitic cavities, 

 some albite occurs in the shear zones as water-clear colorless crystals 

 coating basalt, especially in specimens of one lot from along an east- 

 west fissure. The specimens in which the albite occurred contained 

 also chlorite, laumontite, amphibole, calcite, etc., and the albite ap- 

 peared to be older than all of these. It is in druses of crystals which 

 are uniformly small, rarely reaching 1 mm. in length. They are 

 prismatic by elongation on the vertical axis and are untwinned. The 

 prismatic faces are vertically striated. A typical crystal was meas- 

 ured, yielding the following angles: 



Measurements of albite crystals from vein.. 



The drawing, figure 5, shows the habit and appearance of these 

 vein albite crystals. The albite is optically biaxial positive, 2V 

 medium, /3= 1.530, indicating pure soda feldspar. 



The diabantite occurring as "varnish" on slickensides has already 

 been noted and that coating the fragments of brecciated rock asso- 

 ciated with zeolites from along north-south shears is in no wise 

 different. 



In several specimens from an east-west shear near the middle of 

 the quarry face in October, 1922, the earliest deposit coating the 

 rock fragments is a soft micaceous gray-green or blue-green layer 

 underlying a film of the "mountain-leather'' hornblende, above 

 which is much stilbite and laumontite. Under the microscope this 

 chlorite is seen to be made up of transparent flakes, occasionally 

 with a suggestion of hexagonal outline, aggregated into rosettes. It 

 is. biaxial and negative ( — ) but with 2V approaching zero. The 

 acute bisectrix is perpendicular to the basal cleavage. Refractive 

 indices, oc = 1.625, ^ = y= 1.630, 7— oc = .005. Pleochroism X, pale 

 yellow-green, Y and Z = bluish green, absorption X less than Y = Z . 



