340 



University of California Publications. [Geology 



The general trend of the outcrop is north 6-4° west. In the 

 cut the western part dips about 68° north, while in the face it 

 dips 65° north. At the east end of the mineralized zone the 

 greenstone with incipient schistosity and the barren veins of 

 natrolite lying in these planes strike north 59° west and dip 

 75° north. In other words, the zone turns slightly to the south 

 before dying out. 



GENEEAL EELATIONS OF MINEEALS IN VEINS. 



The most abundant mineral of the veins is natrolite, which 

 occurs chiefly in granular aggregates. Indications of crystal 

 form are largely limited to the drusy cavities, and even there 

 the natrolite generally forms in peculiar groups, projecting in 

 small roof-shaped ridges or coxcomb-like forms, and only very 

 rarely developing the prismatic forms usually characteristic of 

 natrolite. Some of the druses are filled with very small needles 

 of green or blue-green amphibole, and lying in the midst of 

 the cavity supported by these needles the natrolite often occurs 

 as equant 4 polyhedral aggregates of from 1 to 3 millimeters 

 in diameter, not at all suggestive of the mineral natrolite. Most 

 conspicuous and beautiful in this white ground of the natrolite 

 gangue are the scattered idiomorphic crystals of the blue equant 

 or somewhat tabular benitoite and the brilliant black neptunite 

 prisms, showing here and there a touch of deep red. These 

 minerals are the characteristic and more abundant minerals of 

 the benitoite-bearing veins. 



In plate 30 it is apparent that surrounding the drusy 

 cavity is a layer of white (natrolite) and that it is followed 

 by a layer of darker color. This outer layer is of variable 

 thickness — from a fraction of an inch up to several inches 

 — and is usually present between the white vein material and 

 the more definitely recognizable wall-rock. It has a bluish or 

 greenish tint, and looked at closely is seen to show a granular 

 structure with luster and cleavage much like the vein-stuff. It 

 is indeed natrolite which is loaded with numerous microscopic 



* Used in the sense of equidimensional or nearly so, in contrast to tab- 

 ular or prismatic, as suggested by Cross, Iddings, Pirrson and Washington, 

 Joum. Geol, XIV (Dec, 1906), p. 698. 



