368 



University of California Publications. 



[Geology 



further interchange or passage of material from or to the rocks. 

 The setting up of active deposition, the stopping of metamorpho- 

 genic diffusion into the wall-rock, and the substitution of natrolite 

 for albite as the precipitated molecule, all point to a marked 

 lowering of temperature both of ascending solutions and of the 

 country rock. 



The benitoite and neptunite do not appear anywhere to follow 

 the natrolite into the wall-rock. In a few places the neptunite 

 appears to be partially surrounded at its base with amphibole- 

 natrolite matrix but it is practically limited to the confines of the 

 vein proper. Both benitoite and neptunite are attached directly 

 to the wall so that they often commence to deposit before the wall 

 received any coating of natrolite. They also occur in the central 

 portion of the vein entirely surrounded by natrolite, showing 

 that at these points the latter commenced depositing first. They 

 also occur in the natrolite druses and project freely from the 

 surface, showing that they were among the last materials to 

 separate. We conclude therefore that benitoite, neptunite, and 

 - natrolite were deposited contemporaneously throughout the period 

 of vein filling. 



The order of crystallizing power — the crystalloblastic order — 

 arranged in decreasing magnitude is neptunite, benitoite, natro- 

 lite. Natrolite is found in abundance surrounding neptunite and 

 benitoite, molding itself to suit their form and showing the 

 smooth siirfaces of their imprint when separated. A few cases 

 were observed where benitoite crystals are pierced by neptunite 

 prisms or have partially grown around them. 



The next events recognized are those of descending solutions, 

 decomposition, etching, limonite coatings, etc., as already referred 

 to elsewhere. 



DISTBIBUTION OF MINEEALS IN VEINS. 

 The workings are so shallow at the mine that there are no 

 data at hand to indicate the distribution of the various minerals 

 with depth. But it is of some interest to note that the distribution 

 is not uniform along the surface. The benitoite is most abundant 

 along the east central portion of the zone of veination. Passing 

 eastward the titano-silicates decrease in abundance until the 



