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University of California Publications. 



[Geologi 



croscope the rock is seen to be an augite rock of a texture be- 

 tween an augite andesite and a diabase, yet which in spots 

 is that of a fine-grained augite diorite. The augite is fresh, and 

 in more or less irregular grains or in well formed crystals, with 

 high birefringence, very faint pleochroism, and an extinction 

 angle of from 40° to over 50°. The feldspars are well formed, 

 and crystallized at about the same time as the augite. They all 

 show extinction angles of over 20°, and are labradorite. Some 

 few flakes of biotite and grains of quartz are also present. At 

 1,200 feet in there occurred what appeared to be a thin dyke of 

 diorite one-half an inch wide, intrusive in the dark augitie rock. 

 No well defined boundaries were shown, however. Farther 

 search showed many more such dykes at 1,250 feet, of apparently 

 the usual pinkish diorite, intrusive in the dark rock. The micro- 

 scope revealed the fact that the two rocks differ only in coarse- 

 ness of grain and alteration of feldspar, and that the change 

 from one to the other is gradual. The pinkish diorite was thus 

 found to be an augite rock with some of the large augites altered 

 to uralite, but identical in all particulars but grain with the dark- 

 colored stone. The dyke at 1200 feet seemed to show that some 

 motion took place in the mass when still plastic, as very often 

 happens in igneous magmas, for the transition from the fine to 

 coarse grain was more sudden than in the small dykes at 1250 feet. 

 Beyond 1250 the ordinary type of pinkish, coarse-grained diorite 

 comes in gradually. The microscope proved this to be a true 

 augite rock also, with many crystals of fresh augite, and more 

 such changed to uralite in part, and partly to chlorite, epidote, 

 and magnetite. This pinkish rock continues on until at 1358 feet 

 a small mass of diabase occurs, from six to ten feet wide along the 

 tunnel. One side of this diabase is bounded by a slip and some 

 breeciation ; the inner side passes into diorite very gradually. 



Again at 2650 feet, and at 3725 to 3760 feet, diabase occurs, 

 in the latter case particularly, shading beautifully into diorite by 

 all possible gradations. Specimens taken here are either diabase, 

 or diorite, or both, as one may choose to call them. And the rock 

 is all augitie, though none of this mineral is wholly fresh. The 

 rock is much fissured and jointed, so that the alteration of the 

 minerals has proceeded quite far. 



