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University of California. 



[Vol. z_ 



zoisite has almost the same color as the augite, being slightly more 

 yellowish, and exhibiting a faint pleochroism. It occurs in aggre- 

 gates of slightly divergent rods with parallel extinction. One of the 

 characteristic green dikes in the gabbro showed no primary constit- 

 uents. Hornblende, quartz, and a granular mineral having the 

 appearance of zoisite, are the only constituents. 



As has been stated before, the diabase structure insensibly shades 

 into the gabbroitic, and even in some of the coarser gabbros the 

 diabasic structure is not wholly absent. In typical cases the feld- 

 spar occurs in broad plates, while the augite and hornblende occur 

 more often in the interstitial spaces between the feldspars, as in dia- 

 base. They also occur in elongated individuals penetrating the 

 feldspars and penetrated by them in turn. Quartz is commonly 

 present, occupying either angular areas between the feldspars, 

 replacing them, or imbedded in ragged grains in the green decom- 

 position products. Very fine examples occur of a micropegmatitic 

 intergrowth between the quartz and the feldspar, the surface of the 

 feldspar being dotted with similarly oriented quartz grains. The 

 feldspars in some of the slides are perfectly clear, but appear gener- 

 ally more or less granulated. The extinction angles are more easily 

 measured than in the case of the finer-grained rocks. The angle is 

 high, approximating 36 , thus pointing to the presence of a basic 

 labradorite. In many of the slides the augite has been completely 

 changed to green shreddy hornblende dotted with magnetite grains.. 

 Where the change is not complete, the augite appears surrounded 

 by hornblende, or granulated with hornblende shreds. A slide 

 from the specimen analyzed contained augite and secondary horn- 

 blende in about equal proportions. The augite frequently incloses 

 small crystals of feldspar in a poikilitic manner. In the alteration 

 of augite to hornblende the twinning plane of the former is pre- 

 served as the plane of twinning of the hornblende. Zonal extinc- 

 tion is a very common phenomenon of the feldspars in the gabbro. 

 The transition from the more basic core to the acid exterior is abrupt 

 and takes place near the outer boundary. The basic core is marked 

 by the fact that it is often clouded, while the outer rim is always 

 fresh. The subsequent addition has often been much greater on 

 certain faces than on others, so that the form of the core is not pre- 



