52 



University of California. 



[Vol. 2. 



cases the constituents are present in the vein in the same propor- 

 tion as in the rock adjoining. Toward Point Morrito veins of a 

 similar appearance occur in the olivine pyroxene rocks, which can 

 not perhaps be explained the same, because of the lack of feldspar 

 in these rocks; and it is quite possible that they are true dikes. 



The banded gabbros are finely exposed at many points on the 

 smooth faces of the cliffs. The gneissic structure is a more con- 

 stant feature than the banding proper. By gneissic is not meant 

 the structure due to the parallelism of tabular constituents, but that 



Figure 4. — The banded gabbro. 



due to the drawing out of the aggregates of the different constitu- 

 ents, feldspar, augite, and olivine, into irregular, lenticular forms. 

 Rocks showing this structure shade on the one hand into the mas- 

 sive varieties, and on the other into the banded. The facies in 

 which the banding is most apparent to the unaided eye contain an 

 excess of feldspar, which is often drawn out in the most even and 

 regular lines (Fig. 4). Instances were noted in which the feldspar 

 bands one-half inch in thickness appeared as continuous straight 

 lines for ten feet. As a rule the most regular bands of feldspar are 



