8o 



University of California. 



[Vol. 2. 



troctolite in which the olivine grains are segregated in irregular- 

 patches, there can be traced a gradual transition to elongated 

 aggregates rudely parallel, and to the appearance of more or less 

 continuous bands of the two constituents. In the case of this rock, 

 which is found in a number of places, the phenomenon is the sim- 

 plest and most easily explainable. In the transitions to the normal 

 olivine gabbro the same structure is to be seen in its various stages 

 of perfection. The gneissic structure is more particularly connected 

 with the presence of the stringy olivine aggregates. The most per- 

 fect banding occurs generally in rocks in which olivine is not an 

 important constituent. This banding is first noticeable in the mas- 

 sive gabbros in the partial segregation of the feldspars and the 

 ferro-magnesian silicates in layers, so that on a clean surface there 

 are indistinct bandings reproduced. It is sometimes noticeable only 

 on the weathered surfaces through the removal of the dark constit- 

 uents. In portions of the gabbro where this structure is most 

 common it is far from being uniform, and there are quite abrupt 

 transitions from the obscurely banded or gneissic rock to one in 

 which the banding is comparatively regular. As a general thing 

 the bands are not continuous for any great distance, but thin out 

 and are replaced by others. There is an exception, however, in 

 the case of the gabbros near Point Morrito, where the bands are 

 exceedingly regular for many feet. The variation in thickness is 

 particularly noticeable in the case of the feldspar, while the olivine 

 is always found in very thin bands. In some facies of the rock the 

 banding appears only in the microscopic examination. A dark but 

 apparently massive hypersthene troctolite showed, in thin section, 

 the hypersthene and feldspar together in a narrow band to the total 

 exclusion of the olivine, and another band of feldspar almost free 

 from hypersthene. In this specimen the olivine aggregates show 

 no noticeable lineation. 



A specimen obtained at the foot of the cliffs near the Head is 

 quite remarkable. The rock contains in order of their importance 

 augite, brown hornblende, olivine, feldspar, hypersthene. A hand 

 specimen three inches wide showed the following bands : First, one 

 in which hornblende is the most important dark mineral, then a 

 band of augite one-half inch wide, having on each side a very nar- 



