■jo 



University of California. 



[Vol. 2.. 



Observation both in the field and with the microscope confirms 

 the fact of a continuous petrographic series between the strictly 

 feldspathic and the non-feldspathic portions. Feldspar is widely- 

 distributed in rocks which must really be classed with the perido- 

 tites under our present nomenclature. As an example of this may 

 be cited a rock consisting of olivine, hypersthene, and feldspar. 

 The olivine forms about half of the rock, while the remainder is 

 equally divided between the other two. The olivine is largely 

 decomposed to a reddish brown mesh with fresh cores, while the 

 comparatively fresh feldspar is distributed through it in small 

 crystals. The hypersthene occurs in relatively large, almost por- 

 phyritic individuals, also quite fresh and free from interpositions. 



An examination of the literature upon serpentines and perido- 

 tites shows that much confusion exists. The usage is so different 

 by different authors that it becomes very difficult to reduce to any 

 system of classification the great variety of these rocks in the area 

 under consideration. Much of the peridotite is quite fresh, the 

 greatest amount of alteration of course being confined to that rich- 

 est in olivine. The augite may be either normal or show all stages 

 of change to diallage, the larger crystals being generally diallagic r 

 while the smaller are normal augite. As many authors have based 

 the classification on the character of the monoclinic pyroxene, 

 whether augite or diallage, it adds much to the difficulty of making 

 a classification which shall conform to usage. 



Under the head of lherzolite are grouped those rocks which con- 

 tain olivine, augite or diallage, hypersthene or enstatite, and often a 

 small amount of feldspar. These rocks are chiefly confined to 

 Point Morrito, where they are intimately associated with non- 

 feldspathic peridotites. Except for the occasional patches of feld- 

 spar, hypersthene is the most striking constituent in the hand spec- 

 imen, appearing in porphyritic crystals one-fourth to one-half inch 

 in diameter, imbedded in a base of olivine and minute feldspars. 

 Under the microscope the olivine appears to have decomposed to 

 an almost opaque network of magnetite and reddish oxides, scat- 

 tered through which are fresh olivine cores. The feldspar is com- 

 paratively fresh but seamed. The larger individuals of hypersthene 

 show no crystal boundaries, but are fresh, and without interposi- 



