2 4 



University of California. 



[Vol. 2. 



The rock varies in structure from holocrystalline, approximately 

 panidiomorphic, to ophitic. In the ophitic varieties the separate 

 augite patches appear optically and crystallographically orientated 

 the same over large areas, sometimes more than an inch in length 

 and nearly as broad. Gradations take place from this type, in which ♦ 

 the augite individuals are so disseminated as to almost meet and 

 have an ophitic character, to one in which the portions are united 

 to form true poikilitic crystals inclosing scattered feldspars. In 

 the panidiomorphic facies the feldspars occur as long laths or in 

 tabular form and much more coarsely crystallized than in the 

 ophitic. The order of crystallization has been as follows in the 

 panidiomorphic facies: augite, apatite, feldspar, magnetite, and the 

 mineral, probably nepheline, which the analcite has replaced. In 

 the ophitic facies the order is, apatite, feldspar, magnetite, augite, and 

 analcite. 



The apatite is recognized in all facies of the rock as very slender 

 prisms, extinguishing parallel and possessing a hexagonal cross- 

 section. It penetrates both the feldspar and analcite, but not the 

 augite. 



The feldspar is very similar to that from Cuyamas, showing the 

 same zonal structure and alteration products. The extinction angles 

 obtained in the centers of the crystals indicate a basic labradorite. 



Nearly all the sections prepared from the coarser material, in 

 which the feldspars appear as short laths and broad plates, showed 

 very marked zonal extinctions. The crystals consist of a mixture in 

 varying proportions of the anorthite and albite molecules, for, while 

 the centers are basic labradorite, there is a constantly increasing 

 acidity toward the outer edge where the albite molecule is present 

 to the exclusion of the anorthite. It would appear from the exam- 

 ination of many slides that the core of the crystals, embracing about 

 two-thirds of the total diameter as viewed in lath-shaped sections, 

 extinguished nearly simultaneously, but that then there occurs a 

 sudden transition to an acid plagioclase. Taking a section in which 

 the centers of the adjoining twin lamellae extinguish symmetri- 

 cally at +26 on opposite sides of the cross hair, the extreme outer 

 rim became dark at — 4 , that is, on the opposite side of the cross 

 hair. The alteration of the feldspars exhibited in the different types 



