University of California. 



LYol. I. 



this rock has been serpentinized, it is easily possible that olivine 

 may have been originally an abundant constituent, but it is now 

 almost wholly removed by differential decomposition. 



The olivine occurs only in very small grains or fragments 

 imbedded in serpentine. These fragments usually have a common 

 orientation over considerable areas, showing the olivine to -laave 

 originally formed individuals of large dimensions. It is colorless 

 and has the rough shagreened surface characteristic of olivine, so 

 that it stands out strongly from the surrounding serpentine. It 

 exhibits no trace of cleavage, and as the boundaries of the grains 

 are wholly irregular, no observations for extinction were obtained. 

 The interference colors are very brilliant. - Some difficulty was 

 found in distinguishing olivine from certain sections of diallage, but 

 the following characteristics seem to clearly separate them. As 

 the olivine exhibits no cleavage, sections without cleavage may of 

 course be found which show no interference figure in convergent 

 polarized light. But sections of diallage parallel to the orthopin- 

 acoid, which, as the prismatic cleavage is frequently not apparent, 

 would exhibit no cleavage, are at right angles to the plane of the 

 optic axes, and show the emergence of an optic axis. As this is 

 the only section of diallage in which cleavage is not visible, it may 

 thus be distinguished from olivine. The alteration of olivine to 

 serpentine takes place in the manner described by so many authors. 

 The meshes of the resultant serpentine frequently contain at their 

 centers residual grains of olivine, and magnetite is abundant in the 

 cracks, as well as limonite. 



Chromite occurs in scattered, irregular grains imbedded in the 

 serpentine, but not, so far as observed, included in the original con- 

 stitutents of the rock. It exhibits the characteristic high relief and 

 dark brown color of chromite, and gives a strong reaction for 

 chromium in a borax bead. Magnetite is found in irregular grains 

 and in octahedral crystals, mostly occupying veins and cracks in 

 the serpentine, but also occurring as inclusions in the original min- 

 erals of the rock. No feldspar nor any mineral or aggregate which 

 could be referred to feldspar for its origin, could be discovered in 

 any phase of the rock. 



The phase of the massive rock above described is restricted to 



