I9I0.] COAST RANGES OF CALIFORNIA. 339 



named in the foregoing are olivine, enstatite, bronzite, diallage, 

 picotite and chromite. 



Olivine is present only in the freshest variety of the rock and is 

 even there not discernible in the hand specimen. Under the micro- 

 scope it shows high relief, and is colorless. Cleavage cracks tra- 

 verse it, which are often at rectangular position to each other and 

 are usually filled with black opaque masses of magnetite. Crossed 

 nicols show that in these cleavage cracks serpentinization has taken 

 place and given rise to the mesh structure with small rounded 

 fragments of the olivine occupying the central part of a mesh. A 

 number of these rounded fragments usually extinguish together 

 and thus indicate the size of the original crystal. Its lack of 

 cleavage, its high relief and its bright interference colors distinguish 

 it from the pyroxenes. 



Enstatite is seen in the hand specimen as coarse prismatic crystals 

 of a dark green color and with shiny cleavage surfaces which are 

 parallel to the longer axis. Bastite is a pseudomorph after it, but 

 can be recognized from it in that the crystals are easily scratched 

 by a knife and are of a lighter green color. Under the microscope 

 enstatite is seen in coarse platy crystals with a medium relief and a 

 slight pleochroism from colorless to a light green. The cleavage is 

 prominent. When basal sections are present, they sometimes pre- 

 sent a dim prismatic cleavage. 



Interference colors are of the first order, usually a bright yellow, 

 the extinction is parallel and the slower ray is parallel to the elon- 

 gation of the crystal. Plates parallel to the principal cleavage do not 

 give an interference figure, which distinguishes it from bastite. 

 Intergrowths with diallage are frequently observed. 



Diallage is not quite as abundantly represented as enstatite. 

 Under the microscope crystals of irregular outline are seen which 

 resemble enstatite in cleavage lines and pleochroism. Crossed nicols 

 however reveal bright interference colors, red and blue of the second 

 order, and an oblique extinction. The maximum angle made with 

 the principal cleavage was found to be 42 degrees. The slow ray is 

 parallel to the elongation of the crystals. Besides the perfect (100) 

 pinacoidal cleavage a well developed prismatic cleavage is seen in 

 basal sections. Of enstatite and diallage the former appears to be 



