University of California. 



[Vol. i. 



rather indistinct and interrupted. That parallel with the orthopin- 

 acoid, on the other hand, is sharply marked by numerous bold 

 straight lines. The maximum extinction angle observed made an 

 angle of 44° with the prismatic and orthopinacoidal cleavages, and 

 several observations gave readings close to this angle. Careful 

 search in two slides failed to reveal the presence of any olivine or 

 rhombic pyroxene, nor was any indication of the "mesh structure" 

 visible in the serpentine, which probably makes up from one-fourth 

 to one-fifth of the slide, and has evidently resulted from the altera- 

 tion of the diallage. Although in some few instances a crystal of 

 diallage may be observed permeated through and through with the 

 serpentine, only a mere skeleton of the brightly polarizing pyroxene 

 remaining, yet this is comparatively rare. The crystals are usually 

 fresh and uncracked, and the serpentinization has been confined to 

 their peripheries. Magnetite, and possibly chromite, occur very 

 sparingly as original inclusions in the diallage, but, on the other 

 hand, they are often very abundant in the accompanying serpentine. 

 This fact, taken in connection with the manner in which the strings 

 of magnetite granules arrange themselves with reference to the 

 boundaries of the irregular areas of serpentine, shows that these 

 minerals are mainly secondary, having been formed from the excess 

 of iron and chromium oxides present in the pyroxene molecule and 

 not required in the formation of serpentine. 



There can be little doubt that the whole of the serpentine has 

 been derived from a holocrystalline igneous rock composed wholly 

 or mainly of diallage. Fragments of this rock are not confined to 

 the northern end of the serpentine area, but have been found in two 

 other places near the middle portion of the dyke. It would be a 

 little unsafe to say that the rock just described is identical with that 

 which has furnished most of the serpentine, since its present fresh 

 condition may signify that it was a somewhat local facies, and thus 

 was not so susceptible to change as the rest of the rock. But that 

 the difference was not great, and that this facies is capable of trans- 

 formation into serpentine indistinguishable from the main mass, is 

 certain. 



C ontact M etamorphism. — The contact phenomena of the serpen- 

 tine are of the same general character as those of the fourchite, but 



