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University of California. 



[Vol. 2. 



mass, some twenty feet long by six in thickness and five in width, 

 illustrates the occurrence of the mineral in situ. This formed an 

 exceedingly compact rock, composed for the most part of the rela- 

 tively large crystals of the blue hornblende arranged without any 

 common orientation and associated with garnet, chlorite, actinolite, 

 titanite, and albite. The chlorite and actinolite were frequently 

 localized in certain portions of the rock, forming areas of as much 

 as ten centimeters in thickness; the other minerals were more gen- 

 erally distributed through the entire mass. It was from this aggre- 

 gate that the specimen of actinolite already described was obtained; 

 it also furnished one of the specimens used for the study of the blue 

 hornblende and of the chlorite described later on. 



General Description. — Weathered surfaces of the rock were lus- 

 terless and of a dark gray color, but freshly-exposed portions pre- 

 sented a blue-black color, and showed no evidences of decomposi- 

 tion. Associated with the hornblende were crystals and angular 

 fragments of titanite, often several millimeters in length. The latter 

 mineral, however, was badly decomposed, consisting of a light yel- 

 low opaque mass, though the inner portions of some of the larger 

 crystals were transparent and of a characteristic reddish-yellow color. 

 Sections of the rock showed a complex intergrowth of the horn- 

 blende crystals, together with small amounts of actinolite, chlorite, 

 and titanite. The hornblende appeared in either rectangular or 

 rhombic areas, and were easily distinguished by their strong pleo- 

 chroism. Inclusions of titanite were sometimes observed. The 

 second specimen examined was a compact polished boulder, com- 

 posed of fine needle-shaped crystals arranged with their longest 

 axes in approximately the same plane. Sections of this showed 

 the entire absence of other minerals either as inclusions or as 

 intergrowths. 



From the first specimen a few perfect, columnar crystals, some 

 three to five millimeters in cross section and two centimeters in 

 length, which had partially weathered out from the rock mass, were 

 obtained. These presented only prismatic and clinopinacoidal 

 faces. Though too dull to give good reflections, an approximate 

 measurement of the prismatic angle gave the value 1 2 5 ° 14'. For 

 the mineral crossite, Palache obtained the value 126 6', and for 



