28o 



University of California. 



[Vol. i. 



extinction between the inner and outer portions, the angle decreas- 

 ing from the center outward. Twinned sections have a less marked 

 zonal structure or none at all; a fact which might be explained by 

 the more rapid growth of the crystal in 00P00 (010) than in the 

 direction normal to that plane. There are scarcely any individuals 

 but which are more or less decomposed, so that the correct deter- 

 mination of the extinction angles is difficult. The attempt to obtain 

 cleavage plates which should show twinning was not successful, 

 and the classification had to be based on sections cut haphazard. 

 Taking all the sections together, about thirty-five individuals were 

 found which extinguished either simultaneously or approximately 

 so on either side of the twinning plane. A little more than half of 

 these gave angles of extinction between 24 and 36 , the greater 

 portion being less than 30 . This would seem to indicate, for a 

 portion of the feldspar at least, a basicity not less than that of labra- 

 dorite. The large variation in the angle of extinction on sections 

 cut parallel to ooPSo must be interpreted as meaning an increasing 

 acidity from the center outward. Judging from the manner of 

 decomposition and general physical character, the feldspars must all 

 belong; to the same series, but it is difficult to assign their exact 

 position on account of some apparently conflicting evidence. The 

 paucity of calcite as a decomposition product in the rock, and the 

 change to a soda rich mineral (analcite) indicate a soda feldspar; 

 but the undoubtedly high extinction angle of a large proportion of 

 the individuals would place them in the basic portion of the series. 

 After digesting it long with concentrated hydrochloric acid the pul- 

 verized feldspar was apparently partly decomposed, thus allying it 

 to labradorite. As a possible means of aiding the determination, a 

 separation was attempted, using Klein's solution for the purpose of 

 getting the specific gravity. A portion of the rock was pulverized 

 sufficiently fine to be passed through an 80-mesh screen, and 

 that which passed through the 100-mesh was rejected. Upon being 

 placed in the solution the augite and magnetite immediately fell. 

 The solution was then diluted until the separation of apparently 

 pure feldspar was obtained. This was corroborated by a micro- 

 scopic test. The specific gravity of this was found to be 2.51 1. A 

 second separation was made at a greater density, using the material 



