Fairbanks.] 



Geology of Point Sal. 



29 



a long, irregular area of analcite, partly bounded by small feldspars, 

 projected into an augite crystal, its shape being determined by the 

 crystal, planes of the augite on either side and by the feldspar 

 crystals terminating it (Plate 2, Fig. 6). An illustration of a pecul- 

 iar occurrence of analcite, or rather its decomposition product, 

 natrolite, is given in Plate 2, Fig. 5. The augite contains aggre- 

 gates of jagged feldspar crystals, and in one spot, lying parallel 

 to the vertical axis, an area of natrolite nearly one millimeter 

 wide, and about six long. Its sides are formed of crystallographic 

 planes, while feldspars divide it into several parts. It seems to 

 the author that no better indirect evidence could be desired of the 

 original presence of some soda rich mineral. The alteration of 

 the feldspars in many portions of the rock seems to bear a direct 

 ratio to the abundance or paucity of the analcite. Where it is 

 abundant between the augite crystals the feldspars are extremely 

 altered, but within the poikilitic individuals they are often perfectly 

 fresh. 



Calcite and pyrite are occasionally present. The rarity of the 

 former is hard to understand in a rock rich in a lime-soda feldspar. 

 The composition of the secondary products derived from both the 

 analcite and the feldspar must be quite varied and their physical 

 character is such that exact determinations are impossible. 



Chemical Character. — A study of the chemistry of the rock is 

 connected with many difficulties, owing to the abundance of sec- 

 ondary products, but at the same time it is believed that results of 

 importance have been obtained. A specimen of rather coarse texture 

 and panidiomorphic structure was selected for analysis (I). It bore 

 a considerable resemblance to that from Cuyamas, the analysis of 

 which, by Mr. Lehner, was given in the previous paper, but which 

 will be introduced here for comparison (II). 



