294 FINLAV 



Fe203 and FeO vary between wide ranges. The rocks are all 

 comparatively high in lime, some of them, as the diorite and 

 camptonite, abnormally so. They are high in the alkalies, and 

 in every one soda is two or three times as great as potash. 



High soda and alumina, with low silica, makes necessary the 

 formation of orthoclase and nephelite, with segirite in the 

 tinguaites, and light green soda augite in the nephelite syen- 

 ites. Here also the dark brown hornblende, rich in iron, 

 which appears in the more basic varieties, is edged v/ith green 

 by relative increase in the amount of soda present in the later 

 stages of crystallization from the rock magma. Magnesia is 

 very low in the nephelite syenites and tinguaites. Biotite is al- 

 most entirely lacking in these rocks. It only enters into the 

 most basic of the nephelite syenites. The mineral is, however, 

 present in the diorites where MgO is relatively high. 



The character of the augite in all the rocks of the district is 

 very uniform. It is a light green variety, poor in iron and mag- 

 nesia and rich in soda. Its appearance and optical properties 

 are much the same in the normal nephelite syenite, in the ande- 

 site, in the diorite, and even in the camptonites. 



As stated above, the rocks of the San Jose district, so far as 

 the proofs are at hand, fall into three divisions, from the earliest 

 to the most recent. 



1. Nephelite syenite and diorite. 



2. Andesite and dacite. 



3. Basalt, tinguaite, camptonite, vogesite and limburgite. 

 There are represented effusive and deep-seated rocks which 



vary widely in chemical composition. The analyses, however, 

 when placed side by side in the order in which they have been 

 printed are seen to follow in a definite sequence. They are not 

 in a ** series," according to Brogger's use of the term. They 

 are related after the manner of his *' faciessuite." The tingua- 

 ites when examined with the others are found to present 

 striking differences toward them. It is possible that they are 

 part of a tinguaite ** series." The ratios between the sum of the 

 alkalies and silica, and between soda and silica are nearly the 

 same in the two tinguaites, but the alkalies function together in 



