260 FINLAY 



Inclusions of another rock were not observed. The Mexican 

 nephehte syenite is a dense compact rock and its specific gravity 

 is high. Descriptions of the four chief types in the San Jose 

 district follow below. 



I. Nephelite Syenite. 



(a) Baril Type. 



This variety of the nephelite syenite is exposed in the Baril 

 range just east of the mountain of that name. In the hand 

 specimen it is dark gray, with the colorless constituents in ex- 

 cess. It is fine-grained, and exhibits the usual non-porphyritic 

 granitoid texture. Nephelite is present in small amounts only, 

 and this variety of the rock lacks the characteristic greasy look 

 which that mineral commonly imparts to it. White feldspar is 

 a prime constituent, and with it are flashing black crystals of 

 amphibole and pyroxene, and honey yellow grains of titanite. 



Examination with the microscope shows that this variety of 

 the nephelite syenite contains orthoclase, nephelite, hornblende, 

 augite, titanite, magnetite, apatite and pyrite. Of these the first 

 two make up perhaps 65 per cent, of the rock. The orthoclase, 

 which is far more abundant than the nephelite, has, as a rule, a 

 glassy habit. It is often much clouded over with inclusions, 

 which are seen with high powers to be stubby prisms, brown 

 and yellowish-green. They lie in the direction of the basal 

 cleavage. A second cleavage at right angles to this is well 

 developed. There is evidence that all the orthoclase crystals 

 were not contemporaneous. Now and then partly automorphic 

 individuals may be observed standing against others quite dif- 

 ferent from them in habit, owing to incipient kaolinization. 

 Intergrowths between the feldspar and the nephelite are fre- 

 quently seen, with irregular boundary lines between the two 

 minerals. 



The nephelite cannot readily be distinguished from the ortho- 

 clase except by its interference figure. It lacks the two systems 

 of cleavage cracks, however, which show in the feldspar, and 

 such cracks as do occur are very irregular. The mineral is 

 clear and glassy. It is often filled with inclusions essentially 

 like those in the orthoclase. (See PI. IX, fig. i.) . 



