284 FINLAY 



Microscopic CJiaracteristics. — Under the microscope two tex- 

 tures are observed. Besides the usually strictly porphyritic 

 phase of the rock, a fine-grained variety occurs in which all the 

 -crystalline masses are of approximately equal size. Squares 

 and hexagonal sections of nephelite, grains of aegirite and 

 titanite and carlsbad twins of glassy sanidine are the constitu- 

 ents, with accessory minute needles of apatite. These are the 

 commoner minerals which make up the rock. Analcite and 

 rare augite should be mentioned to complete the list. 



The orthoclase is in extremely long narrow carlsbad twins 

 (1.2 mm. by .15 mm.), resembling the smaller ones common in 

 the normal trachyte ground-mass, or again in thick tablets, 

 colorless and glassy, but dusted over with extremely minute 

 rod-like inclusions which tend to lie parallel to the prism 

 .directions. 



Nephelite, in six-sided and rectangular sections .6 mm. by 

 ."S mm., is very common. This mineral gives at times a satis- 

 factory uniaxial interference figure. It is often white, inclining 

 to buff color, ^girite inclusions are observed in it with zonal 

 arrangement. Minute inclusions of the same pyroxene, in sharp 

 lines, produce an effect of ruling which is much like the ap- 

 pearance of definite cleavage cracks. The nephelite is altered 

 but little. Analcite, chiefly, results by weathering. 



Augite is the usual pyroxene. Three generations in all of 

 the mineral are observed. It occurs in short stout prisms 1.2 

 mm. by . 5 mm. as phenocrysts. The extinction angle is nearly 

 45°. Poikilitic intergrowths of the augite, tinged with a violet 

 color, and included emerald green patches of aegirite are very com - 

 mon. Zonal structures are seen at times in such crystals, the 

 several shells having unlike orientation optically. Much apa- 

 tite and some magnetite are associated with the larger individuals. 



Titanite, showing no unusual characteristics, is found in 

 bladed crystals 1.2 mm. long. Patches of biotite were noted, 

 but the mineral is present only in extremely small amounts. 

 Grains of magnetite are scattered through the rock and pyrite 

 is sometimes present. 



The ground-mass is very rich in laths of orthoclase feldspar. 

 These and aegirite rods make up almost the whole of it. 



