GEOL. VOL. I.] SMITH SANTA CAT A LIN A ISLAND. 33 



features of several slides which have very few phenocrysts. 

 It amounts in one to perhaps one-third as much as the feld- 

 spars of that slide, and equals or very slightly exceeds the 

 amount of pyroxene. The magnetite occurs as inclusions 

 in all the other phenocrysts. 



The augite is usually idiomorphic, though where not well 

 developed as phenocrysts it occurs as minute flakes. It is 

 very pale green, almost colorless, and without noticeable 

 pleochroism. Its habit is prismatic and the resulting forms 

 are generally octagonal. The crystal outlines are usually 

 very sharp and clear, though rounded and resorbed sections 

 are not uncommon. The cleavage, in general, is not visible 

 except with higher powers. Cracks are common, traversing 

 the crystal in every direction. The augites generally are 

 remarkably clear and free from alteration products. A few 

 of the larger sections, however, are much dulled and 

 cracked. Twinning parallel to the orthopinacoid is com- 

 mon. Liquid inclusions are numerous, occasionally reaching 

 a diameter of .02 mm. Some inclusions of magnetite also 

 occur. 



The hypersthene differs but little from the augite in gen- 

 eral appearance, in form, habit, inclusions, or its relation to 

 the essential minerals, and cannot always be readily distin- 

 guished from it. The former, however, is very slightly 

 pleochroic, and all sections give parallel extinction. It has 

 a somewhat weaker double refraction than the augite, and 

 the interference colors of the sections are therefore in general 

 lower, showing yellow of the first order in the majority of 

 cases. Further, favorable sections give characteristic inter- 

 ference figures. Though its habit is like that of the augite, 

 the prismatic faces occasionally are little developed, or 

 rarely are entirely wanting. The two minerals are occa- 

 sionally intergrown. 



Whenever the two pyroxenes are developed as definite 

 phenocrysts the hypersthene is always in excess of the 

 augite. The number of the pyroxene phenocrysts in the 

 rocks is always much smaller than that of the feldspars, the 

 ratio being about one to four or five. 



( 3 ) January 9, 1897. 



