GEOL. VOL. I.] SMITH SANTA CAT A LIN A ISLAND. 57 



particularly in the Little Harbor region, apparently confined 

 to the neighborhood of the areas of the amphibole- and talc- 

 schists and serpentine. It is probable that here, as else- 

 where in California, these blue amphibole-schists are due to 

 local contact metamorphism occasioned by the intrusion of 

 basic irruptives. 1 



Microscopic Characters. Only one slide was made of 

 the quartzites, which, however, is doubtless typical of the 

 purer quartzites in general. It consists almost entirely 

 of a mosaic of clear quartz grains of irregular shape and 

 size. Many of them are flattened in a direction parallel to 

 the plane of schistosity, thus giving frequently very much 

 elongated sections. Their boundaries interlock in an ex- 

 tremely intricate manner. Occasional pale pink garnets 

 occur as inclusions in the quartz, averaging a little less than 

 .1 mm. in diameter. They are for the most part rounded, 

 though two or three present crystal boundaries. Long nar- 

 row sections of what is probably sillimanite are compara- 

 tively numerous, nearly all arranged with their longer axes 

 parallel to the plane of schistosity or to the direction of the 

 flattening of the quartz grains. The terminals taper more 

 or less gradually to a point. No cross -sections were seen. 

 The mineral is colorless and has a moderately high index 

 of refraction, somewhat higher than that of quartz, and it 

 may therefore be readily distinguished from the latter in 

 ordinary light. The double refraction is considerable, giv- 

 ing brilliant, though somewhat mottled, polarization colors. 

 The extinction is in all cases parallel and perpendicular to 

 the longer axis of the mineral. In all the sections observed 

 the longer axis is the axis of less elasticity as shown by the 

 quartz wedge. No optical figure was obtained. 



1 " A Contribution to the Geology of the Coast Ranges," by Andrew C. Lawson. Am. 

 Geol., Vol. XV (June, 1895), p. 352. 



" The Geology of Angel Island," by F. Leslie Raiisome. Bull. Dept. Geol., Univ. Cal., 

 Vol. i, No 7. 



