GEOL. VOL. I.] SMITH SANTA CAT A LIN A ISLAND. 37 



yellow. Such stained cracks show, under crossed nicols, a 

 faint polarization, as if from some radially arranged sec- 

 ondary product. The microlites are capillary in form, 

 averaging .01 mm. in length, and exhibit a slight polariza- 

 tion. They also show a pronounced flow structure. In 

 smaller amount are the margarites and trichites, the latter 

 in the form of tufts and wisps, and sometimes curled at the 

 ends. The margarites, also, are occasionally gathered into 

 loose tufts, radiating in all directions. 



Basaltic Fades. It was stated (page 32) that the speci- 

 mens examined were all pyroxene andesites, with but two 

 exceptions. One of these was found on the minor ridge 

 bounding the valley of Whitley's Cove on the north, not far 

 from the contact between the basement rocks and the vol- 

 canics. The other is from the slopes to the east of Isthmus 

 Cove. The rocks differ from those already described, in 

 containing iddingsite. 1 The specimen from near the isthmus 

 is dark gray in color and very much altered, but the iddings- 

 ite occurs here in good crystal forms, while in the other 

 specimen, which is fairly fresh, the crystal boundaries show 

 more or less resorption. As the structure of the latter rock 

 differs considerably from that usually found in the andesites, 

 it will be described in some detail. 



The rock is purplish and compact, but passes into a black 

 vesicular facies, apparently differing microscopically from 

 the compact form only in the size of the component minerals 

 and in the amount of magnetite contained. The minerals 

 are much smaller in the vesicular portion of the specimen, 

 and it is almost black with magnetite, in the form of grains, 

 long, roughly bordered rods, and irregular areas. The 

 glassy ground-mass of this portion of the rock is filled with 

 minute black dots, doubtless magnetite. The compact por- 

 tion of the specimen has very little glass, though it contains 

 a large amount of secondary silica, chiefly in the form of 

 opal. The rock is nearly holocrystalline, and contains 



1 See "Geology of Carmelo Bay," by Andrew C. I^awson. Bull. Dept. Geol., Univ. 

 Cal., Vol. i, pp. 31-36. 



