GEOL. VOL. I.] SMITH SANTA C ATA UNA ISLAND. 23 



individuals. The feldspars are molded on the augites, 

 and sometimes completely enclose the smaller sections. 

 The augite shows no definite cleavage, but is usually more 

 or less traversed by cracks. In this slide no hornblende 

 was seen, though there are considerable areas of chlorite. 

 No doubt hornblende was at one time present, being now 

 represented by the chlorite, for the augite in the slide has 

 no border of chlorite, nor is the latter seen along the cracks 

 of that mineral. When near or touching chloritic areas the 

 augites have as sharp boundaries and appear quite as fresh 

 as those sections which have no chlorite near them. In 

 weathering the augite alters to a granular, dirty brownish 

 product, more or less opaque. In one slide, from the 

 northern side of Avalon canon, what was originally augite 

 with a short prismatic habit, is entirely altered to hornblende 

 (see page 22) . The sections are idiomorphic, with the forms 

 characteristic of augite, but with the cleavage of horn- 

 blende . In vertical sections the cleavage is very pronounced , 

 showing in part as open cracks. This hornblende is pleo- 

 chroic, a being a very pale yellow, fc pale yellowish green, 

 c greenish brown. The mineral is more or less dull in 

 appearance, and the polarization colors are not clear. It is 

 quite unlike the fibrous hornblende in character. One of 

 the sections shows indistinct twinning lamellae parallel to 

 the orthopinacoid. The augites contain as inclusions occa- 

 sional magnetite grains. 



The magnetite varies greatly in amount, being almost 

 entirely absent from some of the sections. In the rock 

 containing abundant augite there is considerable magnetite 

 in small, irregular patches or needle-like forms, at times in 

 or cutting across the feldspars, or projecting into the augites. 

 One slide from Pebbly Beach shows a few small grains with 

 the crystal form of magnetite, but altered to limonite. 



The granular ground-mass of the porphyrite is composed 

 of usually allotriomorphic feldspar and quartz, the larger 

 proportion being of the former. At times, however, the 

 feldspars tend to lath-shaped or rectangular forms. The 

 borders of the grains in the ground-mass frequently inter- 



