GEOL. VOL. I.] SMITH SANTA CATALINA ISLAND. 21 



siderably in number. In some slides there are comparatively 

 few, while in others they constitute the larger portion of the 

 slide. The ground-mass is never glassy, and does not show 

 any flow structure, except to a slight extent in specimens 

 from one or two dikes. Quartz is seen sometimes among 

 the phenocrysts, though this is a very rare occurrence. 

 Magnetite occurs as an accessory, but usually in very small 

 amounts. No apatite was seen, and in the rock of which 

 the analysis is given below no phosphorus was found. 

 Most sections show little evidence of disturbance. 



The porphyritic feldspars occur in idiomorphic forms, 

 which are somewhat tabular parallel to the brachypinacoid. 

 The majority of the crystals show good boundaries, except 

 where their growth was interfered with by the growth of 

 other phenocrysts. Many of the sections, however, show 

 boundaries which are more or less irregular or rounded, 

 and due, in part, at least, to resorption. The sections vary 

 in size from about .2 mm. to nearly 3 mm. Zoning is 

 common. The twinning is in accordance with the albite 

 and Carlsbad laws. Pericline twinning rarely occurs. As 

 in the diorite, the feldspar lies between a basic oligoclase 

 and an acid labradorite. Many of the sections are con- 

 siderably cracked. Some of the feldspars are fairly fresh, 

 but most of them are more or less clouded by decomposi- 

 tion products. This cloudiness is due largely to a kaolini- 

 zation of the mineral. Considerable areas are sometimes 

 altered to calcite, with more or less epidote. The decom- 

 position is such at times as to destroy, partly or wholly, 

 the traces of twinning. Occasional inclusions of hornblende 

 or chlorite are seen in the feldspars. One section was 

 seen with a zone of chloritic material not far from the bound- 

 ary, arranged in threads and fibres parallel to the longer 

 direction of the feldspar. Small magnetite grains are rarely 

 included in the feldspar. 



The hornblende is prismatic in habit, and usually either 

 shows resorbed boundaries, or the original outlines are more 

 or less obliterated by alteration products. No terminal planes 

 were seen in any section. In two or three of the slides 



