Geol— Vol. I.] SMITH— SANTA CATALINA ISLAND. 1 5 



Microscopic CJiaractcrs. — Under the microscope the dio- 

 rites are found to have a nearly even-grained, holocrys- 

 talline structure, and to be composed essentially of a lime- 

 soda-feldspar, hornblende and occasional biotite, with free 

 quartz always present in varying amounts. Augite is also 

 present in some of the slides, and in nearly all magnetite 

 is an important constituent. No apatite was observed in 

 any of the sections. There is an occasional tendency to 

 a porphyritic development among the feldspars. Mineral- 

 ogically considered, the rock is a quartz-hornblende-diorite, 

 with a tendency to lath-shaped forms among the feldspars. 



The feldspars are in general allotriomorphic, and tend to 

 develop crystal faces only occasionally, where they come in 

 contact with quartz. In some of the slides many of the 

 feldspars are fairly clear and free from inclusions or de- 

 composition products. Aside from these the majority are 

 clouded by alteration products, which, in some cases, have 

 partly or wholly obliterated the traces of the twinning lamel- 

 lae, which are clearly shown in the fresher material. This 

 cloudiness is apparently due in part to a kaolinization of the 

 plagioclase, but also to calcite, which occurs in small, 

 irregular patches and threads in many of the sections. This 

 product is also found, in some instances, in lines along the 

 twinning planes. Twinning takes place according to the 

 Carlsbad, albite and pericline laws. Pericline twinning is 

 the least frequent, and is usually seen under crossed nicols 

 as a series of very fine lines. 



Excellent zonal structure is occasionally seen, but is in- 

 frequent. The varying optical orientation in such cases 

 shows that the mineral grows more acid from the center 

 outward. Inclusions in the feldspars are not common, but 

 rarely one of the largest crystals contains from one to a 

 number of smaller feldspars which are without definite orien- 

 tation toward their host, and without good crystal boun- 

 daries. Inclusions of small ragged flakes of hornblende 

 or chloritic material are of much more frequent occurrence. 

 Several feldspars occur packed with small, irregularly 

 bounded sections of what appears to be primary hornblende. 



