GEOL. VOL. I.] SMITH SANTA CATALINA ISLAND. 2>J 



parallel, the range being from N. 25 E. to N. 65 E. Prob- 

 ably all of these dikes are of nearly the same age as the 

 porphyrite, though many, or possibly most of them, are a 

 little later, judging from the fact that the area of the por- 

 phyrite itself is penetrated by them. Most of the specimens 

 obtained are very much altered, and contain a comparatively 

 large amount of calcite, but there is enough of the original 

 structure left to show definitely that the rocks are porphyrite. 

 In general nothing further than this could be determined, 

 though most of the specimens appear to be not much unlike 

 the rocks of the main mass. 



A somewhat different structure is shown in two or three 

 of the slides, only one of which will be described. This is 

 from a dike on the northern coast, at a point about a third 

 of the distance from the isthmus to the extreme northwest- 

 ern end of the island. The dike is nearly vertical, and has 

 a width of eight feet. Specimens were taken from near the 

 margin and central portion of this dike, the two being en- 

 tirely different in appearance. That from near the center 

 is a pale, even gray, while the other is a darker, mottled 

 gray, with an intergrowth of light and dark areas. The 

 darker parts appear to be compressed in a given plane, and, 

 as seen with a lens, have usually a minute central cavity. 

 The rock from the middle of the dike also contains here and 

 there very minute thread-like cavities. 



Although the hand-specimens differ so much in general 

 appearance the contrast is not so great under the microscope. 

 Phenocrysts are not very numerous and are wholly of labra- 

 dorite, which is quite fresh. They are considerably resorbed, 

 and seldom show crystal boundaries. The ground-mass is 

 almost wholly filled with lath-shaped feldspar microlites of 

 various sizes, which show a pronounced flow structure in 

 the slide of the marginal rock, this being less noticeable in 

 the specimen from the center. A majority of these micro- 

 lites have indented terminals. In the marginal rock slide 

 there are two distinct types of areas in the ground-mass, in 

 both of which microlites occur, one somewhat yellowish, and 

 the other dark in color, from minute particles contained in 



