Ransome.] 



Geology of Angel Island. 



229 



a zone of brown or green hornblende, which in turn has its outer 

 edge fringed with a bristling array of the sharp, slender actinolite 

 needles, which penetrate into the surrounding feldspar. Similar 

 needles are scattered profusely all through the clear plagioclase, 

 either singly or in parallel aggregates with bristling ends. 



Other outcrops present macroscopic variations from the rocks 

 just described, but the micro-structure is in all cases similar, or pre- 

 sents but trifling differences. Close to the road, as it crosses the 

 saddle going northward from the barracks, a mass of decomposed 

 rock, weathering like the ordinary fourchite of Point Stuart, is 

 imbedded in the serpentine, and traversed by one or two narrow 

 veins of a mineral which appeared in the field to be serpentine, but 

 which was subsequently determined to be an aggregate of zoisite. 

 As nearly as can be made out under the microscope, it is merely 

 a decomposed facies of the rock just described. 



Another inclusion is in the wedge-shaped southern end of the 

 large northern division of the dyke. Portions of this outcrop 

 exhibit a more or less foliated structure, are brittle and resonant 

 when struck, and present the glittering fracture of many -fine-grained 

 diorites; but the microscope shows that they are merely facies of 

 the rock forming the inclusion first described, the augite having 

 wholly disappeared, and the resulting hornblende being mostly of 

 the brown variety. The groundmass in the two cases is identical. 



In the ravine which separates the main northern division of the 

 dyke from the southern part, and just south of the preceding out- 

 crop, the relations of the various rocks are obscure. A little knoll, 

 almost in the line of the dyke, is occupied mainly by a rock of 

 similar character to that described as forming masses in the serpen- 

 tine, consisting mainly of hornblende and clear, secondary plagio- 

 clase, the latter being thickly crowded with needles of actinolite. 

 Closely associated with this rock is the San Francisco sandstone, 

 fragments of which showing various degrees of alteration occur 

 scattered over its surface, although the actual exposures of the rock 

 in situ are extremely poor. Many of these fragments have a dark- 

 ened and baked appearance, and when one piece with a particularly 

 cindery aspect was examined microscopically, it was found to retain 

 the original clastic structure of sandstone; but between the grains 



