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University of California. 



[Vol. i. 



tions. The actinolite in this, and in one or two other cases, was not 

 at first readily recognized as such, owing to the fact that when the 

 needles are small and thin, and particularly when the section is cut 

 in certain directions, they do not give the usual bright interference 

 colors of hornblende, and it is difficult to determine their oblique 

 extinction. The pleochroism in such cases is often very faint; and 

 they might be mistaken for zoisite. The study of some inclusions 

 in the serpentine, presently to be described, left, however, no doubt 

 of their identity. 



About two hundred feet farther east along the beach, the rock 

 is plainly a sheared sandstone, with the ordinary gray color, and 

 showing scales of secondary mica in the shear planes. Under the 

 microscope it shows a typical sheared structure, with brilliantly 

 polarizing wisps of muscovite lying in parallel lines. The original 

 clastic structure is still present, but recrystallization has gone on, 

 surrounding the remnants of the original grains by a fine mosaic 

 of quartz and feldspar. Still farther away from the serpentine, the 

 sandstone resumes its regular bedding, and shows no alteration 

 beyond the appearance of having been subjected to unusual pres- 

 sure, which characterizes it over the greater part of the island. 



A closely similar schist to that just described is found on each 

 side of the northern end of the dyke. That on the western side 

 appears to take the place locally of the usual glaucophane schist. 

 It is characterized macroscopically by abundant small glistening 

 cubes of pyrite. 



Sonic Peculiar Inclusions in the Serpentine. — At various points 

 in the main mass of the serpentinized dyke, there occur inclusions, 

 often of considerable size, and possessing a peculiar petrographic 

 character. These were first noticed on the hill slope to the east 

 of the barracks, as two isolated masses of rock, much harder and 

 tougher than the serpentine, which have been quarried for road 

 metal. The exact contact between the two rocks, although not 

 always apparent at a casual glance, can in every case be determined 

 with absolute precision as a clean-cut line of division, which shows 

 that the masses are surrounded on all sides by serpentine, and have 

 evidently once been covered by it also. Near the center of each 

 mass the rock is a fresh gray color, heavy, even-grained, and exceed- 



