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University of California. 



[Vol. i. 



which is macroscopically identical with the preceding, differs in 

 having the augite in rather smaller plates, which have been almost 

 wholly transformed to hornblende. Only by careful search can 

 here and there a remnant of the augite be found occupying the 

 central portion of a crystal of hornblende. The feldspathic base is 

 similar to that in the preceding rock, but even more thickly crowded 

 with needles and prisms of actinolite. Not all of the hornblende 

 of this slide is of the green variety, however. Many of the larger 

 irregular areas, resulting from the conversion of the augite, are 

 occupied by a strongly pleochroic brown variety, showing, in 

 sections transverse to the prism, the usual amphibole cleavage. 

 Assuming that the optical scheme is the same as for ordinary horn- 

 blende, then the pleochroism is a = light dirty green, 6 = deep chest- 

 nut brown, and c = light yellowish green. One crystal was noted 

 twinned in the ordinary manner, parallel with the orthopinacoid. 

 There is no apparent regularity in the distribution of the brown 

 variety, for the individuals are often mottled green and brown. 

 The larger grains of hornblende, especially the brown, appear to be 

 perfectly compact, and yet there can be little doubt that it is all 

 secondary, and can be seen fringing off on the borders into needles 

 of actinolite. Any doubt that might have existed as to the second- 

 ary origin of the clear feldspar, and its inclosed actinolite crystals, 

 was wholly dissolved by the discovery, in a slide from another 

 inclusion, of fine cracks traversing the slide, and filled with the 

 same minerals as the matrix or groundmass of the rock. These 

 are very apparent in ordinary light, owing to the greater clearness 

 of their filling, but the distinction vanishes almost completely be- 

 tween crossed nicols. 



Nearly all the slides show small areas of gray, cloudy material, 

 occasionally surrounding a dark opaque grain. These are evidently 

 leucoxene, resulting from the decomposition of ilmenite. Small 

 patches of pyrite are also not uncommon. 



A slide cut from a specimen taken about two feet from the con- 

 tact with the serpentine, deserves notice for the exceptional oppor- 

 tunity it affords of studying the development of the actinolite 

 needles. The large grains of augite are generally surrounded by a 

 border of brown, cloudy material, of no great width; then follows 



