i9o6] SMITH— PARAGENESIS OF MINERALS. 203 



is a massive rock made up of large dodekahedral red garnets more 

 than a centimeter in diameter, thick prisms of black pargasite, thin- 

 ner prisms of bluish carinthine, and plates of margarite, abundant 

 blue prisms of glaucophane, a few small crystals of lawsonite, a 

 little colorless epidote, and small crystals of rutile. This rock grades 

 over into pargasite and margarite schists. Associated with it are 

 ordinary glaucophane schists. 



11. Pseitdodiahases, Pseiidodiorites, and Greenstones, 



Becker^ introduced the terms pseudodiorite and pseudodiabase 

 for certain basic metamorphic rocks, which he thought were meta- 

 morphosed sediments, in which the minerals were all products of 

 recrystallization. These rocks contain plagioclase, hornblende, 

 glaucophane, epidote, and zoisite, and, rarely, white mica. They 

 occur in the form of dykes and sills, and often show a distinct 

 porphyritic texture, even the ophitic structure of diabase being often 

 visible in thin slides. Chemically, too, they are identical with nor- 

 mal diabases or metabasalts. But Becker- says that they were 

 originally ordinary sandstones made up of the residue of decomposed 

 granites, and that the magnesia was introduced into them, and all 

 the minerals formed by hydrothermal action. Becker thought that 

 this same process even made the serpentines of the Coast Ranges, 

 where the magnesia was in great excess. Palache,^ however has 

 demonstrated that the serpentines of the Coast Ranges are made 

 from genuine peridotites, and the writer has examined slides of 

 serpentine from many parts of California showing abundant olivines 

 and orthorhombic pyroxenes as original constituents. It is now 

 recognized by all petrographers in California that these serpentines 

 were originally genuine igneous rocks, and that in the process of 

 serpentinization nothing has been added to them but water. And 

 Ransome^ has shown that the pseudodiabases and pseudodiorites of 

 Becker are merely basic igneous rocks, metabasalts or " fourchites," 



' Mon. XIIL, U. S. Geol. Survey, p. 94- 



=^ Mon. XIIL, U. S. Geol. Survey, p. I35- 



' " The Lherzolite-serpentine and Associated Rocks of the Potrero, San 

 Francisco," Bidl. Dept. Geol. Univ. of California, Vol. I., pp. i6i-i79- 



*"The Geology of Angel Island," Btdl. Dept. Geol. Univ. of California, 

 Vol. L, pp. 207 and 233. 



