i9o6,l 



SMITH— PARAGENESIS OF MINERALS. 205 



they are usually massive, although they grade over into glaucophane 

 schists and eclogites. Since they are usually of a greenish color, 

 from the actinolite, diopside, epidote and chlorite in them, they have 

 commonly been called "greenstones," and although this term has 

 been misused in many vague senses, the writer prefers to call them by 

 that name, restricting them to holocrystalline metamorphic massive 

 rocks, regardless of whether they were made out of diabases, dio- 

 rites or tuffs. In them the metamorphism has gone far enough to 

 destroy entirely the original feldspars and pyroxenes ; and diopside, 

 epidote, zoisite, actinolite, carinthine, glaucophane, white mica, 

 sphene and even occasionally lawsonite have been formed by the 

 redistribution of the chemical constituents, but no garnet has been 

 formed in them. They grade over on the one side to the little altered 

 pseudodiabases and pseudodiorites, and on the other into schists and 

 eclogites. The formation of the schists has taken place under what 

 Van Hise' calls mass-mechanical conditions, and the eclogites were 

 formed under mass-static conditions. When the metamorphism was 

 less intense than in either case, the greenstones were made, and 

 when the alteration was only slight, affecting only those minerals 

 that were unstable even at comparatively low pressure in the presence 

 of water, the pseudodiorites and pseudodiabases were formed. 



A fine exposure of typical greenstone is seen on the Hellman 

 ranch about two miles west of Redwood. It is massive, but grades 

 over into glaucophane schist. The minerals that compose it are 

 short prisms of diopside, rather slender actinolites and glaucophanes, 

 abundant pale epidote, and patches of titanite. Pyrite is disseminated 

 through the mass. Where the rock grades over into schist, glauco- 

 phane is more abundant, and rectangular sections of lawsonite occur 

 as porphyritic constituents. In places this rock might even be called 

 a lawsonite gneiss, on account of the massive banded structure, and 

 the abundance of lawsonite. No analysis was made of the Hellman 

 ranch greenstone, but a similar rock was described by Becker^ as a 

 pseudodiorite composed of actinolite, white mica, rutile, zircon and 

 titanite. 



' Mon. XLVII., U. S. Geol. Survey, p. 698. 

 ^Mon. XIII., U. S. Geol. Survey, p. loi. 



