224 SMITH— PARAGENESIS OF MINERALS. [Octobers. 



The albite schists and gneisses, described above, at least have the 

 composition of soda syenites, and must have been made from such 

 rocks, or from tuffs or arkose derived from them. 



Chemical Readjustment in the Metamorhism of the Glauco- 



phane-bearing rocks. 



Igneous Rocks. — The original igneous rocks and the metamorphic 

 glaucophane-bearing rocks made out of them have the same chemical 

 constitution, for there has been little addition. But the two groups 

 invariably differ mineralogically, for even' when the same mineral 

 species occurs in the two groups, it is represented by different varie- 

 ties, one a characteristic product of crystallization out of a fused 

 magma, and the other the product of solutions under pressure. 

 When the metamorphism has not been intense, some of the original 

 minerals have persisted, but in most cases every mineral in the 

 glaucophane rocks is a product of recrystallization. 



The feldspars, hornblendes and pyroxenes have been the chief 

 factors in the readjustment. The orthorhombic pyroxenes have 

 invariably decomposed, none being found in any of the metamorphic 

 rocks of the glaucophane series. The monoclinic pyroxenes have 

 fared little better, having usually been uralited to form some variety 

 of amphibole, with the probable addition of soda in most cases. 

 More rarely they have formed diopside, and in one instance secondary 

 diallage has been observed, in the altered glaucophane-bearing 

 gabbro of the Junction School-house near Healdsburg. 



The original hornblendes and pyroxenes of the igneous rocks 

 of the Coast Ranges of California have often been titaniferous, but 

 the secondary minerals resulting from them by metamorphism 

 generally contain no titanium. Hence, when glaucophane, epidote 

 zoisite, or garnet was formed there was a certain amount of titanic 

 oxide set free, which either crystallized out as rutile, or took up lime 

 and formed sphene or titanite. Fringes of titanium minerals are 

 common around the original titaniferous hornblendes or pyroxenes 

 where the readjustment has begun. 



In the feldspars the readjustment has been still more fundamental, 

 for the material of which they were composed has been distributed 

 in the formation of entirely different minerals. The original feld- 



