Page:The Journal of geology (IA journalofgeology11893univers).pdf/437

The second division or green schists include foliated gabbro, diabase, variolite, serpentine and various pyroclastic deposits now filled with new epidote, uralite, chlorite, saussurite and other secondary products. They show many points of analogy with the greenstone schist areas of the Marquette and Menominee districts on Lake Superior. Of more than usual interest are Schmidt's descriptions of the chloritic ferruginous oölite of Callovien. This was once a glauconitic oölite of Jurassic age, whose spherical particles have, by dynamometamorphism, been flattened, while their iron has crystallized as magnetite and hematite, and their glauconite changed to chlorite. The process of metamorphism in the Bündnerschists is summed up by Schmidt as follows: "The first stage of the metamorphism of the sediments always consists in the development of rutile microliths, as well as isolated and usually skeleton crystals whose nature depends on the composition of the metamorphosed material. These new phenocrysts gradually increase both in number and size; they are always filled with abundant inclusions of the groundmass whose sedimentary arrangement is not destroyed within the newly formed phenocryst. Finally, the clastic groundmass is transformed into an aggregate of crystalline minerals; and, where the metamorphism is most intense, the contrast between new phenocrysts and groundmass is least distinct." (l. c., p. 71.)

As a result of both his own and Schmidt's work, Heim concludes (l. c. p. 488): 1) that all the demonstrable orographic disturbance, and hence all the dynamometamorphism within his area, is post-Eocene, and much of it post-Miocene; 2) that two sorts of metamorphism must be distinguished. The recent dynamic metamorphism which was caused by, and hence was synchronous with orographic movement; and the much more ancient and probably still continuous metamorphism due to heat, moisture, and simple pressure without motion, which he calls diagenetic metamorphism (statical metamorphism of Judd). He contrasts the effects of mechanical metamorphism upon highly crystalline and sedimentary rocks, in that the same cause crushes the former into a finely granular schistose series, and recrystallizes the latter by developing large phenocrysts within them. He attributes these results in his particular Alpine region entirely to dynamic action, since he can find no trace of eruptive material which could have produced contact metamorphism in rocks of tertiary age.