Page:The Journal of geology (IA journalofgeology11893univers).pdf/438

The regret expressed by Professor Heim at the time of writing his text that no one had been found to thoroughly investigate the dynamic phenomena of the Verrucano seems about to be removed by the work now being published by Dr. L. Milch of Breslau. He has recently offered as his habilitationschrift the first part of his petrographical study of the Verrucano rocks of Graubünden, which deals with the historical development of the knowledge of this formation and the dynamic metamorphism of the eruptive rocks occurring in it. The second part, to be published later, will treat of the metamorphosed sediments and chemical aspects of the whole subject. The basic carboniferous eruptives of the region investigated are all melaphyres belonging to the three types: olivine-weisselbergite, navite and tholeiite; in other words old basalts. Some of them are well preserved, and show clearly the progressive effects of metamorphism with increasing mechanical disturbance. Some of the rocks are massive and others amygdaloidal, but the effect of the pressure is finally to destroy all of their original characters and to produce from them chloritic, epidotic, sericitic, or carbonate schists, which could just as well have originated from sediments of the proper composition. The mechanical action differentiates the originally homogeneous rock into portions of very different mineralogical composition, which in the most squeezed parts of a fold form fine parallel layers, but in the less compressed areas are intimately interlaced. Thus the same orographic force, while it may produce the same result from rocks genetically very distinct, can also, on the other hand, produce highly diverse rocks from one and the same mass.

The acid Carboniferous eruptives of the area studied are quartz-porphyries, or old rhyolites. Some of these form an important part of the pebbles in the Verrucano conglomerates, while others occur in situ as a contemporaneous part of this formation. The latter rocks show many points of resemblance with the Windgälle porphyries, considered by Heim and Schmidt (N. J. B. BB. IV., 1886) as pre-Carboniferous. Milch distinguishes two categories of metamorphic changes exhibited by these acid eruptives. The first is mainly mechanical, consisting of crushing and granulation, and producing fine-grained, jaspery schists; the second is mainly chemical, producing sericite from the feldspathic constituents which forms interlacing membranes. There is then here observable a complementary relation between the mechanical and chemical work of dynamometamorphism,