854 THE JOURNAL OF GEOLOGY. 



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 petro- 

 graphical study of the Verrucano rocks of Graubiinden, 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 carbon- 

 iferous eruptives of the region investigated are all melaphyres belong- 

 ing 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 geneticallv 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 Windgalle porphyries, con- 

 sidered by Heim and Schmidt (N. J. B. BB. IV., 1886) as pre-Carbon- 

 iferous. 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 mem- 

 branes. There is then here observable a complementary relation 

 between the mechanical and chemical work of dynamometamorphism, 



