REVIEWS. 64 1 



In a discussion of regional metamorphism two divisions must be 

 recognized, pressure and dislocation (dynamic) metamorphism. The 

 former alters all rocks, while the latter is dependent on the nature of 

 the rocks, and it is confined to certain areas. Both build similar 

 minerals and they have a common cause for mineral alteration. The 

 tests for distinguishing regionally metamorphosed rocks are varied. 

 While Credner and Dathe emphasized the point of the presence or 

 absence of mechanical phenomena as the clue, Milch regards the 

 absence of such effects as no proof against dynamic metamorphism. 

 A second distinction is the entrance of magnesian minerals of the mica 

 and chlorite groups. In regionally metamorphosed rocks chlorite is 

 more important, while in the true crystalline schists biotite is the 

 important mineral. 



Clastic structures are often lost and so cause an outward resem- 

 blance of many gneisses to eruptive rocks, but that this is only external 

 is easily proven by the chemical composition. 



In pressure metamorphism, the pressure is at first weak but 

 increases slowly through the successive geological periods, and it pro- 

 duces especially kamptomorphous forms. The condition for dislocation 

 metamorphism is a sudden pressure working in a limited time with 

 varying intensity and direction. This pressure does not form gneisses 

 but mica schists. In a rock under a slight pressure, clastic phenomena 

 are abundant, but as this pressure is increased, the forms become 

 kamptomorphotis. Finally, with great loading and small motion, 

 gneisses are formed resembling those produced by pressure metamor- 

 phism. Thus the two divisions are bound together by numerous tran- 

 sitions. The great force is pressure which sometimes produces greater 

 mechanical changes, under other conditions, more chemical alterations. 

 Mechanical changes, especially the crushing and formation of small 

 components, characterize dislocation (dynamic) metamorphism. 



Dr. Milch, in the second article, presents a new rock classification 

 which depends essentially on the source of the material of the rock 

 constituents and the cause of their present form. It is fourfold and 

 conformable to the four divisions of igneous rocks, chemical precipi- 

 tates, mechanical sediments, and crystalline schists. Its advantage 

 over this old system is the placing of the products of contact and 

 regional metamorphism in a fixed place. 



Group 1) includes rocks composed of components whose material 

 separated out of the original molten sea, in the form of the 



