314 DYNAMICAL GEOLOGY. 



is actually a vein-formation. Granite eruptions have no more metamorphic power over 

 the adjoining rocks than those of trachyte ; for granite in the melted state is identi- 

 cal, essentially, with melted quartz trachyte, and conforms to the same principles as 

 regards cooling. The walls of the dike, or mass, will rapidly chill against the cold inclos- 

 ing rock, and fail of the coarse crystalline texture of granite ; and the dike, or mass, will 

 take the coarse texture only under conditions as to thickness of mass that admit of 

 extreme slowness of cooling. The crystallizing of adjoining rocks to any great distance 

 by ejected granite is as improbable as the same by ejected trachyte. The alleged exam- 

 ples of such change in which the walls retain their coarse crystallization but little altered 

 (when at all), and where the metamorphic schists adjoining are supposed to afford an 

 example of what ejected granite can do, are, probably, either examples of cotemporaneous 

 metamorphism, and the contact minerals some of the products made by the process in the 

 transition region between the terranes or strata ; or of metamorphism in overlying 

 beds that were upturned and thrust against the preexisting range of granite, and which 

 became altered or crystalline as a consequence of the friction. 



Regional Metamorphism. 



Regional metamorphism is here considered under the following heads : — 

 (1) Incipient metamorphism, that of the lower or incipient stages ; 



(2) Crystallinic, or that in which there is simply change in crystallization ; 



(3) Paramorphic, or that of a change in crystalline form and not in com- 

 position, as when pyroxene is changed to hornblende, or aragonite to calcite ; 



(4) Metachemic, in which there is change in chemical constitution (also 

 styled metasomatic, which means change in the body of the rock, a general 

 fact under metamorphism) ; (5) Endo-crystallic, or effects of pressure in 

 modifying the structure of crystals, or in fracturing them. Finally, after 

 considering the metamorphic effects produced in uncrystalline rocks, those 

 occurring in crystalline rocks are described. 



The general effects of metamorphism are the following: — 



In the lower or incipient stage it discolors, dries, consolidates. In 

 higher stages it crystallizes the constituents of rocks ; it often produces also 

 chemical changes, making new minerals in the mass ; and, as a result, oblit- 

 erates fossils. Under the high temperature, which may attend it, all the 

 methods of mineral chemistry in nature have a chance for work according to 

 the conditions. The heat may reach that of fusion, producing effects that 

 cannot be distinguished from those of fusion from heat of other sources. 



The obliteration of fossils comes in an early part of the changes ; for 

 shells are seldom a twentieth of an inch thick, while the grains rendered 

 crystalline by the change are seldom so small as this. Large crinoid stems 

 have the best chance among calcareous fossils for preservation. But no 

 calcareous fossils can withstand the chemical action of siliceous solutions 

 at high temperatures ; for even strata of limestone are thinned down by it. 

 Trilobites, and other fossils whose tests are phosphatic, resist longer than 

 the calcareous. 



The uncrystalline rock-materials that undergo regional metamorphism. — It 

 has been stated that fragmental rocks are the chief kinds. But it is to be 



