766 PRINCIPLES OF STRATIGRAPHY 



atiire of the country rock adjacent to the igneous mass, such rise 

 in temperature reaching even fusion point. Three stages of influ- 

 encing the country rock are recognized : baking, fritting, and vitri- 

 fication. As a result of baking, in(hiration of unconsoHdated ma- 

 terial will occur without fusion. Hydrous minerals are dehydrated 

 with a corresponding change, such as limonite to hematite, gypsum 

 to anhydrite, etc. From carbonates the CO. is given off, changing 

 limestone and dolomites to lime and magnesium oxides, respectively, 

 which have a caustic reaction. Volatile gases are driven off, as in 

 the case of coal which is changed to coke. 



Fritting is partial fusion, carried to the point where the silica 

 begins to act on the bases, forming an imperfectly melted or fritted 

 mass. Vitrification results from complete fusion, the mass being 

 transformed into a glass. 



The effects of pyrometamorphism on clastic rocks are various. 

 "Sandstones are decolorized and often fritted to a glistening enamel, 

 like a porcelainic mass ; where the cement is of a calcareo-argilla- 

 ceous nature, this is melted into a glass ; clay and mud are con- 

 verted into porcelainite or brick, with marked change of color in 

 many cases ; tuft's and phonolites are so far vitrified as to acquire a 

 character resembling that of obsidian; brown coal is altered into 

 seam coal or anthracite, and these in other cases into a substance 

 more resembling graphite, while in others (probably under less 

 pressure) the coal is converted into coke; a prismatic structure is 

 developed not only in clays and marls, but even in sandstones, in 

 brown coal, in seam coal and in dolomite ; limestones are altered 

 into crystalline marble, often with complete effacement of their 

 stratification and even of all traces of their fossils ; the finer varieties 

 of grauwacke and its associated shales are converted into horn- 

 stone, as in the classical region of the Brocken." (Irving-19: 7<5.) 

 The effects of dry heat are very limited in extent, usually penetrat- 

 ing the rocks only for a short distance. Where hydro- and gaseo- 

 (atmo-) metamorphism are also active, as is almost universally the 

 case in pyrometamorphism, the alterations will be more extensive 

 and widespread. The endomorphic effects, or those on the igneous 

 mass itself, are largely confined to a more rapid solidification and 

 hence the production of finer crystallization along the contact, owing 

 to the chilling effect of the cool wall rock. The presence of water 

 in the latter, of course, contributes to a much modified result, as 

 already indicated in an earlier chapter (p. 312). 



2. H ydrometamor phistn. Contact with rising waters, whether 

 hot or cold, carrying mineralizers, produces the second type of con- 

 tact metamorphism to which the term hydrometamorphism may be 



