CONTACT METAMORPHISM. 489 



In the belt of weathering contact metamorphism comprises (1) the 

 direct contact effect and (2) the indirect effect through gaseous solutions. 

 This second is ordinarily called the action of fumaroles and solfataras. 



DIRECT CONTACT EFFECT. 



The direct contact effect is produced by conduction of the heat of the 

 magma into the surrounding rocks. The temperature of the rocks 

 immediately adjacent to the igneous rocks may become very high — indeed, 

 approach that of fusion, and possibly reach a fusion temperature locally. 

 Under these circumstances the complex mixtures of minerals characteristic 

 of the belt of weathering, including the hydrates, silicates, and other forms, 

 are likely to be baked, and thus indurated. The hydrous minerals may be 

 dehydrated, and thus to this extent the ordinary reactions of the belt of 

 weathering are reversed. The zeolites, serpentines, talcs, chlorites, etc., 

 where dehydrated, are destroyed. Limonite may be changed to hematite, 

 gypsum to anhydrite, etc. From the carbonates the carbon dioxide may 

 be driven off or calcination take place, and thus lime and magnesia be 

 produced. This gives the rocks an alkaline reaction, so that contact action 

 on such rocks in the belt of weathering is sometimes called "caustic" 

 action. 



The general contact effects of dry heat have been called, depending 

 upon the degree of action, baking, fritting, and vitrification. Irving gives 

 the following illustrations of these processes: 



Sandstones are decolorized and often fritted to a glistening enamel-like or por- 

 celanic mass; in other cases, where the cement is of a calcareo-argillaceous .nature, 

 this is melted into a glass; cla} ;r and mud are converted into porcelanite or brick, 

 with marked change of color in many cases; tuffs 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, the various 

 shades of metatropic change of brown coal into anthracite, carbon-glance, bituminous 

 coal, and black coal being observed in some cases in the same section through several 

 meters of the mass; 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 asso- 

 ciated shales are converted into hornstone, as in the classical region of the Brocken. 



