CONTACT METAMORPHISM 765 



the silica in solution. Here the flints mark the original distribution 

 of the organic remains in the strata. The source of the silica is to be 

 found in the organic structures of silica scattered through the mass 

 of the chalk. These are dissolved by the circulating waters and 

 redeposited around the organic nucleus. 



Chert concretions occupy the same relation to limestone that 

 flints do to chalk. They too are derived from the organic silica 

 enclosed in the deposit, and redeposited in favorable places. Con- 

 fluent concretions of chert produce a more or less continuous chert 

 bed such as is common in the Devonic limestones of eastern North 

 America. Chert concretions may enclose organic remains, but they 

 do not necessarily form around a visible nucleus. Van Hise holds 

 that the heavier chert bands are formed first by the original segre- 

 gation of siliceous organisms and the subsequent enrichment by 

 silica-bearing grotind waters of these siliceous strata. 



VII. Hydration and Dehydration. 



Hydration, or the union with water of originally anhydrous de- 

 posits, may produce profound results. Thus anhydrite is changed 

 to gypsum with a corresponding swelling of the entire mass (see 

 ante, p. 177) and the production of deformative structures. Dehy- 

 dration of gypsum, on the other hand, produces a corresponding 

 shrinking of the entire mass. 



CONTACT METAMORPHISM OR ^THOBALLISM. 



I. Pyrometamorphism. When rocks come in contact with 

 heated igneous masses, as a result of subterranean intrusions, or of 

 surface flows, they are more or less altered, especially along the 

 contact, this alteration gradually decreasing in intensity away from 

 the igneous mass, until its effect has been entirely lost. Such a 

 phase of the contact metamorphism may be called igneo- or pyro- 

 metamorphism. It is especially manifested in the formation of 

 new minerals along the contact zone and in the introduction of 

 mineral substances from the igneous mass. A zonal arrangement 

 is commonly formed, different alteration products arising at dif- 

 ferent distances from the igneous mass. Effects are felt by both 

 the intruded and the intruding (or overflowing) rock; the former 

 is exomorphic, the second endomorphic. The exomorphic effect 

 from contact with dry heat is first of all a raising of the temper- 



