GEOLOGY. 



METAMORPHIC ACTION. 



Prof. David Forbes, in "Journal of the Chemical Society," 

 June, 1868, classifies the facts on this intricate subject under six 

 difi'erent sections. 



1. By pressure alone. — Alterations by compression and indura- 

 tion, often inducing cleavage, as in clay-slates. This metamor- 

 phic change is trul}' mechanical ; but occasionally a slight amount 

 of chemical change (combination apparentl}") has taken place, 

 which possibly might have arisen from the indirect conversion of 

 mechanical into chemical action. 



2. By heat alone. — Many soft beds, like shales and cla3's, be- 

 come strongly indurated and converted into porcelanite, etc., in 

 near contact with basaltic dikes. Beyond the mere expulsion of 

 water, the change is similar to the baking of stone-ware. Such 

 rocks, in situ, are found generally to be more compact also from 

 pressure at the same time. 



3. By heat, in conjunction with chemical action and crystalliza- 

 tion. — Thus, if a calcareous and ferruginous shale or clay be sub- 

 jected to a heat much below fusion, whether in nature or artifi- 

 cially, we have a complete change in its appearance and mineral 

 composition, produced by the recombination of its constituents, 

 and the formation of new minerals, such as epidote and garnet, 

 both of which are silicates of alumina, iron, and lime. This will 

 explain the common occurrence of such minerals in the rocks at 

 the point of contact with igneous eruptions, as most of the sedi- 

 mentary rocks contain these constituents. When constant, but 

 not necessarily great, pressure is added, very striking conversions 

 take place, often due to recr3^stallization. To a similar cause may 

 be attributed many of the metamorphic crystalline limestones and 

 marbles, when the contact with igneous eruptions may have 

 heated them suflSciently to bring about such molecular changes. 



4. By aqueous action, either inducing crj'stallization, by intro- 

 ducing or dissolving out certain mineral substances, by forming 

 hydrated compounds with others, or by effecting chemical changes 

 through the agency of the gases or solids, or both, held in solution 

 in the water, all of which effects may be more or less assisted by 

 heat and pressure. Thus, the structure of organic limestone may 

 be obliterated, and this be converted into crystalline rook, as the 



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