98 



METAMORPHISM. 



TJPPEE 



Gaxlbby. 



Wall-case 45 



more 



or less crystalline, the rocks (32 to 35) have more 

 of an igneous than of an aqueous character. 



The specimens which illustrate the gneissic series are 

 necessarily imperfect, the geological survey not yet having 

 been much engaged on metamorphic districts. The forces 

 which produced these rocks are of a different kind from 

 those above adverted to. It will be observed that mica- 

 schist, gneiss, &c, possess a laminar arrangement in which 

 various minerals are arranged generally in wavy lines 

 more or less distinct. The most typical gneiss consists of 

 alternating layers of quartz, felspar, and mica, these being 

 the same constituents that form granite. Perfect and most 

 gradual passages may often be traced from common shahs 

 slates, and grits, into true mica-schist and gneiss, and gneiss 

 sometimes passes imperceptibly into granite. The evidence 

 may therefore be considered as perfect, that gneissic rocks 

 have been metamorphosed, or that they have assumed dif- 

 ferent mineral characters from what they possessed in their 

 original sedimentary forms. This has been understood and 

 believed by most geologists since the days of Hutton,* but 

 the causes that produce these changes are yet but imper- 

 fectly explained. 



The subject involves much discussion, but it may be briefly 

 stated that enough is known to make it certain, that, when 

 under metamorphic action, distinct minerals are developed 

 that do not appear in the unmetamorphosed strata, it is 

 not that new substances are created there, but as a rule 

 simply that the rocks themselves originally contained 

 certain constituents which chemically re- arranged them- 

 selves according to their affinities, probably in all cases 

 under the influence of slow and long applied heat and 

 moisture. The only cases in which new substances could 

 appear, would be by the occasional infusion of gases and 

 moisture containing new ingredients, and it is probable 

 that the development of minerals in metamorphic rocks 



* « 



Theory of the Earth." 



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