704 DYNAMICAL GEOLOGY. 



3. METAMORPHISM. 



1. General characteristics. 



The process of metamorphism is simply a process of change or 

 alteration, such as has occurred among many of the strata of the 

 globe after their original deposition. The term is applied especially 

 when the changes have affected great series of strata, producing, 

 as an extreme result, a crystallization of the rocks, and, as a more 

 moderate effect, simple consolidation, and where it is evident that 

 some degree of heat above the ordinary atmospheric temperature 

 has been concerned. 



of local alteration of structure and crystallization are common, modify- 

 ing the composition of isolated crystals or masses. But such changes come 

 mostly under the term pseudomorphism (from -nvEv&os, false, and jiopiprt, form). If, 

 however, as is not unusual, they occur over considerable areas, or near dikes or 

 veins, and are not due simply to ordinary mineral solutions infiltrating through 

 a rock or seam, or to some similar local action, but to a wider cause analogous to 

 that crystallizing the metamorphic rocks and requiring some elevation of tem- 

 perature, they are then examples of true metamorphism. Still, it is often difficult 

 to draw the line between the two series. 



Examples of metamorphic rocks in part fossiliferous are mentioned on pages 

 270, 391, 392. The famous Carrara marble is an altered Jurassic limestone 

 underlaid by talcose and mica schist and gneiss. The crystalline rocks of the 

 Alps are largely of the same age : according to Charpentier, Lardy, and Studer, 

 Belemnites occur near St. Gothard in a micaceous schist containing garnet • 

 and in the Grisons, Murchison observed a nummulitic rock turned into a kind 

 of gneiss. Crystallized limestones in the Urals still retain in places their 

 Palseozoic fossils. In the Vosges, corals are said to occur in a hornblendic 

 rock changed, without a change of form, to hornblende, garnet, and axinite. 



The various kinds of metamorphic rocks have been described on 

 pages 74-84 ; and examples of the results on a large scale have been 

 presented at length in the case of rocks of the Azoic age on pages 

 138-142, and of those of the Palseozoic ages on pages 409, 410. The 

 pages referred to are a proper introduction to the review of the 

 subject and the additional explanations which are here given. 



2. Effects of metamorphism. 



The principal effects of metamorphism upon rocks are the follow- 

 ing: — (1) Consolidation ; (2) Loss of water or other vaporizable in- 

 gredients ; (3) Change of color ; (4) Obliteration of fossils ; (5) Crys- 

 tallization, with or without a change of constitution. 



1. Consolidation. — Ordinary atmospheric or subterranean waters, 

 however prolonged their action, do not necessarily produce solidifi- 



