724 DYNAMICAL GEOLOGY. 



would have made doleryte, when fused. If then a liquid dolerytic 

 fire-sea has, for such a range of eruptions, been made by the transform- 

 ation of motion into heat, the material fused must have been the un- 

 derlying first-formed crust of the globe; and this must then be doleryte 

 in constitution. On this point see, further, page 735. 



4. METAMOKPH1SM. 



1. General Characteristics. 



The process of metamorphism is a process of change in texture and 

 often in mineral constitution, such as has occurred among many of the 

 strata of the globe, after their original deposition. The term is ap- 

 plied 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. 



Cases of local alteration of structure and crystallization are common, modifying the 

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

 term pseudomorphism (from <//ev6^s, false, and ^op 1 /"), 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 meta- 

 morphic rocks, and requiring some elevation of temperature, 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 237, 256, 

 432. The famous Carrara marble is an altered Jurassic limestone, underlaid by talcose 

 and mica schist and gneiss. Extensive strata of limestone, gneiss, and mica schist 

 in the Green Mountains are Lower Silurian, and others in the Connecticut Valley are 

 Lower or Upper Helderberg (p. 256). The gold-bearing slates of the Sierra Nevada 

 are Triassic or Jurassic, as proved by the presence of fossils in some cases. 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 66-74 ; and examples of the results on a large scale have been 

 presented in the case of rocks of the Archaean age on pages 151-156, 

 and of those of the Paleozoic ages on pages 214, 400. The pages re- 

 ferred 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. 



