706 DYNAMICAL GEOLOGY. 



the carbonic acid is not given out if the material is under heavy 

 pressure. If this be true of carbonic acid, it will be so also of 

 other ingredients less easily expelled. 



3. Change of color. — Compact limestones are usually of grayish, 

 yellowish, brownish, and blackish colors. From the metamorphic 

 process they often come out white. The original color in these and 

 argillaceous beds is often due to carbon from ancient plants or 

 animal matters ; and, when so, this carbon is removed and the rock 

 blanched, — -just as the limestone in a lime-kiln turns white. When 

 oxyd of iron in any form is present, the blanching does not take 

 place unless the oxyd is thrown into some new state of combination 

 in the crystallizing process. When there is only a partial meta- 

 morphism, its presence generally causes a change of color to red. 



4. Obliteration of fossils. — Eocks that have been subjected to the 

 metamorphic process have usually lost all their original fossils. 

 Where the metamorphism is partial, the fossils may in part remain, 

 only obscured. The Devonian coral limestone of Lake Memphre- 

 magog contains some nearly perfect corals ; but most of them are 

 much flattened and indefinite in outline, and others are only 

 patches of white crystalline carbonate of lime in a bluish-gray 

 limestone rock, which is itself hardly at all crystallized. A step 

 further in the process, and the limestone would have become a 

 whitish rock of uniform granular texture, with no traces of the 

 fossils, except, it might be, in white veinings and blotches. 



5. Crystallization. — The variety of crystalline rocks formed by the 

 metamorphic process, and the wide extent of the regions over 

 which they have been formed, will be learned from the pages 

 already referred to in the earlier part of this volume. They occur 

 in all parts of the world, underlying sedimentary formations, if not 

 at the surface, and they are of various ages, from the Azoic to the 

 Tertiary. While the Appalachian crystallization and that of New 

 England took place before the Mesozoic era, that of the Sierra 

 Nevada in California, according to Whitney, occurred as late as 

 either the commencement or end of the Cretaceous period ; and 

 that of portions of the Alps, after the Jurassic or Cretaceous. 



The crystallization, in some cases, involves no change of com- 

 position. This is the fact with most limestone ; the ordinary com- 

 pact rock may be simply changed by the process to a crystalline- 

 granular condition, and bleached in color. 



In other cases the constitution is altered, new mineral species 

 being formed. Argillaceous shales are changed to mica schists, and 

 argillaceous sandstones to gneiss or granite. Even in the case of 

 limestone, the impurities are turned into crystalline minerals of 



