42 PHYSICAL GEOLOGY 



gases of the atmosphere, of wind and water, of plants and 

 animals, and of other agents. They commence at once to 

 break up and decay, their constituents forming new combina- 

 tions suited to the new conditions. As in the cases suggested, 

 metamorphic changes in general are in the nature of adapta- 

 tions to a new environment. 



Although metamorphism, strictly speaking, includes all 

 changes in all rocks, and may be destructive in its effects, as 

 well as constructive, yet in common usage it implies radical 

 changes of the latter type, in consolidated rocks. Such 

 changes take place within the earth, especially at great 

 depths. The processes of metamorphism are treated hi later 

 pages (78-83). 



In working out its physical history, it is frequently impor- 

 tant to determine whether the metamorphic rocks of a given 

 region were derived from igneous or from sedimentary 

 rocks. The answer is sometimes given by bodies of unaltered 

 or little changed rocks within the metamorphic rocks. In 

 some cases, more or less distorted pebbles of various kinds 

 indicate that the parent rock was a conglomerate whose finer 

 material has been changed greatly, while the larger pebbles 

 were merely flattened and lengthened. Or again, it may be 

 possible to trace the gradation from the metamorphic rocks, 

 through less and less changed rocks, into the unaltered rocks 

 of a neighboring area. The origin of many metamorphic rocks 

 may be detected, too, by microscopic examination or by 

 chemical analysis. 



Gneiss (pronounced "nice") is a crystalline rock, in many 

 cases containing the same minerals and having the same general 

 appearance as granite, except that it is distinctly banded 

 (Fig. 18), due to the partial arrangement of unlike minerals 

 in separate layers. The bands may be bent and twisted in a 

 way that suggests intense crumpling (Fig. 6). Granitic 

 rocks usually become gneisses when metamorphosed. Gneiss 

 may be made, however, from various other kinds of rock. 



Schist is in general more closely and regularly banded than 



