DEPTH THE MOST IMPORTANT GEOLOGICAL FACTOR. 43 



movement. Furthermore, the alterations which occur under dynamic con- 

 ditions are far more profound than those which take place under static 

 conditions. For instance, very ancient sedimentary rocks which have been 

 undisturbed by orogenic movements may be in almost the original condition 

 in which they were deposited. On the other hand, rocks of comparatively 

 recent age which have been in mountain-making areas and been deeply 

 buried may be profoundly modified. Little metamorphosed rocks of great 

 age are illustrated by the St. Peter sandstone of Wisconsin and the uncon- 

 solidated Cambrian sands of Russia. Profoundly metamorphosed rocks 

 of comparatively recent age are illustrated by the Eocene and Neocene 

 rocks of the Coast Range of California and the Eocene of the Alps. 



Depth. — Rocks at or near the surface of the earth are ordinarily under 

 conditions of slight pressure and low temperature. Rocks at some depth 

 below the surface are under conditions of considerable pressure and 

 temperature. It will be shown that the alterations of a given rock under 

 these varying conditions are very different. Therefore depth is a matter of 

 great consequence in the consideration of metamorphism. Indeed, depth 

 is believed to be the most important of the influences which determine the 

 character of the alterations of rocks. Therefore the geological factor 

 which in this treatise will serve as the primary basis for a classification of 

 metamorphism is the dominant factor of depth. On this basis metamor- 

 phism will be classified into (1) alterations in the zone of katamorphism 

 and (2) alterations in the zone of anamorphism. The zone of katamor- 

 phism is subdivided into (a) the belt of weathering- and (b) the belt of 

 cementation. The zone of katamorphism may be defined as the zone in 

 which the alterations of rocks result in the production of simple com- 

 pounds from more complex ones. The zone of anamorphism may be 

 defined as the zone in which the alterations of rocks result in the pro- 

 duction of complex compounds from more simple ones. The belt of 

 weathering is the belt which extends from the surface to the level 

 of ground water. The belt of cementation is the belt which extends 

 from ground-water level to the zone of anamorphism. 



It is to be noted not only that this classification is geological, but that 

 the factor is one which is universally applicable. Geological factors of 

 different kinds, such as movement, contact action, etc., are not introduced. 

 It is therefore clear that the proposed classification follows one law of all 



