594 A TREATISE ON METAMORPHISM. 



VARIABLE MATERIALS AND CONDITIONS OF BELT OF CEMENTATION. 



Within the belt of cementation the materials and conditions are variable, 

 but not nearly so variable as in the belt of weathering. The materials 

 within the belt of cementation include all the classes which exist in the 

 belt of weathering (see pp. 429-430); that is, igneous rocks, sedimentary 

 rocks, surficial rocks, metamorphosed rocks, and their variations and 

 gradations; but ordinarily the alteration products are not nearly so com- 

 plex in the belt of cementation as in the belt of weathering, and therefore 

 the alteration forms of the belt are much less varied than those of the belt 

 of weathering. 



Water solutions are the main active agents at work in the belt of 

 cementation. Gaseous solutions and organic compounds, which are so 

 important within the belt of weathering, pla}^ a comparatively small part. 

 The water solutions may contain any of the bases and acids which occur in 

 the belt of weathering. (See pp. 457-458.) They may also contain gases 

 in solution, of which oxygen and carbon dioxide are the more important, 

 but others are of considerable consequence. In the belt of cementation the 

 temperature varies with depth and also varies greatly in consequence of 

 mechanical action and igneous intrusions. A great range of temperature is 

 much more common than in the belt of weathering, but the speed of change 

 at any place is slow, so that for a long time at any one place the tempera- 

 ture is relatively constant. The work of the belt of cementation is 

 accomplished by mechanical and chemical agents, precisely as in the belt 

 of weathering, but the character of the work performed is very different 

 from that of the belt of weathering. 



WORK IN BELT OF CEMENTATION. 



The rocks in the belt of cementation are modified by mechanical work, 

 by chemical work, and by igneous work. 



MECHANICAL WORK. 



The mechanical work of the belt of cementation occurs in connec- 

 tion with deformation. The mechanical work of wind, water, and ice, of 

 changes in temperature, of plants and animals, so important in the belt of 

 weathering, plays no part. The chief force producing' deformation is gravity 



