430 A TREATISE ON METAMORPHISM. 



original igneous rocks of eveiy variety, from plutonic to volcanic, including 

 both lavas and tuffs; rocks varying from the most dense to the most porous, 

 such as pumice; rocks varying from the most basic to the most acid, and 

 having the most variable mineralogical composition. A second class of 

 rocks which has even a more widespread occurrence in the belt of weath- 

 ering - than the igneous rocks are the sedimentary rocks. These include 

 mechanical sediments of all kinds, chemical and organic sediments of all 

 kinds, and all possible gradations between them. A third class which is 

 present in the belt of weathering is the metamorphosed rocks of igneous 

 and sedimentary origin. These rocks may have been altered either in the 

 zone of anamorphism or in the belt of cementation. Moreover, the amount 

 of alteration is variable, ranging from rocks which have been but slightly 

 affected to those which have been mashed and granulated or recrystallized 

 throughout. Fourth, there is every variety of surficial rocks produced by 

 the transporting- agents of wind, water, and ice. Moreover, all these four 

 classes of rocks are present in the belt of weathering in various stages 

 of alteration under the conditions of that belt, so that the products vary 

 from those in which the forces of weathering have produced but slight 

 effects to those in which the}' have produced final results. 



Within the belt of weathering all the agents of metamorphism are 

 present. These include g-aseous solutions, water solutions, and organic 

 compounds. Oxygen, carbon dioxide, water, nitrogen, and other gases are 

 present in variable proportions. Of these the first three are of greatest 

 consequence. In humid regions, at times of abundant precipitation, the 

 belt of weathering becomes nearly saturated with water, and these gases 

 may be nearly or quite excluded except as they are contained in the water 

 solutions. In desert regions there may be an almost complete lack of water, 

 even that held by imbibition being evaporated by the heat of the sun near 

 the surface. At such places the gases dominate. In the lower part of the 

 belt, in consequence of the variation of the level of ground water, is a layer 

 which has alternately conditions of saturation and conditions in which water, 

 carbon dioxide, and oxygen are together in important amounts and in various 

 proportions. Therefore the conditions are those ranging from saturation by 

 water solutions to saturation by gaseous solutions. 



It follows from the above that the amount of gaseous solutions is 

 inversely as the amount of water solutions. When the water solutions 



