CHANGES IN CHEMICAL COMPOSITION. 765 



also occurs. The water and carbon dioxide eliminated from the chemical 

 combinations are largely squeezed out of the zone of anamorphism (see 

 pp. 665-667), but a subordinate amount may remain as inclusions in the 

 minerals developed. (See pp. 667-678.) The absolute losses of these 

 compounds is considerable. The loss of hydrogen, oxygen, and carbon 

 dioxide increases somewhat the percentage of the other elements, but does 

 not affect their relative proportions. Excluding hydrogen, the relative pro- 

 portions of the bases are substantially the same before and after alteration. 

 The replacement of carbon dioxide by silica reduces the amount of carbon- 

 ates, increases the quantity' of silicates, and lessens the amount of free 

 quartz ; but the absolute quantity of silica remains the same. In discrimi- 

 nating the metamorphosed sedimentary from the metamorphosed igneous 

 rocks, it will be shown on pages 914-915 that in many cases the pro- 

 portions of the important chemical elements furnish the best criteria for 

 separating the two classes of rocks. 



While in the absence of intrusives there is small change in chemical 

 composition of the rocks in the zone of anamorphism, there may be great 

 changes in the chemical composition where injection is an important factor 

 in the metamorphism (see pp. 711-720), for by injection material different in 

 chemical composition from the injected rock may be intruded in various 

 amounts and in various degrees of division for extensive areas. (See pp. 

 708-711.) The changes in chemical composition of the rocks where injec- 

 tion is important may be as great as or greater than in the belt of 

 cementation. 



The difference in the amount of chemical changes in the rocks in the 

 belt of cementation and in the zone of anamorphism under ordinary condi- 

 tions, and the difference in the amount of change in the latter zone where 

 injection is absent and where it is present, explain some of the differences 

 of opinion which are prevalent among- geologists. One geologist says that 

 metamorphism does not alter the composition of the rocks; another says 

 that metaniorphism greatly alters the chemical compositions of the rocks. 

 Each has a part of the truth. Probably the facts with which each is familiar 

 A T erify his point of view, but each makes a mistake in supposing that a nar- 

 row set of facts covers the entire field. The truth of the matter is, as 

 explained, that there are profound changes in the chemical composition of 

 the rocks in the belt of weathering, great changes in the belt of cementa- 

 tion, and considerable changes where intrusion is important in the zone 



