Geology and Mineralogy. 151 



ing metaniorphism and its products the author divides the crust 

 of the earth into three zones whose relations to the various 

 factors of metamorphism are shown in this table. 



Temperature Hydrostatic Stress Chief effect 



pressure of pressure 



Upper zone moderate small strong mechanical 



Middle zone higher stronger very strong chemical 



Lowest zone very high very great small chemical 



Each zone is chai*acterized by certain minerals ; thus the upper 

 by chlorite, sericite and chloritoid ; the middle zone by horn- 

 blende, staurolite and muscovite, and the lower by pyroxene, 

 feldspars, etc. ; the minerals of the last division are more nearly 

 like those of the igneous rocks. Some minerals, like biotite, are 

 found in all three zones, others appear in two only. Now if a 

 given kind of material, and it makes no difference what its origin, 

 whether igneous or sedimentary, be subjected to metamorphism, 

 while its chemical composition in mass remains the same, it will 

 yield rocks quite different in mineral composition and texture in 

 each of the three zones. Thus a diabase becomes a greenstone 

 schist, an amphibolite or an eclogite in descending order, and in 

 the same way a clay shale becomes a phyllite, a mica-schist or a 

 gneiss. The gneisses are of course mostly in the lowest zone. 



The first step in the classification is a purely chemical one ; all 

 the material, whatever may be its origin, which can be subjected 

 to metamorphism, is considered and divided into twelve groups 

 based on chemical composition. This grouping is accomplished by 

 following essentially the same methods proposed by Osann for the 

 igneous rocks, and his projection of the analyses is also followed 

 for illustration. This demands that a chemical analysis of 

 the rock must be made in order to definitely place it, though 

 of course comparisons with well-studied types will often answer. 

 To carry out his plan the author has had made under his direc- 

 tion a large number of analyses of selected specimens, the 

 results of which are given. 



Each of the groups thus made is then divided into three 

 orders, according as the rock belongs to one of the three zones 

 mentioned above. Thus the crystalline schists are divided into 

 thirty-six orders belonging to twelve groups, each group charac- 

 terized by a certain general chemical composition and each order 

 by characteristic minerals, textures, etc. The whole scheme has 

 been most carefully thought out and developed and is very clearly 

 presented. Criticism ot it will probably be directed to two 

 features. The first is the initial difficulty encountered in requir- 

 ing a chemical analysis for every rock to be classified, though 

 much of course can be done by comparison with well investi- 

 gated types. It can be suggested also that the method of micro- 

 scopic analysis, introduced by Rosiwal and nowadays being so 

 much developed in the study of igneous rocks, will prove just as 

 serviceable in the study of the crystalline schists. 



