596 PETROLOGY AND MINERALOGY 



is true French investigators hold the contrary view, believing that 

 even in the transformations in the usual contact metamorphism, 

 e. g., of clay slate into hornfelt, (Fruchtschiefer, Garbenschiefer) , 

 new materials contributed to the sub-strata play a part. Chemical 

 analyses early proved this to be true for contacts of intrusive dia- 

 bases. There can be no doubt, either, that when the host of certain 

 granitic intrusives shows, beside the usual alterations, repeatedly 

 recurring mineralizations with newly formed tourmaline, topaz, cas- 

 siterite, axinite, and fluorine mica, that the formation of these min- 

 erals, so often connected with fissures, must point to a fumarole-like 

 exhalation of fluorine and boron vapors accompanying the eruption of 

 the granite. In other words, they must prove that there took place 

 an actual infusion of foreign chemical materials into the surrounding 

 rocks. 



There is another kind of rock metamorphism. The mountain- 

 building forces have compressed, folded, and crushed the rocks over 

 broad regions. Thus they have acquired a different and usually more 

 schistose structure, while at the same time they have developed a new 

 mineral composition. There now arises the important question what 

 are the chemical characteristics of these products of pressure meta- 

 morphism. Based on insufficient material and limited to specially 

 favorable hypothetical conditions, the law has been pronounced that 

 even in cases of the most thorough transformations of structure and 

 mineral composition there can have been no noteworthy chemical 

 change. By means of a comprehensive series of analyses, Reinisch has 

 shown this to be an erroneous generalization. He has shown that 

 the granite orthoclase rocks and diabases, when subjected to pressure 

 metamorphism, undergo a regular and very considerable chemical 

 alteration. There may be so great a difference between the composi- 

 tion of the normal rock and the composition of the same rock after 

 undergoing dynamic metamorphism that it is no longer possible to 

 speak of the rock as being chemically unimpaired. This is not un- 

 natural, since a greatly crushed rock offers a great number of new 

 points to the attack of the subterranean water. It is thus no longer 

 justifiable to attempt to determine, as was formerly done, the original 

 rock by means of the chemical analysis of its representative among 

 the products of dynamic metamorphism, for all resemblance to the 

 original has been obliterated. 



These examples show how indispensable in petrographic problems 

 is the aid offered by chemical analysis. The obligation does not lie 

 all upon the one side, however. Cases could be cited where chemistry 

 has reason to make acknowledgments to petrography for having 

 demanded increased refinement or broadening of existing methods. 

 Charged with the problem of determining the presence of even those 

 elements which occur in scarcely traceable quantities in the terres- 



