RELATIONS TO OTHER SCIENCES 595 



out the relative proportions of the individual components as shown 

 by their percentages of the total weights calculated according to the 

 individual molecular weight. Loewinson-Lessing, Pirsson, Michel- 

 Levy, Miigge, Brogger, Becke, Iddings, Osann, have made special 

 suggestions in this broad field of chemico-classificatory formulae, 

 graphics, and topics. 



The second great aim of the chemical analysis of rocks is to prove 

 the existence of changes in the substance of certain rock-material 

 by comparing it with other material which has not undergone such 

 alteration. Thus the methods of analytic chemistry have accumu- 

 lated a great mass of knowledge concerning the regular course of 

 simple weathering and of the complicated alterations caused by 

 the universally active agencies of weathering aided by the carbonated 

 and silica-bearing solutions which are the first products of that 

 process. Our great master, Gustav Bischof , has rendered the immortal 

 service of introducing order into our comprehension of this silent 

 play of chemical relationship and of the mutual exchange of material 

 within the rocks and strata of the earth. 



But the science of chemistry must also come to our aid in explain- 

 ing other more local transformations which take place in the min- 

 erals of the earth's crust. And first of all, regarding the changes in 

 those regions where as the result of the intrusion of eruptive masses 

 the bordering rock-strata have often been altered over broad areas 

 into that changed condition known as contact metamorphism. As 

 far as the effect of the active eruptive rock upon the passive country 

 rock can be recognized in these aureole-like areas of metamorphism, 

 from the actual line of contact where the metamorphic energy is 

 most intense, even to the extreme circumference where the last 

 traces die out in the unaltered host, --the affected rock-mass is 

 found changed according as it is more or less sensitive to such changes; 

 but yet hundreds of localities widely scattered over the earth's sur- 

 face show that the change in mineral content and even in rock-texture 

 has always taken place along the same general lines. It is now a 

 problem for the chemist to decide whether such a change represents 

 simply the molecular rearrangement of materials already present in 

 the host, or whether the latter has undergone an essential change 

 in its chemical composition, having taken up materials which separ- 

 ated out from the intruding rock as it solidified. Long series of 

 comparative analyses seemed to support the former explanation, at 

 least for the case of the plutonic rocks. These analyses indicate 

 that as a rule the phenomena of contact metamorphism take place 

 without either addition or loss of material, that the active eruptive 

 rock produces the phenomena of metamorphism simply through the 

 agency of changes in pressure and temperature accompanying the 

 intrusion quite independently of its own peculiar composition. It 



