Properties of the Feldspars. 95 



The application of measurable high pressures at the higher 

 temperatures has never been successfully accomplished, and 

 until something can be done in this direction, our knowledge 

 of the rock-forming minerals and in fact all the generalizations 

 relating to equilibrium between the states of matter, which 

 have been established for moderate temperatures, must be 

 regarded as more or tess tentative and subject to eventual re- 

 vision. We have been accustomed to assume, both in geology 

 and physics, with rather more confidence than scientific exper- 

 ience justifies, that established relations for ordinary tempera- 

 tures and pressures will hold in comparable ratio for the higher 

 temperatures and pressures also. Experimentation under 

 extreme conditions is slow and technically difficult, and it is 

 therefore not strange that simple relations which are verifiable 

 within easily accessible conditions should now and then be 

 accorded the dignity of natural laws without sufficient inquiry 

 into the more remote conditions. 



General Plan. — Our plan on entering this field was to study 

 the thermal behavior of some of the simple rock-making min- 

 erals by a trustworthy method, then the conditions of equili- 

 brium for simple combinations of these, and thns to reach a 

 sound basis for the study of rock formation or differentiation 

 from the magma. Eventually, when we are able to vary the 

 pressure with the temperature over considerable ranges, our 

 knowledge of the rock-forming minerals should become suffi- 

 cient to enable us to classify many of the earth-making pro- 

 cesses in their proper place with the quantitative physico- 

 chemical reactions of the laboratory. 



Relation to Geological Research. — The relation which this 

 plan bears to general geological research may perhaps be 

 expressed in this way. Geological field research is essentially 

 a study of natural end-phenomena, of completed reactions, with 

 but a very imperfect record of the earlier intermediate steps 

 in the earth-making processes. The records of the splendid 

 laboratory experiments in rock synthesis which have already 

 been made are also of this character. The final product has 

 been carefully studied, but the temperatures at which partic- 

 ular minerals have separated out of the artificial magma, and 

 the conditions of equilibrium before and after such separation, 

 have not been determined. In fact, except for a limited num- 

 ber of determinations of the melting points of natural minerals, 

 no exact thermal measurements upon minerals or cooling mag- 

 mas have been made, and it is in this direction that a beginning 

 is to be attempted. The temperatures of mineral reactions 

 under atmospheric pressures are nearly all within reach of 

 existing laboratory apparatus and methods. 



