422 Wright and Lav sen — Quartz as a Geologic Thermometer. 



heit or Reaumur. For higher temperatures melting points of 

 pure chemical elements, as gold, silver, copper, etc., serve as 

 the standard fixed points. 



Similarly in the study of rocks and minerals and their tem- 

 peratures of formation, it is expedient to choose and to deter- 

 mine certain definite points of reference which serve to 

 establish limits within which observed reactions must have 

 been effected. Geologic phenomena take place at different 

 temperatures, but at present very little exact information on 

 the entire subject is available, and in many instances the tem- 

 peratures are merely guessed at. Direct temperature measure- 

 ments are seldom possible, and even then only rough approxima- 

 tions can be obtained because of the disturbing factors enter- 

 ing into the problem. Experiment and laboratory tests must, 

 therefore, be largely relied upon for exact data bearing On 

 such problems. Geologic problems are often complex and 

 involved and require for satisfactory solution evidence from 

 all possible viewpoints, especially data on the geologic mode of 

 occurrence and on the physical and chemical relations of the 

 rocks. The present tendency to consider eruptive rocks as the 

 products of chemical systems and rock-making minerals as 

 components of such systems or magmas, and, therefore, sub- 

 servient to the laws of equilibrium governing physical chemical 

 systems, is the natural result of applying the exact methods of 

 physics and chemistry to geologic problems of a certain kind 

 which have thus far been investigated in many respects only 

 in a general qualitative or reconnaissance way. The geologic 

 mode of occurrence of a rock or mineral is an exceedingly 

 important fact to be carefully observed, since from it the • 

 original conditions of formation and consequent history may 

 be inferred ; but it is equally important to test such inferences 

 and to render them precise if possible by using, wherever 

 feasible, the exact methods of attack of physics and chemistry. 

 For this reason accurate data, relating to equilibrium condi- 

 tions and stability ranges of different minerals and aggregates 

 of minerals over different temperatures and pressures, are of 

 prime importance. 



To illustrate: crystallized calcium metasilicate (wollastonite or 

 pseudo-wollastonite) fuses at 1512°,* but at 1190° passes from 

 wollastonite to pseudo-wollastonite, which on cooling does not 

 revert in the solid state to wollastonite. The temperature of 

 inversion, 1190°, can, therefore, be used as a point on the 

 geologic thermometer scale, for the appearance of wollastonite 

 in a rock signifies at once that at the time of formation of the 

 wollastonite the temperature of the magma or solution was 

 below 1190°, otherwise pseudo-wollastonite, the form stable 



*This Journal, xxi, 101, 1906. 





