SUMMARY AND CONCLUSION 177 



of crystallization. Practically the beginning may be delayed and the rate 

 variable without limitation, and the assignment of boundaries to the melt- 

 ing interval be therefore wholly arbitrary. The manner of participation 

 of such minerals in rock formation, either in nature or in the laboratory 

 is usually dependent upon the presence of certain volatile ingredients 

 (mineralizers?) which, even when present in very small quantities, appear 

 to increase the molecular mobility of the mineral to an extraordinary de- 

 gree, and thus to enhance its activity greatly. In dealing with minerals 

 of this class, the duty of the laboratory is clearly to follow nature by re- 

 placing those volatile ingredients of which insignificant traces are still 

 found in most of the mineral deposits, and to establish with equal care 

 the progress of the reactions in their presence. 



What has been said of the melting temperature is true in somewhat less 

 degree of the inversions of solid crystalline minerals from one crystal 

 form to another. It happens that the energy required for such transfor- 

 mations is relatively small compared with that required for melting, and 

 so but few such have hitherto been discovered. The application of meth- 

 ods of great precision in the Geophysical Laboratory has developed a 

 number of these where none were suspected before, and therefore greatly 

 enriched our knowledge of the development of individual minerals at rela- 

 tively low temperatures. Furthermore, many of these transformations 

 occur so punctually at a characteristic temperature as to serve an unex- 

 pectedly useful purpose in the establishment of a system of geologic ther- 

 mometry. 



In a number of recent publications in this laboratory, the behavior of 

 quartz has been cited in illustration of the facility with which geologic 

 temperatures can be established through the agency of such transforma- 

 tions. We are now able to differentiate sharply, for example, between 

 quartz formed above 575 degrees and quartz formed below that tempera- 

 ture ; and this, in turn, is radically different from the cristoballite, which 

 forms above 800 degrees. In each of these cases unmistakable records 

 have been left on the minerals themselves of the transformation through 

 which they have passed. Several other such_ inversion temperatures are 

 known for the minerals already studied, and together they form the begin- 

 ning of an accurate system of temperatures of which immediate and ex- 

 tensive application can be made in the study of natural rocks. 



The problem of building up a geologic thermometer is a straightfor- 

 ward one. We must select those substances which respond promptly to 

 changes of temperature, and determine for these the temperatures of their 

 changes of state. It may happen that a given mineral will offer but a 

 single reference point of this character, or, like quartz, a single mineral 





