178 A. L. DAY MINERAL RELATIONS FROM LABORATORY VIEWPOINT 



may yield four or more sharp physical changes which recur promptly at 

 the appropriate temperatures. In our experience it has happened that 

 inversions, or changes of crystal form in the solid state, occur more 

 promptly and are often more definitely recognizable than the melting 

 temperature, even though the quantity of energy involved in the inver- 

 sion is considerably less. Furthermore, inversion temperatures are much 

 less dependent upon volatile constituents. Pure quartz, for example, 

 melts very slowly over a long range of temperature. Its crystallization 

 does not occur under laboratory conditions without the assistance of vola- 

 tile substances. On the other hand, its inversion at 575 degrees takes 

 place sharply under the thermal microscope, with never a delay greater 

 than .5 degree, either on the heating or the cooling curve. Wherever 

 such transformations, which are known to occur promptly at a particular 

 temperature, leave an intelligible record behind, we have an absolute 

 bench-mark in geologic thermometry, and the establishment of a suffi- 

 cient number of such will place in the hands of the geologist a tempera- 

 ture scale of universal application. Nearly every mineral studied yields 

 one or more such points, and hardly more than a beginning has yet been 

 made in this field. 



