256 day: geophysical eesearch 



operate in the not far distant future to develop an entirely new 

 conception of the science of mineralogy. 



As the number and scope of such exact measurements increase, 

 we gradually build up what may be called a geologic thermometer. 

 Just as the location of fossils offers a basis for estimating geologic 

 time, it often happens that a mineral takes on a variety of differ- 

 ent crystal habits, according as it happened to form at one tem- 

 perature or another. Quartz, for example, which is one of the 

 commonest of natural minerals and one of the most familiar, 

 undergoes two changes in its crystal form which leave an inefface- 

 able record. One occurs at 575° and the other at 800°. An 

 optical examination of even a minute quartz fragment from the 

 mountainside will reveal to the skilful petrologist whether the 

 crystal formed at a temperature below 575°, between 575° and 

 800°, or above 800°. And if we could, have at our disposal a 

 great body of such exact measurements of the temperature region 

 within which particular crystals originate and remain stable, 

 we could apply that directly to terrestrial formations in which this 

 mineral occurs, and read therein the temperature which must 

 have obtained during their formation. All this will not be done 

 in the first year, and perhaps not in the first decade ; but the ulti- 

 mate effectiveness of this method of procedure in establishing 

 the relations between the minerals and the valuable ores is now 

 as certain of success as the operations of any of the sciences which 

 have now come to be characterized as exact, as opposed to descrip- 

 tive. 



There is one important difference between the great laboratory 

 of nature and its feeble human counterpart. Nature operated 

 with large masses, mixed with a generous hand, and there was 

 always plenty of time for the growth of great individual crystals, 

 at which we marvel whenever we encounter them, and which we 

 have sometimes come to regard highly as precious stones. To 

 carry these processes into the laboratory is necessarily fraught 

 with certain limitations. The quantities must remain small and 

 the time and available financial resources will always be limited. 

 So long as we are able to ascertain the optical character of a crys- 

 tal with equal exactness whether the crystal is of the size of the 



