PRESSURE AS A FACTOR IN THE FORMATION OF 

 ROCKS AND MINERALS 



JOHN JOHNSTON 



Geophysical Laboratory, Carnegie Institution of Washington 



It is not surprising that application of the knowledge comprised 

 in the older chemistry should have yielded comparatively little 

 information as to the way in which rocks and minerals actually 

 form; and for this reason. The older chemistry dealt in the 

 main with the formation and behavior of substances at tempera- 

 tures confined to a very small range and practically at a single 

 pressure only — that of the atmosphere ; in other words, it dealt with 

 the merest slice of the surface which would represent the behavior of 

 the substance throughout the range of temperature and pressure 

 within which it is capable of existence. At the same time there was 

 a failure to recognize that its work was so limited in character, and 

 consequently the endeavor to deduce regularities from the behavior 

 of substances at ordinary temperature and pressure — under arbi- 

 trary conditions, in other words — was but partially successful. The 

 reason for this is clear if we consider how our ordinary chemistry — 

 that of the 20° C. level — would be changed at the 200° level, for a 

 large number of compounds would then have become unstable ; and 

 again, that the number of compounds persisting at the 1,000° level, 

 and a fortiori at the 2,000° level, would be but a small fraction of 

 those which exist at ordinary temperatures. 



Igneous rocks and minerals have formed under conditions much 

 removed from 20° and i atm. pressure; so that we are little likely 

 to ascertain much about their formation except by thorough and 

 extensive investigations over a wide range of temperatures and 

 pressures, investigations which would at the same time throw much 

 needed light on a number of very important chemical problems. 

 Incidentally it may be observed that the variables temperature and 

 pressure are completely analogous; that besides the ordinary 

 freezing-point of a liquid — in stating which we imply a definite 



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