408 JOHN JOHNSTON AND PAUL NIGGLI 
accordance with this we have observed that a pressure even of 15,000 
atmospheres (equivalent to a depth of perhaps 30 miles) is incom- 
petent at room temperature to convert marcasite (sp. gr. 4.9) into 
pyrite (sp. gr. 5.0) nor did this change take place to an appreciable 
extent at 425° under 2,000 atmospheres pressure, although under 
ordinary pressure it does take place about 450°.7 
Geologic thermometer—The use of transformation and other 
points has been suggested as fixed points on a geologic temperature 
scale,” that is, as points of reference by means of which we may be 
enabled to fix the range of temperature within which certain pro- 
cesses have occurred or certain rocks or minerals have been formed. 
Such points should be chosen and used with caution. The only 
points free from objection are those transformations solid-solid 
which take place at a definite point with appreciable velocity and 
are not much influenced by pressure; in other words, we must choose 
rapid transitions which are accompanied by a small change of 
volume (or in the rarer case, by a large heat effect; or by both 
together). This is a serious limitation upon the number of satis- 
factory points, a limitation which must, however, be retained so 
long as our ignorance of the magnitude and character of the com- 
pressive stresses to which rocks have been subject remains complete, 
as in effect it now is. Melting-points could be used as points on 
the geologic thermometer only by postulating the purity of the 
substance and on the further improbable assumption that the com- 
pression has always been uniform; for, as we hope to show in a 
later paragraph, the effect of unequal pressure (shearing stress) 
upon the melting-point is so great that no conclusion of any value 
can be drawn except from such transition points solid-solid as 
remain unaffected by compression of any kind. 
The use of transitions involving a gas or vapor phase is alto- 
gether indefensible. ‘Take for instance the dissociation of calctum 
carbonate, according to the equation CaCO,=CaO+CO,; to each 
temperature there corresponds a definite pressure of CO.,, a pres- 
1G. Spezia (Atti. Accad. Sci. Torino, XLVI [1911], 1) has made a number of simi- 
lar observations all of which are in complete harmony with the above statements. 
2See Wright and Larsen, Am. Jour. Sci., XX VII (1909), 421; Z. anorg. Chem., 
LXVIII (1010), 338; J. Koenigsberger, Newes Jahrbuch Min., Beilage Band XXXII 
(1911), tor, in which he makes a number of statements which require some reservation, 
as will be evident from what follows in the text. 
