140 CARNEGIE INSTITUTION OF WASHINGTON. 



New determinations of the a-(8 inversions of the several species have given 

 the following results: 



a-quartz — >/3-quartz 575°. 

 /3-quartz — > a-quartz 570°. 



a-tridymite — > /3i-tridymite 117°. 

 /3i-tridymite — >/32-tridymite 163°. 

 Reversions on cooling not very sharp. 



a-cristobalite — > ^-cristobalite 275° to 220°, 

 depending upon previous heat-treatment. 



i(3-cristobalite — > a-cristobalite 240° to 198°, 

 depending upon previous heat-treatment. 



A study of the remarkable variations in the temperature of inversion of 

 a- into /S-cristobalite has led to the conclusion that this mineral is made up 

 of two different molecular species of silica A\dthin the same crystal. Various 

 other properties of the silica minerals seem to have considerable bearing 

 upon theories of the structure of molecules and crystals. The nature of the 

 radical differences existing between the two difTerent types of inversion has 

 been discussed in some detail. 



The melting-point of cristobalite has been found to be close to 1625°. 

 Quartz melts at least 155° lower. 



(23) Die Stabilitatsverhiiltnisse der ICieselsiiure Mineralien. C. N. Fenner. Z. anorg. 



Chem., 1914. (In press.) 



A German translation of "The stability relations of the silica minerals" 

 (Am. Jour. Sci. (4), 36, 331-384. 1913). Reviewed under No. 22 above. 



(24) The general principles underlying metamorphic processes. John Johnston and 



Paul Niggli. J. Geol., 21, 481-516, 588-024. 1913. 



This paper is an endeavor to set forth the most important general prin- 

 ciples concerned in rock metamorphism — a general term which includes a 

 number of special cases (e. g., contact metamorphism), all of which, however, 

 differ only in the degree of predominance of one (or more) of a definitely 

 limited group of effective factors. These factors are : temperature, uniform 

 pressure, stress (non-uniform pressure), and gross composition of the system 

 at the time of metamorphism; the same, namely, which determine the equi- 

 librium of the relatively simple chemical systems hitherto investigated 

 experimentally. The knowledge gained from a study of these simple 

 systems may be used as a basis for a prediction of the general character and 

 significance of metamorphic processes; though in applying the principles 

 one must always bear in mind those circumstances which oppose the attain- 

 ment of a state of true equilibrium, such, for example, as slowness of reaction 

 or the formation of metastable intermediate products. 



Now, although the general character of the process may be predicted, no 

 particular statement as to the effects produced in a given system by change 

 of any of the above factors can yet be made, owing to lack of the requisite 

 quantitative data. In this connection, it is to be noted that the general 

 application of experimental results which obtain for a given system under 

 given external conditions, to another system under similar conditions, or 

 even to the same system under widely differing conditions, is subject to 

 considerable limitation. Conclusions drawn from such extrapolation of 

 experimental evidence mil commonly be of little value, and may be alto- 



