ABSTKACTS: GEOLOGY 527 



elusion, the importance of further and more detailed systematic study 

 of volcanoes, the need of numerous chemical analyses, the importance 

 of the application of physico-chemical research to petrological problems, 

 and the magnitude and complexity of such future investigations, are. 

 insisted on. H. S. W. 



GEOLOGY. — The general principles underlying metamorphic processes^ 

 John Johnston and Paul Niggli. J. Geol. 21: 481-516. 1913. 



This paper is an endeavor to set for'th the most important general, 

 principles concerned in rock metamorphism — a general term which in- 

 cludes a number of special cases 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 equilibrium of the relatively simple chemical systems hitherto in- 

 vestigated 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; tho in 

 applying the principles one must always bear in mind th se circum- 

 stances which oppose the attainment of a state of true equilibrium,, 

 such, for example, as slowness of reaction or the formation of metastable 

 intermediate products. 



Now, altho 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 con- 

 ditions, is subject to considerable limitation. Conclusions drawn from 

 such extrapolation of experimental evidence will commonly be of little 

 value, and may be altogether misleading; moreover, one may as well guess, 

 the final result as arbitrarily choose the data required in calculating it. 

 From this we see that the application of the above simple principles,, 

 which determine rock metamorphism, to the complicated rock systems, 

 will be no simple matter, but will require extended experimental investi- 

 gation and a long time. In such investigation the first thing necessary 

 is a definite conception of the general processes of rock metamorphism ; 

 this it was the purpose of the authors to present. The choice of par- 



