Dr. J. W. Evans — Presidential Address. 559 



There are, however, other phenomena that require investigation and 

 that involve a great difference of level in their operation, but do not 

 take place at such elevated temperatures. Such are some of the 

 processes of ore deposition or transference, especially secondary- 

 enrichment. Here, with the friendly assistance of mining engineers, 

 but at the cost of considerable expenditure, it might even be possible 

 to experiment with columns several thousand feet in vertical height. 



In any attempt to reproduce the processes of metamorphism other 

 than those of a purely thermal or pneumatolytic character, or to 

 imitate the conditions that give rise to primary foliation, we must 

 consider the effects of non-uniform or differential pressure involving 

 stresses that operate in definite directions and result in deformation 

 of the material on which they act. Unlike uniform -pressure which 

 usually raises the crystallization point, differential pressure may 

 lower it considerably and thus give rise to local fusion and subsequent 

 recrystallization of the rock. 1 At the same time it profoundly 

 modifies the structure, resulting in folds and fractures of every 

 degree of magnitude. One of the most pressing problems of geology 

 at the present moment is to determine the effects of non-uniform 

 pressure in its operation at different temperatures, and in the 

 presence of different amounts of uniform pressure, a factor which has 

 probably an important influnce on the result, which must also depend 

 on the pi'oportion and nature of the volatile constituents which are 

 present, as well as on the time during which the stresses are in 

 operation. There seems no reason why valuable information should 

 not be obtained on all those points by properly conducted experiments. 



The time element in the constructive or transforming operations 

 of nature cannot, of course, be adequately reproduced within the 

 short space of individual human activity, or, it may be. that of our 

 race ; but I am inclined to think that, even in the case of meta- 

 morphic action, the importance of extrernety prolonged action has 

 been exaggerated. 



In attempting to imitate the natural processes involved in the 

 formation and alteration of rocks and mineral veins, we require some 

 means of ascertaining when we have approximately reproduced the 

 conditions which actually prevailed. It is not sufficient to bring 

 about artificially the formation of a mineral occurring in the rocks or 

 mineral deposits under investigation, for the same mineral can be 

 reproduced in many ways. It is, however, probable that a mineral 

 produced under different conditions is never identical in all its 

 characters. Its habit, or the extent to which its possible faces are 

 developed (a function of the surface tension), the characters of the 

 faces which are present, its twinning, its internal structure, inclusions, 

 and impurities, all vary in different occurrences, and the more closely 

 these can be reproduced, the greater the assurance we obtain that an 



1 See Johnston & Adams, Journ. Am. Chem. Soc, vol. xxxiv, p. 563, 1912 ; 

 Am. Journ. Sci., vol. xxxx, p. 206, 1913 ; Harker, Proc. Geol. Soc, vol. lxxiv, 

 pp. 75-7, 1919. It is interesting to note that similar principles apply to the 

 pseudo-fluidity induced in clay by non-uniform pressure. See Crosthwaite, 

 Proc. Inst. C.E., December 19, 1916, p. 149; Journ. and Trans. Soc. Eng. t 

 vol. x, pp. 82-6, 92-4; Ackermann, ib., pp. 37-80, 102-7. 



