184 TRANSACTIONS OF SECTION C. 



A much more formidable obstacle in realising the conditions under which 

 rocks are formed is the small scale on which our operations can be carried on. 

 There are important problems connected with the differentiation of magmas, 

 whetlier in a completely fluid or partly crystallised state, rmder the action of 

 gravitation, for the solution of which it would seem for this reason impossible 

 to reproduce the conditions under which Nature works. Instead of a reservoir 

 many hundreds of feet in depth, we must content ourselves in our laboratory 

 experiments with a vertical range of only a few inches. 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 experi- 

 ment with cohmms several thousand feet in vertical height. 



In any attempt to reproduce the processes of metaniorphism other than those 

 of a purely thermal or pnemnatolytic character, or to imitate the conditions 

 that give rise to primary foliation, we must consider the effects of non-uniform 

 or directed pressure involving stresses that operate in definite directions 

 and result in deformation of the material on which they ac-t. Unlike uniform 

 pressure which usually raises the crystallisation point, directed pressure may 

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

 lisation of the rock.-' At the same tim.e 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 directed pressure in its operation at different temperatures, and in the 

 presence of different amounts of imiform pressure, a factor which has probably 

 an important influence on the result, which must also depend on the proportion 

 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 transfoiming 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 metamorphic action, the importance of extremely 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 pre- 

 vailed. It is not sufficient to bring about artificially the formation of a mineral 

 occurring in the rocks or mineral deposits imder investigation, for the same 

 mineral can be reproduced in many waji^s. It is, however, probable that a mineral 

 produced nnder 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 fares 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 artificial mineral has been formed under the same 

 conditions as the natural product. 



For this purpose it is above all necessary that there should be in the first 

 place a systematic comparative study of these characters and of the association 

 in which they are found. The results thus obtained should be of the greatest 

 value in indicating the directions along w-hich experimental work would be most 

 probably successful. They should, of course, be supplemented by laboratory 



" See J. Johnston and L. H. Adams, Jour. Am. Chem. Soc, vol. xxxiv., 

 p. 563 (1912) ; Am. J. ScL, vol. xxxv., p. 206 (1913) ; A. Harker, Proc. Geol. ,'^oc., 

 vol. Ixxiv., pp. 75-77 (1919). It is interesting to note that similar principles 

 ap'ply to the pseudo-fluidity induced in clay by directed pressure. See 

 P. M. Crosthwaite, Proc. Inst. C.E., December 19, 1916, p. 149; Journ. and 

 Trans. Soc. Eng., vol. x., pp. 82-86, 92-94, 1918; Alfred S. E. Ackermann, ih. 

 pp. 37-80, 102-107. 



