io6 



NATURE 



[October 2, 1919 



origin, were widely different from those with which 

 we are familiar. In such circumstances the ultimate 

 physical principles are the same, but the so-called 

 constants have to be determined afresh, and a new 

 chemistry must be worked out. It is necessary, there- 

 fore, so far. as possible, to reproduce the conditions 

 that prevailed — a task which has been courageously 

 undertafien and, to a considerable extent, accom- 

 plished by the geophysical laboratory of the Carnegie 

 Institution of Washington. 



Bv artificial means temperatures and pressures have 

 been already produced far higher than those that 

 were in all probability concerned in the evolution of 

 anv of the rocks that have been revealed to us at the 

 surface by earth-movements and denudation, for it is 

 unlikelv that in any case they were formed at a 

 greater depth than five or six miles, corresponding with 

 a uniform (or, as it is sometimes termed, hydrostatic) 

 pressure of, 2000 or 2400 atmospheres, or at a greater 

 temperature!, than 1500° C. Indeed, it is probable 

 that the vast majority of igneous and metamorphic 

 rocks, as well as mineral veins, came into existence 

 at considerablv less depths, and at more moderate 

 temperatures. It is true that most of the rock-forming 

 minerals crystallise from their own melts at tem- 

 peratures between 1100° C. and 1550° C, but they 

 separate out from the complex magmas from which 

 our igneous rocks were formed at lower temperatures. 

 It has been found possible at the geophysical labora- 

 tory to maintain a temperature of 1000° C. or more 

 under a uniform pressure of 2000 atmospheres for so 

 long a time as may be desired, and, what is equally 

 important, the temperature and pressure attained can 

 be determined with satisfactory accuracy, the tem- 

 perature within 2° C, and the pressure within 

 5 atmospheres. 



It has been ascertained that such uniform pressure 

 as would ordinarily be present at the depths men- 

 tioned does not directly affect the physical properties 

 of minerals to anything like the same extent as the 

 difference between the temperature prevailing at the 

 earth's surface and even the lowest temperature at 

 which igneous rocks can have been formed. It has, 

 however, a most important indirect action in main- 

 taining the concentration in the magma of a con- 

 siderable proportion of water and other volatile con- 

 stituents" which have a far-reaching influence in 

 lowering the temperature at which the rock-forming 

 minerals crystallise out — in other words, the tem- 

 perature at which the rock consolidates — and in 

 diminishing the molecular and molar viscosity of the 

 magma, thus facilitating the growth of larger crystals 

 and the formation of a rock of coarser grain. They 

 must also be of profound significance in determining 

 the minerals that separate out, the order of their 

 formation, and the processes of differentiation in 

 magmas. 



It is, therefore, obvious that any conclusions derived 

 from the early experiments which were carried out 

 with dry melts at normal' pressures must be received 

 with very considerable caution. Nor does much 

 advance appear to have been made, even at the geo- 

 physical laboratory, in experiments with melts cori- 

 taining large amounts of volatile fluxes, and yet, if 

 we are to reproduce even approximately natural 

 conditions, it is absolutely necessary to work with 

 magmas containing a proportion of these constituents, 

 and especially water, equal in weight to at least one- 

 third or one-half of the silica present. This will 

 obviously present considerable difficulties, but there 

 is no reason to doubt that it will be found possible to 

 surmount them. 



\ much more formidable obstacle in realising the 



« John Johnston, Journ. Franklin Inst., Januaty, 1917, pp. 14-19- 



NO. 2605, VOL. 104] 



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, whether in a completely 

 fluid or partly crystallised state, under 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 ver- 

 tical range of only a few inches. There are, how- 

 ever, other phenomena that require investigation and 

 that involve a great difference of level in their opera- 

 tion, but do not take place at such elevated tempera- 

 tures. Such are some of the processes of ore deposi- 

 tion or transfereTice, 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 meta- 

 morphism other than those of a purely thermal or 

 pneumatolvtic character, or to imitate the conditions 

 that give rise to primary foliation, we must consider 

 the effects of non-uniform or "directed" pressure in- 

 volving stresses that operate in definite directions and 

 result in deformation of the material on which they 

 act. Unlike uniform pressure, which usually raises 

 the crystallisation point, directed pressure may lower 

 it considerably and thus give rise to local fusion and 

 subsequent recrvstallisation of the rock. .\t 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 

 directed pressure in its operation at different tem- 

 peratures, and in the presence of different amounts of 

 uniform 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. 



The time elements in the constructive or trans- 

 forming operations of Nature cannot, of course, he 

 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 in- 

 volved in the formation and alteration of rocks_ and 

 mineral veins, we require some means of ascertaining 

 when we have approximately reproduced the condi- 

 tions which actually prevailed. It is not sufficient to 

 bring about artificially the formation of a mineral 

 occurring in the rocks or m.ineral 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, ;ill vary in different occur- 

 rences, 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 com- 

 paratii'e study of these characters and of the associa- 

 tion in which thev are found. The results thus ob- 

 tained should be of the greatest value in indicating 

 the directions along which experimental work would 



