PRESIDENTIAL ADDRESS. 188 



presented by the endogeiietic rocks " which have come into existence by the 

 action of the forces of earth's interior, for the conditions of temperature and 

 pressure under which they were formed, whether they are igneous rocks in 

 the narrower sense, or mineral veins, or metamorphic in origin, were widely 

 different from those with which we are familiar. Under 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, 

 therefore, as far as possible, to reproduce the conditions that prevailed — a 

 task which has been courageously undertaken and to a considerable extent 

 accomplished by the Geophysical Laboratory of the Carnegie Institute at 

 Washington. 



By artificial means temperatures and pressures have been already produced 

 far higher than those that were in all probability concerned in the evolution of 

 any of the rocks that have been revealed to us at the surface by earth-move- 

 ments and denudation, for it is unlikely that in any case they were formed at a 

 greater depth than five or six miles, corresponding to a uniform (or, as it is 

 sometimes termed, hydrostatic) pressure of 2,000 or 2,400 atmospheres, or at 

 a greater temperature than 1,500° C. Indeed, it is probable that the vast 

 majority of igneou.s and metamorphic rocks, as well as mineral .veins, came into 

 existence at considerably less depths, and at more moderate temperatures. It 

 is true that most of the rock-forming minerals crystallise from their own melts 

 at temperatures between 1,100° C. and 1,550° C, but they separate out from 

 the complex magmas from vvhich our igneous rocks were formed at lower 

 temperatures, rarely much exceeding 1,200° C and frequently considerably 

 less.-" 



It has been found possible at the Geophysical Laboratory to maintain a 

 temperature of 1,000° C. or more under a imiform pressure of 2,000 atmospheres 

 for so long a time as may be desired, and, what is equally important, the tem- 

 perature and pressure attained can be detennined with satisfactory accuracy, 

 the temperature 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 mentioned does not directly affect the physical proper- 

 ties 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 maintaining the concentration in the magma of a considerable 

 proportion of water and other volatile constituents -^ which have a far-reaching 

 influence in lowering the temperature at which the rock-forming minerals 

 crystallise out, in other words, the temperature at which the rock consolidates, 

 and in diminishing the molecular and molar viscosity of the magma, thus facili- 

 tating 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 differentia- 

 tion in magmas. 



It is, therefore, obvious that any conclusions derived from the early experi- 

 ments 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 Geophysical Laboratory, in experiments with melts 

 containing large amounts of volatile fluxes, and yet, if we are to reproduce even 

 approximately natural conditions, it is absolutely necessary to work with magmas 

 rontaining 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. 



'« T. Crook; Min. Mag., vol. xvii., p. 87, 1914. 



-" It is probable that the temperatures recorded in some lavas higher than 

 the melting point of copper, which is well over 1.200° C, are due to chemical 

 reactions, such as the oxidation of hydrogen, carbon monoxide, ferrous oxide, 

 and perhaps sulphur. See Dav and Shepherd, BiiJl. Geol. Soc. Amer., vol. xxiv.. 

 pp. 599-601, 191.3. 



^' John Johnston, Joiirn. Franl-lhi Iih?t., Jan. 1917, pp. 1419. 



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