PRESIDENTIAL ADDRESS. 371 



As regards the former, the movement which I have noticed is merely a return 

 to the standpoint of Sorby, the father of modern petrology. It is true indeed 

 that, before his time, the problem of the origin of igneous rocks had engaged the 

 ingenuity of Scrope and Darwin, of Bunsen and Durocher, and many others; and 

 the bold speculations of the heroic days of geology have justly exercised a lasting 

 influence. The petrologist of to-day, however, has at his command a much 

 ampler range of information than was possessed by his predecessors. In addition 

 to the rich store of petrographical data already mentioned, he can press into 

 service on the one hand the results of physical chemistry and on the other much 

 additional knowledge which has been gathered concerning the structure of the 

 earth's crust and the distribution of various rock-types, both in space and in 

 time. Either of these branches of the subject would furnish material for a much 

 longer address than my assurance could venture or your complacence would 

 endure. I have chosen the geographical aspect of petrology; but t before pro- 

 ceeding to this, I will say a few words concerning the experimental side. 



Data from the Experimental Side. 



That the modern developments of physical chemistry, starting from the phase 

 rule of Willard Gibbs, must in theory furnish all that is necessary to elucidate 

 the crystallisation of igneous rock-magmas, has long been perceived by some 

 petrologists. This recognition is in itself an advance. Natural rock-magmas, 

 •however, are far more complex solutions than those which chemists have em- 

 ployed in working out their laws, and the problem in its entirety is of a kind 

 almost to daunt inquiry. Despite the courageous attempt made by Professor 

 Vogt, whose enthusiastic lead has done so much to inspire interest in the sub- 

 ject, it seems clear that the application of the laws of chemistry to the particular 

 class of cases with which the petrologist is concerned demands as a prerequisite 

 a large amount of experimental work in the laboratory. The high melting- 

 points of the rock-forming minerals, their extreme viscosity, and other specific 

 properties render such work extremely difficult and laborious. That most of the 

 practical difficulties have now been overcome is due in the first place to Dr. A. L. 

 Day and his colleagues of the Geophysical Laboratory at Washington, who have 

 thus opened out what is virtually a new field of investigation. The methods of 

 high temperature measurement have been perfected and the thermometric scale 

 standarised up to 1550° C, thus embracing the whole range of rock-formation. 

 Calorimetric measurements have been so far improved that it is now possible, 

 for instance, to determine specific heats, even in the highest part of this range, 

 with an accuracy ten times greater than has hitherto been usual at ordinary 

 temperatures. Incidentally there has been, in the hands of Mr. F. E. Wright, a 

 notable enlargement of the scope of ordinary petrographical methods, since it has 

 been found necessary to devise special means of measuring with precision the 

 crystallographic and optical constants of very minute crystals. 



The American chemists have already determined the temperature-range of 

 stability of numerous rock-forming minerals. Beginning with the simpler 

 cases and working always with chemically pure material, they have established 

 quantitatively the mutual relatione of the various possible forms in a number 

 of two-component systems and in one of three components. So far as these 

 instances go, the mutual lowering of melting-points in a silicate-magma is now 

 a matter of precise measurement, and it is no longer inferred, but demonstrated, 

 that the order of crystallisation of the minerals depends upon their relative 

 proportions in the magma. The perfect isomorphism of the plagioclose felspars 

 lias been finally established, and a certain degree of solid solution between quite 

 different minerals has furnished the explanation of some apparent anomalies, 

 such, for instance, as the variable composition of the mineral pyrrhotite. As 

 a single illustration of how these investigations in the laboratory provide the 

 working petrologist with new instruments of research, I will cite the conception 

 of a geological temperature-scale, the fixed points on which are given by the 

 temperature-limits of stability of various minerals. It is often possible, for 

 example, to ascertain whether quartz in a given rock has crystallised above or 

 below 575° C, this being the inversion-point between the a- and /3- forms of the 

 mineral. At about 800° there is another inversion-point, above which quartz 

 is no longer stable, but gives place to cristobalite. In like manner we know that 



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