September 22, 1911] 



SCIENCE 



355 



laborious. That most of the practical diffi- 

 culties 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 investiga- 

 tion. The methods of high temperature 

 measurement have been perfected and the 

 thermometrie scale standardized up to 

 1550° C, thus embracing the whole range 

 of rock-formation. Calorimetric measure- 

 ments 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 de- 

 termined the temperature-range of stabil- 

 ity of numerous rock-forming minerals. 

 Beginning with the simpler cases and 

 working always with chemically pure ma- 

 terial, they have established quantitatively 

 the mutual relations of the various possible 

 forms in a number of two-component sys- 

 tems and in one of three components. So 

 far as these instances go, the mutual low- 

 ering of melting-points in a silicate-magma 

 is now a matter of precise measurement, 

 and it is no longer inferred, but demon- 

 strated, that the order of crystallization of 

 the minerals depends upon their relative 

 proportions in the magma. The perfect 

 isomorphism of the plagioclose felspars 

 has been fLaally established, and a certain 

 degree of solid solution between quite dif- 

 ferent minerals has furnished the explana- 

 tion of some apparent anomalies, such, for 



instance, as the variable composition of the 

 mineral pyrrhotite. As a single illustra- 

 tion of how these investigations in the lab- 

 oratory provide the working petrologist 

 with new instruments of research, I will 

 cite the conception of a geological tempera- 

 ture-scale, the fised 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 crystallized above or 

 below 575° C, this being the inversion- 

 point between the a- and ;S-forms of the 

 mineral. At about 800° there is another 

 inversion-point, above which quartz is no 

 longer stable, but gives place to cristo- 

 balite. In like manner we know that wol- 

 lastonite in a rock must have crystallized 

 below 1190°, pyrites below 450°, and so for 

 other cases. "We may confidently hope 

 that, with the aid of such data, we shall 

 soon be enabled by simple inspection, to 

 lay down in degrees the temperature-range 

 of crystallization of a given igneous rock. 

 There are now several laboratories where 

 high-temperature research, of the rigorous 

 order indicated, is being carried out; but 

 the work is peculiarly arduous and results 

 come slowly. Some branches of the in- 

 quiry, notably those involving high pres- 

 sures, and again the investigation of sys- 

 tems into which volatile components enter, 

 are as yet virtually untouched. For these 

 reasons it would be premature to hazard 

 at this stage any more detailed forecast of 

 the services to be rendered to petrology by 

 synthetic experiment. I will accordingly 

 leave this attractive subject and pass on 

 from the laboratory to the field. 



GEOGRAPHICAL DISTRTBtTTION OP IGNEOUS 

 ROCKS 



Here the existing situation is very dif- 

 ferent. Instead of following out definite 

 lines already laid down, we are concerned 



