Alumina with Silica, Lime and Magnesia. 333 



and chemical data, and this can only be supplied by experi- 

 ment, since the amount of exact field evidence along these 

 lines which has been obtained up to the present time is exceed- 

 ingly slight. 



A rock has been defined as a geologically independent part 

 of the earth's lithosphere. It owes its position to the action 

 of certain geologic forces and stands in causal relation to these. 

 These forces are in part physical and crystallographic and in 

 part chemical, and the rock as it appears to the geologist is the 

 resultant end-product of a certain chemical system acted upon 

 by the geologic forces ; such forces, however, have not always 

 remained the same throughout the history of any given rock, 

 but have changed from time to time, either slowly or abruptly, 

 and each change has brought with it new conditions of equi- 

 librium in the crystallized mass, and possible consequent 

 readjustment of mineral composition and texture. Such 

 readjustment in texture and composition, however, is rarely 

 complete, and the imprints or scars 'of each period of geologic 

 activity are often clearly marked in the rock and to the trained 

 eye serve to indicate its past history. The geologist has to 

 rely chiefly on field evidence in his interpretation of the history 

 of the earth, but such evidence is in large measure qualitative 

 and does not of itself yield exact data along certain lines, 

 particularly with reference to rock genesis and the actual 

 character of subsequent transformations. This evidence is 

 best obtained by direct experiment, by studying the crystalli- 

 zation of definite chemical systems under definite and deter- 

 minable conditions of pressure and temperature. The difficul- 

 ties of complicated texture and composition will undoubtedly 

 be much diminished when the simpler chemical systems have 

 been experimented upon and their behavior under different 

 conditions studied. The technical difficulties in such problems 

 are formidable, but once overcome in the simpler systems, 

 they are mastered for all. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 



Washington, D. C, June, 1909. 



