Alumina with Silica, Lime and Magnesia. 329 



temperature and pressure ranges over which they are stable. 

 From such data general laws of equilibrium can be deduced and 

 tested and then applied directly to the rocks themselves, which 

 in effect are the end-products of physico-chemical systems. In 

 the case of igneous and metamorphic rocks we have to deal 

 with chemical systems that have been subjected to certain 

 physical conditions which, in turn, have left their imprint or 

 seal on the end-product or rock now accessible to the geologist. 

 It is the task of the geologist to decipher this seal as he finds it 

 expressed in terms of mineral composition and texture, and 

 from it to infer the conditions of original formation. The 

 actual processes of formation have not been and in general 

 cannot be witnessed by him, and he must base his conclusions 

 on the existing evidence, weighed in the light of his own 

 experience. Such evidence is in part geologic, but in no small 

 degree experimental, and the more evidence of an experi- 

 mental nature there is at hand, the more confident is he of his 

 conclusions. Exact thermal data especially are lacking, but 

 are of fundamental significance, since they indicate limits at 

 which the energy content of the system changes abruptly ; 

 any change of this kind, such as the melting and inversion tem- 

 peratures of compounds, or eutectic temperatures of mixtures, is 

 most important, since it is the outward expression of a shift of 

 the equilibrium of the system, as a result of which profound 

 changes may occur. What before was stable may become 

 unstable, and vice versa ; a rearrangement of forces accom- 

 panies the change in energy content and new stability relations 

 are at once established. 



Under normal conditions, therefore, thermal measurements 

 are adapted to indicate the relative energy content of any 

 preparation at different temperatures. But by so doing they 

 indicate the presence of different compounds in a series and 

 establish temperature ranges over which these compounds and 

 mixtures of the same are stable. 



Optical data. — The microscopic examination of the prepara- 

 tions, at ordinary room temperatures and after the changes 

 have taken place, does not of itself directly prove an energy 

 change in the system. In the thermal microscope such changes 

 can be followed in their effect on the optical properties (melt- 

 ing down of crystal plates, abrupt changes in birefringence, 

 optic axial angle, and the like), but such evidence is used 

 ordinarily only to confirm the purely thermal data. The pur- 

 pose of the microscopic investigation is primarily to determine 

 the compounds present in any preparation (composition, with 

 special reference to homogeneity and crystallization), to study 

 the relation of the different components to each other (texture) 

 and to establish by measurement the degree of departure of 



