Ixiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [vol; lxxiV, 



to petrology is not in itself a new departure. What is new, and 

 is for the first time possible, is the conduct of experiments under 

 conditions quantitatively known and continuously controlled, so 

 that the investigation rests upon a strict scientific basis. Work 

 of this character has already, in a few years, }<ielded important 

 results. Their immediate application; so far, is mainly to the 

 evolution of igneous rocks and the genesis and alterations of 

 ore-deposits ; but the practical difficulties which embarrass investi- 

 gation at high pressures and the study of systems including a 

 volatile phase have been in part conquered, and some of the 

 problems of metamorphism are already brought within the scope 

 of laboratory research. Moreover, apart from actual experimental 

 results, a clear formulation of the principles involved may do 

 much towards clearing the way, and here, too, the chemists have 

 rendered useful service. If then we would realize truly the 

 present standing of the stud}' of metamorphism, we must note 

 that field-observation, microscopic study, and chemical analysis 

 are now being reinforced by a new and powerful weapon of attack. 

 These and kindred considerations warrant, I think, a new 

 orientation towards the subject as a whole. More accurately, it 

 is a reversion to a point of view which has passed too much into 

 abeyance, for in the early days of modern geology the merely 

 descriptive standpoint and the rational or genetic were sufficiently 

 sharply opposed. To the one school the crystalline schists, with 

 gneisses and allied rocks, became simply a class, coordinate with 

 the igneous rocks and the .sedimentary, with peculiar characters 

 to be observed and described. This view does not, of course, 

 preclude further inquiry touching the origin of the rocks ; but in 

 fact such inquiry has only recently and partially entered in such a 

 way as to affect the progress of the study, and it remains broadly 

 true that we have here the last surviving relic of the Wernerian 

 system of geology. The rival school was concerned less with 

 metamorphosed rocks as a class than with metamorphism as a 

 process, which may operate upon rocks of all kinds. Unfor- 

 tunately, from this logical starting-point little progress was made, 

 and the failure must be ascribed mainly to disregard of chemistry. 

 Even the leading British geologists of the time are not free from 

 this reproach ; while for some of their followers metamorphism 

 became a wizard's wand, by which one rock was transformed to 

 another quite irrespective of composition. After these extra- 

 vagances had provoked a reaction, some real steps in advance were 



