2 DEVELOPMENT OF PETROLOGY. 



magmas now form the bases of the various hN'potheses 

 put forward to account for the genesis and diversity of 

 igneous rocks. 



During the present century there have been pubHshed 

 several works of an epoch making nature. 



Amongst the many important pubHcations we find 

 notably : " The Quantitative Classification of the Igneous 

 B/Ock,"" by the four eminent American petrologists, Cross, 

 Iddings, Pirsson, and Washington ; " Die Silikatschmelz- 

 losungen," by Vogt ; a " Treatise on Metamorphism," 

 by Van Hise : "' Die Krystallinen Schiefer," by Gruben- 

 mann ; "" Natural History of Igneous Rocks," by A. 

 Harker ; and the various pubhcations by R. A. Daly 

 cleahng with his "Overhead Stoping Theory" and "Alka- 

 line Rocks" ; by F. E. Wright on "'Microscopic Petrography"; 

 and by Day and others of the Carnegie Institute, Washing- 

 ton, on the physical properties of minerals. 



For many years after the application of inicroscopic 

 methods to the study of rocks, a tremendous amount of 

 descriptive Avork was carried out and was ready for use 

 in the formation of the generaUsations which have been 

 formulated during the present century. 



Such has been the influence of the developments of 

 physical chemistry and the various factors enumerated 

 above, that petrology is now rapidly passing from a purely 

 descriptive into an inductive science. 



Whitman Cross in his " Review of the Development of 

 Systematic Petrography in the Nineteenth Century," sum- 

 marises the position at the close of that century as follows : 



1. There is as yet no comprehensive and properly 

 systematic classification of all rocks 



2. Rocks of igneous origin have been much more 

 thoroughly investigated than others and they have received 

 correspondingly more definite and systematic classifi- 

 cation 



8. The rocks which have been formed on the surface, 

 of the earth b}' the destruction of older rocks may be viewed 



from so many standpoints that no consistent 



arrangement of these objects, deserving the name of a petro- 

 graphic system, has been proposed. 



