VII 



THE RISE OF PETROLOGY AS A SCIENCE 

 By LOUIS V. PIRSSON 



THIS chapter is intended to present a brief sketch of 

 the progress of the science of petrology from its 

 early beginnings down to the present time. The 

 field to be covered is so large that this can be done only in 

 broadest outline, and it has therefore been restricted chiefly 

 to what has been accomplished in America. Although the 

 period covered by the life of the Journal extends back- 

 ward for a century it is, however, practically only 

 within the last fifty years that the rocks of the earth's 

 crust have been made the subject of such systematic 

 investigation by minute and delicately accurate methods 

 of research as to give rise to a distinct branch of geologic 

 science. It is not intended of course to affirm by this 

 statement that the broader features of the rocks, espe- 

 cially those which may be observed in the field and which 

 concern their relations as geologic masses, had not been 

 made the object of inquiry before this time, since this is 

 the very foundation of geology itself. Moreover, a cer- 

 tain amount of investigation of rocks, as to the minerals 

 of which they were composed, the significance of their 

 textures, and their chemical composition, had been car- 

 ried out, concomitant with the growth from early times 

 of geology and mineralogy. Thus, in 1815, Cordier by a 

 process of washing separated the components of a basalt 

 and by chemical tests determined the constituent min- 

 erals. At the time the Journal was founded, and for 

 many years following, the genesis of rocks, especially of 

 igneous rocks, was a subject of inquiry and of prolonged 

 discussion. The aid of the rapidly growing science of 

 chemistry was invoked by the geologists and analyses of 

 rocks were made in the attempt to throw light on impor- 



