Washington — Statement of Rock Analyses. 59 



Art. IX. — The Statement of Rock Analyses ; by Henry 

 S. Washington. 



Of late years the importance of chemical analyses in the 

 study of igneous rocks is generally recognized, and their pub- 

 lication and use is becoming more and more common. 



Apart from improvements in methods and facilities, as well 

 as increase in the number of workers, the main factor in this 

 increase in the number of analyses published is the growing 

 appreciation of their vital importance for a thorough compre- 

 hension of the rocks of the globe. 



This importance of rock analyses to-day lies in their disclos- 

 ing the bearing of the chemical composition of the rock masses 

 on the deeper theoretical problems of petrology. Many of 

 these, such as the differentiation of magmas, the genetic rela- 

 tionships of various rock bodies or parts of a complex to each 

 other, or the true character and meaning of the so-called 

 " petrographic provinces," are only to be solved by means of 

 thoroughly good and trustworthy analyses. 



There is an increasing tendency among some petrographers 

 to regard the magma as one of the main objects of study, of 

 which the solid rocks are simply the consolidated and acces- 

 sible portions, many of their characters being to a great extent 

 fortuitous and dependent on extraneous conditions of solidifi- 

 cation and the like. 



The chemical composition of the magma either persists as 

 such in the rocks formed from it (as in Brogger's aschistic 

 dikes), or is to be inferred if the magma has undergone changes 

 such as those induced by differentiation (as in Brogger's dia- 

 schistic dikes), absorption of country rock or other causes. 

 This chemical composition is, as far as we can tell at present, 

 the single original character of the magma which is open to 

 study after it has undergone the physical and physico-chemical 

 changes which result in the formation of igneous rocks, whether 

 aschistic or diaschistic. 



Hence for obtaining a knowledge of these vast and highly 

 important portions of the earth's crust, of their original char- 

 acters, the conditions under which they exist and the changes 

 which they may undergo, rock analyses are absolutely essential. 

 It is true that physics also comes to our aid in certain direc- 

 tions, but this does not lessen the importance of the chemical 

 investigation. 



The practical use of analyses involves their collation or com- 

 parison one with another, either those of the rocks of one 

 given region, or those of different regions with each other, or 



