AVERAGE CHEMICAL COMPOSITIONS OF IGNEOUS-ROCK 



TYPES. 



By Reginald Aldworth Daly. 

 Presented December 8, 1909 ; Received December 4, 1909. 



Contents. 



Introduction: Purpose of the Paper 211 



Method of Calculation 213 



Sources of Information 214 



Average Specific Gravities of Certain Types 235 



Some Applications 235 



Introduction: Purpose of the Paper. 



The study of the igneous rocks has hitherto largely consisted in an 

 analysis of their mineralogical and chemical composition, with the 

 special intent to produce a satisfactory nomenclature and classification 

 of the rocks as they occur throughout the world. This systematic 

 petrography, though still pursued by a great number of workers, is 

 now rivaled in interest and excelled in importance by its own offshoot, 

 petrogeny. The science of the origin and history of the igneous 

 rocks is reacting on the more purely descriptive subject, and at present 

 petrologists are feeling their way toward a genetic classification of 

 this great series of rock-types. Meantime, the much more numerous 

 class of workers engaged on the problems of economic and general 

 geology, of geochemistry and cosmogony, are raising highly important 

 questions which belong to the field of petrogenesis. The problems 

 thus raised are as fundamental as they are complex and difficult. For 

 many of their solutions recourse must be had to the more modern 

 geological reports and maps. With ever increasing skill and accuracy 

 the distribution and relations of the rocks composing the earth's crust 

 are being recorded by government officers and by geologists working in 

 private capacity. For some thirty years past, as at present, the great 

 body of geologists have mapped and described the igneous rocks in 

 terms of what may be called the German system of nomenclature and 

 definition. In particular, Rosenbusch's monumental treatises on the 



