REPORT OF THE CHIEF ASTRONOMER 679 



SESSIONAL PAPER No. 25a 



or glassy base. Since the holocrystalline rocks are overwhelmingly preponderant, 

 and since nearly all of the glassy and ultra-compact rocks are chemical equiva- 

 lents of respective holocrystalline types, it seems possible to make an essentially 

 complete and highly useful Mode classification of igneous rocks on a quantitative 

 basis. The existing Mode classification is only sub-quantitative and, in the 

 writer's opinion, would be strengthened by closer precision in its definitions. A 

 tentative experiment has shown that Eosenbusch's plutonic types can be usefully 

 defined in terms of limits in the quantities of the essential minerals, and that 

 without causing any marked changes in the names which Rosenbusch has 

 adopted. Leaving further demand to decide the advisability of such a restating 

 of the Mode classification, we may note the quantitative basis of the present 

 system as shown in the average chemical analyses of the now recognized types. 



Average Compositions of Leading Types. 



These averages have been calculated by the writer according to the method 

 described in a special paper, which also hints at some of the uses to which the 

 averages may he put.* More surely than single averages they illustrate the 

 essential chemical nature of the world types as actually encountered in the field. 

 The individuality and objective character of the types are as well shown as in 

 the mineralogical composition of the corresponding rocks. The averages are 

 here entered in the succeeding table (XLIV), in order to facilitate comparison 

 with the many analyses made from the Boundary Survey collection. 



In summary, the writer may state as his reasons for using the Mode classifica- 

 tion, not only that it is the prevailing one, nor simply because it is the best of 

 the available systems for the field geologist, but also because it is a real approxi- 

 mation to the natural classification for geologist and petrogenist. It takes 

 account of the actual mineral composition and of the chemical composition, 

 each of which is a fairly direct expression of magma and of magmatic history. 

 The Mode classification may be somewhat affected in form through the future 

 application of entectic principles, but the leading types of igneous rocks as now 

 usually recognized, will doubtless remain in the system. 



* R. A. Daly, Proceedings American Academy of Arts and Sciences, Vol. 45, 1910, 

 p. 211. 



