678 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



becomes to the trained geologist as characteristic as the features of Caucasian 

 or Mongolian are to the ethnographer. Rock, like man, has a ' habit ' of its 

 own and this is of first aid to the geologist who is mapping an igneous region. 



A Mode classification seems also vital to the investigator in petrogeny. If 

 rock magmas be only moderately superheated — the usual condition in nature — 

 the dissociation of the molecules, which on cooling will form homogeneous 

 crystals, is probably very slight. The actual minerals seen in a crystalline rock 

 are, therefore, so many direct indications of the nature of the magma before it 

 er\ stallized. Since it is becoming more and more certain that the laws of solution 

 govern the phenomena of rock crystallization, it is legitimate, with proper safe- 

 guards, to reason from a rock's actual mineralogical constitution or ' mode/ to 

 the condition of the pre-existing magma. The origin and history of magma is 

 really the core of petrogeny. The professed petrogenist, no less than the field 

 geologist, should regard the Mode classification as fundamental and, in a sense, 

 final. 



A further reason for its retention is found in the fact that a sound petrogeny 

 must be based on an inductive study of the actual igneous terranes of the world. 

 This study is now only possible through the maps and memoirs which, with a 

 very few exceptions, have been composed in terms of the prevailing classification. 

 Inasmuch as the field geologist must map areas according to the visible, minera- 

 logical character of the rocks, the raw material for the comparative petrologist 

 must retain essentially the same character as it has now. It is of the highest 

 importance that the quantities (species) dealt with by geologist and petrogenist 

 should have a common denominator. Where the two part company there is new 

 opportunity for unsound hypotheses concerning the origin of the rocks in 

 nature. 



These are some of the reasons why the Norm classification oi igneous 

 rocks seems bound to be a failure for petrogenist and geologist alike. Harker's 

 destructive criticism in the last chapter of his ' Natural History of Igneous 

 Rocks ' (1909) is hardly to be refuted. The nature of the highly artificial 

 system is doubtless familiar to the reader of the present report and need not be 

 described. While realizing the inefficacy of the system as a direct aid in the 

 problems of rock genesis, it has this residual advantage that the norm calculated 

 from a rock analysis may be used as a guide to the nearest chemical equivalents 

 of that rock among modern types. For the purpose Washington's large compila- 

 tion of analyses, for which the norms, subrangs, etc., have been determined, is of 

 great service. * Largely for this reason the norms of the analyzed types 

 occurring in the Boundary belt have been calculated. 



There is no apparent reason why the Mode classification of the holocrystalline 

 rocks should not be made rather strictly quantitative, somewhat after the manner 

 of calculation by which the position of a rock is found in the Norm system. 

 Thanks to Rosiwal's well-known method the modes can be calculated for most of 

 these rocks. In a measure the same is true for porphyritic rocks with aphanitic 



* H. S. Washington, ' Chemical Analyses of Igneous Rocks published from 1884 to 

 1900/ Professional Paper, No. 14, U.S. Geol. Survey, 1903. 



