356 



SCIENCE 



[N. S. Vol. XXXIV. No. 873 



in reducing to order a great mass of dis- 

 crete facts drawn from many sources. The 

 facts which enter into consideration are 

 those touching the distribution of various 

 igneous rocks in time, in space and in en- 

 vironment, including their relation to tec- 

 tonic features; the mutual association of 

 different rock-types and any indications of 

 law in the order of their intrusion or ex- 

 trusion; and, in short, all observable rela- 

 tions which may be presumed to have a 

 genetic significance. The digestion of this 

 mass of data has already led to certain 

 generalizations, some of which are accepted 

 by almost all petrologists, while others 

 must be regarded as still on their trial. 



Of the former kind is the conception of 

 petrographical provinces, which was put 

 forward by Professor Judd twenty-five 

 years ago, and has exercised a profound 

 influence on the trend of petrological spec- 

 ulation. It is now well established that 

 we can recognize more or less clearly de- 

 fined tracts, within which the igneous 

 rocks, belonging to a given period of igne- 

 ous activity, present a certain community 

 of petrographical characters, traceable 

 through all their diversity or at least ob- 

 scured only in some of the more extreme 

 members of the assemblage. Further, that 

 a province possessing an individuality of 

 this kind may differ widely in this respect 

 from a neighboring province of like date; 

 while, on the other hand, a striking simi- 

 larity may exist between provinces widely 

 separated in situation or in age. It is nat- 

 ural to attribute community of chemical 

 and mineralogical characters among as- 

 sociated rocks to community of origin. 

 The simplost hypothesis is that which sup- 

 poses all the igneous rocks of a given prov- 

 ince to be derived by processes of differen- 

 tiation from a single parent-magma. This 

 may be conceived, for the sake of simplic- 

 ity, as initially homogenous, though doubt- 



less some of the causes which contribute to 

 promote heterogeneity were operative from 

 the earliest stage. Granted this hypoth- 

 esis, it folloM's that the points of resem- 

 blance among the rocks of a province will 

 indicate the nature of the common parent- 

 magma, while the points of diversity will 

 throw light on the causes of differentiation. 

 The observed sequence in time of the vari- 

 ous associated rock-types will also have an 

 evident significance, especially if, as there 

 are good reasons for believing, differentia- 

 tion in igneous rock-magmas is largely 

 bound up with progressive crystallization. 

 Those petrologists, on the other hand, who 

 attach importance to the absorption or 

 "assimilation" of solid rock-matter by 

 molten magmas, are bound to consider 

 both the nature of the chemical variation 

 and the local distribution of the different 

 types with constant reference to the com- 

 position of the country-rocks. The balance 

 of opinion, and I think of argument, would 

 assign the variation, at least in the main, 

 to differentiation; and there are well- 

 known principles, chemical and mechan- 

 ical, which theoretically must operate to 

 produce a diversity of ultimate products 

 from a magma originally uniform. How 

 far these principles are in practise ade- 

 quate to the demands which have been 

 made on them is a question not to be 

 finally resolved without quantitative 

 knowledge which is still a desideratum. 

 Experiment may in time come to our aid. 

 My design to-day is rather to offer some 

 remarks upon a distinct, though allied, 

 problem — viz., that presented by the petro- 

 graphical provinces themselves. 



The geographical distribution of differ- 

 ent kinds of igneous rocks long ago en- 

 gaged the attention of Humboldt, Boue, 

 and other geologists, and the subject has 

 always possessed a certain interest in view 

 of the association of most metalliferous de- 



