September 22, 1911] 



SCIENCE 



357 



posits with igneous rocks. It has, however, 

 acquired a new importance in recent years 

 in connection with questions of petro- 

 genesis which are still under discussion. 

 The problem is, in brief, to account for the 

 existence of petrographical provinces and 

 for the observed facts relative to their dis- 

 tribution. One theory, advocated espe- 

 cially by Dr. G. F. Becker, invokes prime- 

 val differences in composition between 

 different parts of the globe, which have 

 persisted throughout geological time. It 

 involves the hypothesis that igneous rock- 

 magmas result from the refusion of pre- 

 existing rocks within a limited area. In- 

 deed Becker discards altogether the 

 doctrine of differentiation, and conceives 

 the varied assemblage of rocks in a given 

 province as produced by admixture from a 

 certain number of primitive types. These, 

 he says, should be recognizable by their 

 wide distribution and constant character. 

 It is clear, however, that, on the hypoth- 

 esis of admixture, the primitive types 

 must be those of extreme composition. 

 These are, in fact, always the rarest and 

 the most variable, pointing not to admix- 

 ture, but to differentiation as the cause of 

 the diversity. A theory which attributes 

 the special characteristics of petrograph- 

 ical provinces to permanent heterogeneity 

 in the composition of the globe is difficult 

 to reconcile with the small extent and 

 sharp definition of some strongly char- 

 acterized provinces, such as that of Assynt 

 or of the Bohemian Mittelgebirge. A 

 more fatal objection is that petrographical 

 provinces are not in fact permanent. A 

 good illustration is afforded by the mid- 

 land valley of Scotland, an area our knowl- 

 edge of which has been much enlarged by 

 the recent work of the Geological Survey. 

 It was the theater of igneous activity in 

 Lower Old Red Sandstone times and again 

 in the Carboniferous, but, in respect of 



mineralogical and chemical composition, 

 the two suites of rocks present a striking 

 contrast. The Old Red Sandstone lavas 

 are mostly andesites, though ranging from 

 basalts, on the one hand, to rhyolites, on 

 the other, and the associated intrusions are 

 mainly of diorite, quartz diorite and 

 granite, with porphyrites and other dyke- 

 rocks. In the Carboniferous, on the other 

 hand, we find porphyritie basalts, mugea- 

 rites and trachytes (including phonolitic 

 types), with pierites, teschenites, mon- 

 ehiquites, orthophyres and other allied 

 rocks. It would be possible to cite many 

 other cases illustrating the same point. 



THE ALKALINE AND CALCIC BRANCHES 



The two Scottish suites of Upper Paleo- 

 zoic rocks just mentioned fall into opposite 

 categories with reference to what is now 

 becoming recognized as the most funda- 

 mental distinction to be made among igne- 

 ous rocks. The earlier set is typical of the 

 andesitic division and the later of the 

 tephritic; or, using other equivalent 

 names, the one belongs to the calcic (or 

 "alkali-calcic") branch and the other to 

 the alkaline. I will adopt the latter ter- 

 minology as being generally familiar to 

 petrologists ; but the characteristics of the 

 two branches, which are too well known to 

 need recapitulation here, are more clearly 

 definable in mineralogical than in chem- 

 ical language. This two-fold division of 

 igneous rocks is, of course, in no wise a 

 final or exhaustive treatment of the sub- 

 ject; but as a first step towards a natural 

 or genetic classification it seems to be es- 

 tablished beyond question. No third 

 branch in any degree comparable with the 

 two and distinct from them has been pro- 

 posed. The charnoekites and their allies 

 represent but a single rock-series, and 

 Rosenbusch has not made clear his reasons 

 for separating them from the calcic rocks. 



