Septembek 22, 1911J 



SCIENCE 



359 



fact that a given district may form part of 

 a calcic province at one period and of an 

 alkaline one at another, precludes the 

 hypothesis that the composition of igneous 

 rocks depends in any degree upon peculi- 

 arities inherent from the beginning in the 

 subjacent crust. The same objection ap- 

 plies with scarcely less force to various 

 conflicting suggestions based on an assumed 

 absorption or "assimilation" of sedimen- 

 tary rocks by igneous magmas. Thus Jen- 

 sen supposes the alkaline rocks to be de- 

 rived by the assimilation or fusion of alka- 

 line sediments at great depths. Daly pro- 

 pounds the more elaborate, and on a first 

 view paradoxical, theory that alkaline have 

 been derived from calcic magmas as a eon- 

 sequence of the absorption of limestone. 

 These geologists agree in regarding the 

 alkaline rocks as relatively unimportant in 

 their actual development and in some sense 

 abnormal in their origin. For Suess, on 

 the other hand, it is the calcic rocks which 

 owe their distinctive characters to an ab- 

 sorption of sedimentary material, enrich- 

 ing the magma in lime and magnesia. 

 Apart from difficulties of the physical and 

 chemical kind, all such theories fail to 

 satisfy, in that they ignore the separation 

 of the two branches of igneous rocks in dif- 

 ferent regions of the globe, each of which 

 includes sediments of every kind. What 

 then is the real significance of this regional 

 separation 1 The obvious way of approach- 

 ing the question is to inquire first whether 

 the alkaline and calcic regions of the globe 

 present any notable differences of a kind 

 other than petrographical. 



BELATION BETWEEN TECTONIC AND PETEO- 

 GEAPHICAL FACIES 



The close connection between igneous 

 activity and displacements of the earth's 

 crust has been traced by Suess, Lossen, 

 Bertrand, de Lapparent and others, and is 



a fact sufficiently well recognized. We 

 have here indeed two different ways of re- 

 lieving unequal stresses in the crust, and 

 it is not surprising that they show a broad 

 general coincidence both in space and in 

 time. We can, however, go farther. Not 

 only the distribution of igneous rocks in 

 general, but the distribution of different 

 kinds of rocks is seen to stand in unmistak- 

 able relation to the leading tectonic features 

 of the globe. It is very noticeable that petro- 

 graphical provinces, and in particular 

 provinces belonging to opposite branches, 

 are often divided by important orographic 

 lines. This is illustrated by the Cordil- 

 leran chain in both North and South Amer- 

 ica, and again by some of the principal 

 arcs of the Alpine system in Europe. If 

 now we examine the actual distribution 

 more closely, in the light of Suess 's analy- 

 sis of the continents and oceanic basins, we 

 perceive another relation still more signifi- 

 cant. It is that, as regards the younger 

 igneous rocks, the main alkaline and calcic 

 regions correspond with the areas char- 

 acterized by the Atlantic and Pacific types 

 of coast-line, respectively. I briefly drew at- 

 tention to this correspondence in 1896, and 

 a few years later Professor Becke, of 

 Vienna, arrived independently at the same 

 generalization. Recalling the two classes 

 of crust-movements discriminated by Suess, 

 he says it appears that the alkaline rocks 

 are typically associated with subsidence 

 due to radial contraction of the globe, and 

 the calcic rocks with folding due to lateral 

 compression. The greater part of Becke 's 

 memoir is devoted to a comparison of the 

 two branches in respect of chemical compo- 

 sition; but here, I think, he has been mis- 

 led by taking as representative of the 

 whole alkaline "Sippe" or tribe the rocks 

 of one small and peculiar province, that of 

 the Bohemian Mittelgebirge. Some petrol- 

 ogists have followed Becke in adopting the 



