academy of sciences] DISCUSSION AND CONCLUSIONS. 77 



with their associated lamprophyres minette, kersantite, etc., in the Erzegebirge; the alkaline 

 Tertiary (and Devonian ?) rocks of Germany, and the "calcic" lamprophyres of Carboniferous 

 age; the alkaline rocks of the Great Rift Valley of British East Africa, and the underlying 

 complex of ancient paragneisses, and calcic gabbro-granite series; but the vast difference in age 

 of the members of each pair of adjacent rock-series is not noted, nor that in each of the six (or 

 seven) cases the requirements of the tectonico-petrological hypothesis are exactly fulfilled. The 

 Tertiary alkaline rocks in all three regions cited were emitted during the blockf aulting of plateaux 

 (and the semialkaline Devonian igneous rocks of Germany were developed during slow con- 

 tinuous crust-subsidence unaccompanied by folding). The German lamprophyres are obviously 

 connected with the gabbro-granite intrusions that accompanied the Variscan crust-folding, and 

 whatever age may be eventually assigned to the plutonic rocks in the gneisses of the Erzegebirge 

 and East Africa, it can scarcely be doubted that strong lateral pressure accompanied their 

 intrusion. It must, however, be recognized that there are some regions where there is an 

 indubitable association of rocks of calcic and alkaline affinities within the same petrographical 

 province and epoch of vulcanicity (Daly's collection of facts are useful in this connection), and 

 we may share Cross's objection to the attempt to separate into two sharply distinct branches the 

 various types of igneous rocks. The objection seems to be met by the following remark: " The 

 doctrine of two classes of igneous rocks, alkaline and calcic, having a significant geographical 

 distribution in relation to the great tectonic features of the globe, does not, of course, imply any 

 sharp divisions, and perhaps the more philosophical conception is that of two opposite petro- 

 graphic poles, toward which igneous rocks tend as a result of primary differentiation. There is 

 naturally some complication introduced by subsequent processes, giving rise to the great diversity 

 of igneous rocks known to petrographers; and these later processes may sometimes obscure, as 

 regards individual rock-types, the primary characteristics" (Harker '17, p. lxx). Such associa- 

 tions, he says ('11) are to be found among the later derived types 5 referrable to prolonged or 

 repeated differentiation, and are to be expected especially where the initial magma is not very 

 strongly characterized as either calcic or alkaline. It seems thus reasonable to explain such 

 petrographically diversified regions as those in which the subcrustal lateral separation of magmas, 

 or tectonico-petrologic specialization was incomplete, and such regions must inevitably occur if 

 tectonic movements exercise any control on the course of magmatic differentiation; since the 

 crust movements themselves do not in reality fall into two distinct and separate groups. This 

 association of diverse rock-types may well be particularly marked in such a region as the central 

 Pacific island groups (Cross '15) where crustal instability (as shown by the recent investigations 

 of geologists and biologists), but with absence of marked lateral pressure, seems to have long 

 been the dominant tectonic condition. 



The results of the experimental work in the Geophysical Laboratory at Washington, D. C, 

 accord with the view that in those "regions of the earth's crust where tangential extension is 

 the dominant expression of the forces acting (Atlantic structures), the development of alkaline 

 rocks might be a prominent feature, though the conditions requsite to their formation would 

 undoubtedly occur locally elsewhere" (Bowen '20). It is not, however, intended to assert here 

 that differentiation during deformation is the sole process that may be involved in the production 



of alkaline rocks. 



CONCLUSION AND ACKNOWLEDGMENTS. 



Thus we may believe that the evidence here collected tends to the conclusion that each of 

 the several morphological types of basic and ultrabasic rocks is very generally associated with 

 as definite a set of tectonic conditions, and often with as definite a group of less basic igneous 

 rocks. This appears to emphasize the importance of crust-movements in determining, not only 

 the mise-en-place and form of intrusive masses of igneous rocks, but also in actually controlling, 

 in some degree, the processes of magmatic differentiation. 



» See Bowen's reference ('15, p. 60) to Bergeat's discussion of the leucitic lavas of the Aeolian Islas, and compare the same with the occurrence 

 of leucitic lavas in Celebes (Wanner '19) and in Java. (Verbeek and Fennema '96.) 



