ACADBMT of sciences] INTRODUCTION AND GENERAL STATEMENTS. 3 



pegmatites would be concentrated, and he concluded that alkaline rocks could be formed in 

 this manner as the last consolidation products of a subalkaline magma. While not of the 

 opinion that a regional differentiation occurred as the result of tectonic forces, he suggested that 

 it might be possible that under the conditions of comparative tranquillity associated with 

 plateau-block faulting, alkaline differentiates might be produced that could not be formed 

 under conditions of lateral thrusting and folding. He accords no place to the absorption of 

 sediments by the magma. 



Bowen ('15), while considering that there is a comparatively slight alteration of magmatic 

 composition by such absorption of sediment, and thus abandoning a characteristic feature 

 of Daly's doctrine, concurs with him in putting a very great importance upon gravitational 

 differentiation by the sinking of crystals forming in magmas originally of basaltic composition, 

 believing that by this process together with the separation of liquid from crystals brought 

 about by the deformation of a crystallizing magma even alkaline magmas may be developed 

 as the latest products of magmatic differentiation, though possibly granitic, dioritic, gabbroid 

 and ultrabasic rocks had previously been derived from the initial magma. Peridotites on this 

 view are merely the aggregates of subsided crystals of ferromagnesian minerals, though it is 

 held possible that they may have been re-fused in part, or mixed with residual magma to 

 allow them to be independently injected into neighboring rocks. In common with perhaps the 

 majority of American petrologists, he rejected the view that there is any provincial causal con- 

 nection between the type of igneous rocks produced and the tectonic conditions existing before 

 and during their crystallization, holding that the alkaline and alkali-calcic rocks are too closely 

 associated to permit of such a tectonico-petrological connection, but he has since recognized 

 the possibility of such a connection (Bowen '20, p. 162). This association is, however, recog- 

 nized by Harker ('16), who also believes the alkaline rocks to be derived ultimately from 

 the same magmas as the subalkaline, but urges that lateral pressure, is an efficient cause of 

 spatial differentiation, no less than gravitative sinking, in that it causes the molten residuum 

 of a magma to move into the regions of lesser compression, so that orogenic forces bring about 

 a distribution of such fractions drawn from the parent magmas as to produce broad distinctions 

 between petrographic provinces, when operating for a long period of time and over extensive 

 areas, and local complexes of diversified types when acting within more limited areas. Indeed, 

 he holds, as also does Cross ('15), that exposures of gravitationally differentiated masses of 

 igneous rocks are far less common than Daly or Bowen believe. We may also suggest that in 

 regions in which there are intimate associations of the more alkaline with the alkali-calcic igneous 

 rocks, upon which some petrologists have laid much stress, e. g., Cross ('10, '15), local and 

 gravitational (or gaseous) differentiation may have had a greater effect than regional lateral 

 pressure, while in the regions where lateral pressure was strongly effective, leading to provincial 

 specialization, it is still possible for a magma remaining unconsolidated after the great lateral 

 pressure had diminished to be differentiated in the ways Bowen and Smyth described (for 

 these by no means seem to be mutually exclusive), leading possibly to the production of 

 nepheline-syenite, intrusive among the alkali-calcic rocks, as occurs in western Canada (Daly '12, 

 pp. 448-454) and the Ural Mountains. The occurrence, therefore, of provinces of incomplete 

 petrographic specialization appears to the writer not to be a negation of Harker's generalization, 

 but rather as exemplifying a reasonable deduction from it. 



The papers of Smyth and Bowen are of great value in that they suggest definite mechan- 

 ical concepts for the process of differentiation within the magma, and the latter is especially 

 noteworthy in that it deduces from a long experience of experimental research the probable 

 physico-chemical conditions and compounds present in a cooling magma, which make possible 

 the separation of fractions of the magma such as the field evidence indicates to have been suc- 

 cessively differentiated in the production of igneous complexes. They give reason for the 

 belief that very diversified types of rock have been produced from a single magma, probably 

 of basaltic composition, and, as we have suggested, such differentiation may have occurred 



