786 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



explained in at least two ways. The dioritic peripheral phases of many sub- 

 jacent bodies seem to be best accounted for on the view that molar-contact 

 chilling tends to increase viscosity beyond the point where magmatic splitting 

 can take place (See page 772). Many diorite bodies, often slightly older than 

 associated granites, are clearly satellitic injections, such as dikes, sheets, chono- 

 liths, etc. Since these are all relatively small and quickly cooled bodies, it is 

 readily understood that they will preserve the syntectic composition. 



The diorites associated with granodiorites are subject to the same reasoning 

 except that possibly they share with granodiorites certain chemical features due 

 to the assimilation of basic sediments, or basic volcanic material, in addition 

 to granitic rock. The known variability in the diorite family suffices to cover 

 these complex syntectics as well as those formed of primary basalt and the 

 earth's acid shell. 



In the third place, the possibility is recognised that some rocks, fairly called 

 diorites, may themselves be differentiates from special syntectics, or from the 

 primary basaltic magma. Such types seem to be rare. 



Most of .the acid andesites are effusive equivalents of diorites and, on the 

 theory, are to be regarded as similar syntectics, which, however, are generally 

 somewhat differentiated. That the syntectics are here more differentiated is 

 explained by the often favourable conditions for splitting in volcanic vents 

 (pages 700 and 712). The dioritic magma gives granitic magma through splitting; 

 andesitic magma gives liparitic (rhyolitic) magma through splitting. We can 

 thus understand the common association of liparite and andesites in volcanic 

 regions. The theory holds that some liparites may be extreme differentiates of 

 the more basic augite-andesite magma, but typical post-Archean augite andesite 

 is to be considered not as a syntectic but as a polar differentiate of basaltic 

 magma. 



This, in brief, is the writer's interpretation of the andesites occurring in 

 the Irere, Beaver Mountain, Rossland, Phcenix, Midway, Pasayten, Skagit, and 

 Chilliwack volcanic groups. 



The Complementary Dikes and Sheets, and the Pegmatites. — The general 

 theory includes the prevailing conception that these bodies are directly due to 

 differentiation. They fall into groups according to their derivation from 

 primary basaltic magma, from syntectics, or from differentiates of the basalt or 

 the syntectics. The exact processes of the splitting are still largely mysterious. 

 It seems probable that the differentiation is gravitative; the aplitic poles rising, 

 the lamprophyric poles sinking in residual portions of magma. Since none of 

 these bodies is ever large when compared to the parent batholiths, stocks, etc., 

 and since the dikes regularly close batholithic periods, it is fair to conceive 

 that the splitting magma was greatly reduced in volume from the size indicated 

 in the batholith. 



The necessary concentration of juvenile and resurgent fluids in the liquid 

 magma remaining after most of a batholith has crystallized, may be the con- 

 trolling condition. These volatile materials must lower the viscosity of the 

 magma thus left in pockets or sheets within the frozen rock. In the specially 



