REPORT OF TEE CHIEF ASTRONOMER 785 



SESSIONAL PAPER No. 25a 



the Hozomeen range. The whole series represents one narrow belt through the 

 wonderful chain of granodiorite bodies which extends, with many breaks, from 

 Patagonia to Western Alaska. Though batholiths of this chemical type are 

 known in other continents, the home of the type and its greatest development 

 is in the Cordilleras of the two Americas. The intrusive mechanism of the 

 bodies is the same as that of the true granitic. The rather steady chemical 

 contrast to the latter offers a petrogenic problem of special interest. 



Several possibilities are open. These granodiorites may represent a less 

 perfect differentiation of the same kind of syntectic as that from which the 

 average post-Archean granite has been differentiated. Or, secondly, the average 

 syntectics may be conceived as different in the two cases. Many true granites 

 are known which have replaced little else than formations of granitic composi- 

 tion. In apparently all cases the typical granodiorite batholiths have been 

 developed in folded geosynclinals carrying heavy masses of argillaceous rock, 

 or in other terranes lower in silica than true granite. This is true of all the 

 bodies in the foregoing list, as reference to the maps will show. It seems 

 to be true of all the granodiorite batholiths of the Cordillera, as, for example, 

 those of the Sierra Nevada, where huge volumes of Paleozoic and Mesozoic 

 slates have been so evidently replaced. The assimilation of great volumes of 

 argillite must affect the syntectics profoundly, and it is worth while to hold, as a 

 good working hypothesis, that the granodiorite type is the product of a syste- 

 matic differentiation of a syntectic which was formed not only of primary basalt 

 and the pre-Cambrian granitic terrane but, also to an essential amount, of 

 sediments, chiefly argillaceous. 



The Diorites and Acid Andesites. — Four diorite bodies in the Boundary belt 

 have merited special names; the Rock Creek, Lightning Creek, Slesse, and 

 Sumas bodies (pages 392, 490, 532 and 527). All of these are small. The grano- 

 diorites often pass into quartz diorite, both at the Forty-ninth Parallel and 

 throughout the Cordillera. The Boundary section illustrates also the world- 

 wide association of diorite and true granite. A partial list of these associations 

 may recall illustrations : — 



Dioritic Rocks. Associated Granites and Granodiorites. 



Quartz diorite contact phase of Trail Trail granodiorite, granitic f acies ; 



batholith. Sheppard alkaline granite. 



Eock Creek diorite. Rock Creek granodiorite. 



Quartz diorite contact phase of Oso- Granodiorite and granite (original 



yoos batholith. phase?) of Osoyoos batholith. 



Lightning Creek diorite. Castle Peak granodiorite. 



Slesse diorite. Chilliwack granodiorite, granite. 



Sumas diorite. Sumas granite. 



The general theory regards some dioritic rock as the typical crystallized 

 syntectic formed by the assimilation of granite (generally pre-Cambrian) rock 

 in primary basalt. That syntectic has generally been differentiated so as to afford 

 new, secondary granites which, like the original material assimilated, are ' anchi- 

 eutectics.' The failure of differentiation in the case of the diorites may be 



