REPORT OF THE CHIEF ASTRONOMER 477 



SESSIONAL PAPER No. 25a 



GENERAL SUMMARY. 



1. At the Forty-ninth Parallel of latitude the Okanagan mountains and a 

 part of the belt of the Interior Plateaus (the Interior Plateau of Dawson) have 

 been carved by erosion out of an assemblage of plutonic igneous rocks which, in 

 spite of the diverse lithological character of the rocks, should be regarded as an 

 enormous single member of the Cordilleran structure. This plutonic group is 

 named the Okanagan Composite Batholith. The details of its constitution are 

 given in a foregoing resume of its geological history. 



2. This composite batholith was of slow development, beginning with small 

 intrusions in late Paleozoic (or possibly Triassic) time, increased by great batho- 

 lithic irruptions of granodiorite during the Jurassic, and completed by likewise 

 immense irruptions of alkaline hornblende-biotite granite and biotite granite 

 batholiths of Tertiary age, possibly as late as the Upper Miocene or the Pliocene. 

 The satellitic Tertiary stock of Castle Peak in the Hozomeen range (see next 

 chapter), is composed of normal granodiorite. 



3. The local intrusion of a small, composite body of malignites and nephe- 

 lite syenites; the regular basification along the batholith and stock contacts, 

 giving collars of monzonites and diorites; and the sporadic appearance of certain 

 peridotites (hornblendites and dunites) are probably all incidents of magmatic 

 differentiation and do not directly represent the compositions of general sub- 

 crustal magmas. 



4. The composite batholith offers striking testimony to the probable truth 

 of the assimilation-differentiation theory of granitic rocks. 



5. The composite batholith includes two consanguineous series of intrusions. 

 The older one is non-alkaline; the younger, alkaline. They are separated in 

 time by the whole Cretaceous period, at least. 



6. The two consanguineous series nevertheless appear to belong to one com- 

 pound petrogenic cycle. Throughout the cycle batholithic intrusion has followed 

 the usual law of decrease in magmatic density and increase of magmatic acidity 

 with the progress of time. 



7. Exposures of contact surfaces in the Similkameen batholith illustrate 

 with remarkable clearness the downward enlargement of such bodies with depth. 



8. The Similkameen granite bears three roof-pendants. Their distribution 

 suggests that the present erosion surface of this batholith west of the Similka- 

 meen river is not far from coinciding with the constructional, subterranean 

 surface of the batholith. 



9. The Osoyoos and Bemmel granodiorites have been extensively metamor- 

 phosed by orogenic crushing and its attendant processes. The metamorphism 

 was both dynamic and hydrothermal. The granodiorites have been locally, 

 tbough on a large scale, transformed into banded gneisses and schists. These 

 changes have been brought about through the hydrous solution and migration 

 of the original mineral substance of the granodiorites, especially the more basic 

 minerals. The dissolved material has been leached out from the granulated rock 



