REPORT OF THE CHIEF ASTRONOMER 731 



SESSIONAL PAPER No. 25a 



interpretation seems possible by the theory outlined. On the other hand, every 

 observer who has even a small acquaintance with crystalline terranes of the 

 sort, is now and again struck with the evidences that the granitic magmas 

 represented in his field of study are far from being passive in the hands of the 

 eruptive Titan. Their general defiance of structure and composition in the 

 invaded formations, the irregular ground-plans, and the huge finger-like pro- 

 jections sent into the country-rocks, which are undisturbed either in dip or 

 strike, are among the familiar phenomena indicating that such magmas actively, 

 aggressively ' made their way in the world ' by the irregular removal of the 

 invaded formations. The latter look as if they had been, as it were, corroded 

 on a huge scale. 



The ' laccolithic ' hypothesis finds no support in the facts already learned 

 concerning the greater intrusive bodies of the Boundary belt. It is, for example, 

 virtually inconceivable that the Okanagan composite batholith could have been 

 developed in its present size and relations by mere injection. If the older rocks 

 parted successively, to admit tbe huge masses of the Osoyoos, Similkameen, and 

 Cathedral masses, the traces of these scissions must be left, yet none has been 

 discovered. How any one of them could, like a laccolith or like some chonoliths, 

 enter its chamber by lifting its roof without somewhere breaking through to the 

 earth's surface, is a puzzle, to say the least. 



Those who so lightly apply this hypothesis have usually neglected to prove, 

 or even discuss, the nature of the structure which, for batholiths, is the equiva- 

 lent of a weak zone in the Henry Mountains. The typical laccolith was 

 intruded in stratified rock and in an easily-split zone of shale. What 

 is the analogous (sub-horizontal) crustal structure which, on this hypothesis, 

 must be antecedent to the injection of the greater bodies? The granites character- 

 istically appear in the complexly folded terranes which are exceptionally strong. 

 Many large granitic masses, like the Cathedral batholith, (see page 459 and map), 

 have broken through more or less massive plutonics. The laccolithic hypothesis 

 implies the abundant recurrence of a relatively flat plane of weakness a fraction 

 of a mile, or a few miles, below the surface of the mountain ranges. Why it 

 should occur there, or how developed, has never been suggested. 



This hypothesis is powerless to explain the field relations of the Castle Peak 

 stock, the stocks of the Selkirk range in the Boundary belt, or many others of 

 the smaller granitic bodies in the belt. These are small enough to admit of 

 rather complete diagnosis, yet in no case is there any ground for the explana- 

 tion by pure injection. Large or small, batholiths or stocks, the granitic bodies 

 along the Forty-ninth Parallel must be otherwise understood. 



'Marginal Assimilation ' Hypothesis. 



The insufficiency of the pure-injection hypothesis has caused a second 

 school of geologists to emphasize a hypothesis of slow caustic action by magmas 

 that have advanced into the overlying earth-crust by their own energetic solvent 

 action on the walls and roofs. Additional evidence for the truth of this con- 



