2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER XXVI. 



MECHANISM OF BATHOLITHIC INTKUSION. 



The facts to be explained by a final theory of batholiths fall into 

 three classes : field relations, time relations, and chemical relations. At the- 

 risk of repeating statements made in preceding chapters as well as many of 

 those published by other writers, a summary of the leading pertinent facts 

 may well anticipate the theoretical discussion. It will be understood that stocks 

 and bosses are regarded as only small batholiths or as parts of batholiths, and 

 in many cases will not be specially named. 



FIELD RELATIONS. 



There is general agreement that batholiths are to be found only in, or 

 on the immediate borders of, mountain-built regions. This rule is so general 

 that it may be called a law. Almost if not quite as general is the rule that 

 batholithic intrusion to observed levels in a mountain-range, follows the climax 

 of an orogenic paroxysm; though flow-structures and gneissic structure may be 

 induced in the batholith in the closing, weaker stage of the crustal movement. 

 The gneissic structure may be difficult to distinguish from that due to a later, 

 independent period of crushing. Abundant examples of the two rules are noted 

 in the summary geological histories of the Selkirk, Columbia, and Cascade 

 ranges. The Eykert batholith is the only one on the Forty-ninth Parallel which 

 seems to have a well developed primary flow-structure. 



No one has ever seen the bottom of stock or batholith. Owing to the 

 limited amount of possible upheaval of the earth's crust above baselevel, erosion 

 has probably never been able to penetrate more than six miles into these masses 

 and, in general, penetrates less than three miles. In each case, therefore, the 

 observer walks on a surface near the top of the body. Sometimes erosion has 

 not yet uncovered such granitic masses, whose presence is detected by the heavy 

 contact metamorphism so characteristic of batholiths. More often the roof is 

 partly destroyed, leaving broad belts of metamorphosed roof rock about the 

 cupola-like intrusive, and roof -pendants within it. A fine example of a partially 

 uncovered stock is that at the Dewdney trail on the summit of the Selkirks 



(see page 299). Perhaps the majority of exposed batholiths have lost their roofs 

 so far that only small areas of the pendants remain, while the metamorphic 

 aureole has the narrowness appropriate to the wall of a batholith. Not even 

 one roof-pendant is known in the Coryell batholith of the Columbia range. 



(See page 358 and map sheet.) 



With deep erosion and destruction of the roof the molar (main) contact 

 has typically an elliptical ground plan, though it is often irregular. Disregard- 

 25a— vol. iii— 47£ 725 



