10 University of California Publications in Geology [Vol. 8 



discrete blocks. But there may be some doubt as to the reality of 

 the laccolithic structure, and particularly as to the existence of the 

 floor. If Brogger's conception 24 of the intrusive process be correct, 

 then the broken floor of his laccolith can be regarded only as a par- 

 tition separating it from an underlying batholith into which it sank. 

 But this sinking of the floor in blocks is a manifestation of stoping 

 on a large scale ; and we have no assurance that the foundering pro- 

 cess once inaugurated stopped short of the complete annihilation of 

 the partition, by absorption in the depths of the batholith. Thus, 

 while the Christiania mass may have been temporarily laccolithic, as 

 a molten mass, it may by the vanishing of its floor before solidification 

 have merged with the deeper parent magma, and may be now. as a 

 structural feature, only the upper part of a batholith. Doubtless 

 the sagging of large laccoliths is due, as Coleman has argued for the 

 Sudbury case, to the transfer of molten matter from the region below 

 a partition to that above it ; and it is easily conceivable that a batho- 

 lithic magma may in this way be completely drained off by the sub- 

 sidence of the partition till the latter rest upon the bottom of the 

 batholith. So long, however, as the partition remain in large part 

 intact and be not engulfed piecemeal and resorbed by the batholith, 

 the magma transferred to the region above it and migrating laterally 

 over it becomes a true laccolith, be it ever so vast. 



Having thus pointed out that large laccoliths occupy geosynclinal 

 basins, it is interesting to turn again to the Boulder mass and observe 

 that it has precisely this structure. Thus Sales 25 says : 



In a broad way the [Boulder] batholith seems to occupy the trough of a 

 great synclinal basin, in whose dissected sloping sides may be seen remnants 

 of the entire series of sedimentary rocks reaching from the pre-Cambrian slates 

 and shales upward to the coal-bearing sandstones of the late Cretaceous. 



And again : 



There is an abundance of evidence to show that the Boulder batholith did 

 not produce a doming effect on the sedimentary rocks now found along its 

 borders; in fact, the orientation of these intruded rocks does not appear to 

 have been visibly disturbed even where they are in direct contact with the 

 granite. In nearly every instance where such contacts are open to observation, 

 the sedimentaries are found to dip at a steep angle toward the granite; an ex- 

 ception, however, may be noted at Elkhorn, where dips away from the granite 

 appear to be a coincidence and not an effect produced by its intrusion. 



-4 Die Eruptivgesteine des Kristianiagebietes, II, 1895, p. 133. 

 Op. cit., p. 1526. 



