1914] Lawson: Is the Boulder "Batholith" a Laccolith f 



9 



With these notable instances of the vast scale upon which lacco- 

 lithic injections may operate before us, it is clear that the mere size 

 of the Boulder mass can no longer be urged as a valid argument 

 against its laccolithic nature. 



Structural features. — It is a remarkable fact that in all three of 

 the great laccoliths cited above, the Sudbury, the Duluth, and the 

 Bushveld, the floor of the injected mass has the configuration of a 

 geosynelinal sag. In the case of the Sudbury laccolithic sheet the 

 synclinal structure is very pronounced. Coleman" accepts 30° as the 

 correct average dip of the spoon-shaped mass toward its center and 

 on this basis estimates its thickness at a mile and a quarter. He 

 explains the structure as due to collapse : 



The most probable supposition is that the source of the magma was im- 

 mediately beneath the longer axis of the area, which is now basin-shaped, but 

 was then flat and undisturbed. . . . When it ascended and spread out 

 widely between the upper sediments and the less regular rocks beneath, there 

 was a collapse, since the schists and older eruptives below had lost their central 

 support. This shows itself very plainly in the shattered and faulted character 

 of the rocks underlying the sheet. The collapse gave rise to the roughly 

 synclinal basin occupied by the eruptive, while the overlying sedimentary 

 rocks settling into the still plastic magma beneath formed a more regular 

 syncline. 



The sag of the Duluth laccolith is the geosyncline of the southwest 

 arm of Lake Superior. 21 



Molengraaff 's mapping 22 of the Bushveld laccolith shows very 

 clearly that its main mass occupies a synclinal sag no less definite 

 than that of the Sudbury laccolith though on a vaster scale, while 

 another, perhaps smaller, sag accommodates the northern portion of 

 the intrusion. He says: 23 "The strata of the Transvaal System, at 

 a certain period, sank under the weight of the intrusive masses, and 

 it resulted from this subsidence that all around the plutonic basin the 

 strata of the Transvaal System dip towards a common center. In 

 fact the dip, easily observable, especially in the Pretoria beds, is 

 everywhere directed towards the Bushveld." 



Brogger's classic description of the Christiania "laccolith" shows 

 that the collapse of the floor of that intrusive mass was effected not 

 so much by synclinal bending as by the insinking of the bottom in 



20 Loc. cit. 



=i IT. S. G. S. Prof. Paper 52, Plate I, Sect. C-D. 



22 Op. cit., Plate I. 



23 Op cit., p. 51. 



