1914] Lawson : Is th e Boulder 1 ' Batholith ' ' a Laccolith ? 5 



From his discussion and diagrams it is apparent that a laccolith 

 may have a very large degree of irregularity and yet conform to the 

 essential relationships of this type of intrusion. He says : 



The term [laccolith] may be understood to imply a rock body which is 

 distinctly intrusive, occupying a space opened concurrently with its injection, 

 and having a thickness presumably much less than its lateral extent. The 

 essentia] point is that such a mass must possess an under as well as an upper 

 surface; and if strata are truncated by the visible upper surface, we must 

 suppose that their prolongation still exists beneath the intruded mass. The 

 actual upper surface of contact may be of more or less irregular shape, owing 

 especially to portions of the roof having become detached and sunk in the 

 magma. This action has by some geologists been accorded a foremost place in 

 the mechanism of batholithic intrusion. It is the process which Daly styles 

 overhead stoping.9 



While laccoliths in their simple forms may be classed as con- 

 cordant intrusions, as Harker 10 has done, in their more irregular and 

 complex forms they may, consistently with his own comprehensive 

 definition, be transgressive. Intrusive magmas may be injected into 

 previously disturbed strata, making space for themselves concur- 

 rently with their injection, relatively flat at the bottom and arched 

 above, without following bedding planes very strictly. Such in- 

 trusives are in their essential features laccoliths, particularly in re- 

 spect to the fact that they rest upon a floor on which they have been 

 placed by lateral migration of the magma. I know of no reason, 

 moreover, why the reaction of a laccolithic magma upon its roof 

 should not be, under certain conditions, entirely similar to that of 

 a batholith, so that the configuration of its upper surface might be 

 greatly modified by the foundering, faulting, stoping and resorption 

 of its cover, thus increasing its irregularity. For example, the in- 

 trusive mass at Marysville so admirably described by Barrell 11 may, 

 notwithstanding its transgressive relations to its cover, have a floor 

 of lower Beltian or pre-Beltian rocks; and in that sense, the essential 

 sense, be a laccolith. I do not by this statement assert that the mass 

 has such a floor and is, therefore, a laccolith. I merely point out 

 that in view of the evidence the hypothesis of a floor is as good as 

 that of no floor, and that the transgressive relation of the mass to 

 its roof does not preclude the existence of such a floor. I do venture, 



» Op. ext.. p. 84. 

 io Op. cit., p. 63. 

 " U. S. G. S. Prof. Paper, 57. 



