1914] Lawson : Is the Boulder "Batholith" a Laccolith ? 13 



the depth the less will be the modification of the thermal and emanant 

 effect. Thus a large zone of intense metamorphism establishes a pre- 

 sumption in favor of the view that the intrusive mass is a batholith, 

 while a narrow zone of intense metamorphism, or even a wide zone of 

 feeble metamorphism, is indicative, as Termier has pointed out, of an 

 injected mass — a laccolith. 



From what has been written of the contact zone surrounding the 

 Boulder mass, and from personal examination of the zone south of 

 Helena, I am satisfied that the metamorphism which it displays is 

 relatively feeble. It is certainly not in the same class with the "ter- 

 rains cristallophylliens" which Termier 32 holds to be characteristic 

 of batholiths, and it is of just such an intensity as one might expect on 

 the periphery of a large laccolith. 



CONSEQUENCES OF THE LACCOLITHIC HYPOTHESIS 



Magmatic Concentration of Sulphides. — Now it seems to me, from 

 the considerations set forth in the preceding pages, that the batho- 

 lithic origin of the Boulder mass is not proved, and that there is quite 

 as much justification for the view that it is a laccolith. In the liter- 

 ature of economic geology it has been assumed without question that 

 it is a batholith, and certain consequences that flow from this assump- 

 tion have been held to sustain the doctrine of the magmatic origin 

 of the waters concerned in ore deposition. It may be well, therefore, 

 to examine the possibilities inherent in the alternative and equally 

 good hypothesis, that it is a laccolith. If the mass be a laccolith, Ave 

 should expect by analogy with the Sudbury, Duluth, and Bushveld 

 laccoliths that the magma should have undergone differentiation with 

 concentration of its metallic constituents in its lower part. That such 

 differentiation has adually taken place locally is shown by Knopf 33 

 in his description of the magmatic sulphide ore body at the Golden 

 Curry mine, in the Elkhorn District. The mode of occurrence of the 

 oxidized copper ores of the Bullwhacker mine and adjacent territory 

 appears to me to indicate another instance of this sort of magmatic 

 concentration. These ores are mined, in part, in large open pits at 

 the base of the steep slope determined by the Continental fault. For 

 a description of them I cannot do better than quote Sales, 34 under 

 whose guidance I visited them last summer. He says : 



32 Loc. cit. 



33 Op. cit., pp. 137-139. 



