THE ORIGIN OF THE INCLUSIONS IN DIKES 177 



ically metamorphosed rock which is not exposed at the surface. 

 In the Boundary district there occur Tertiary tuffs and conglom- 

 erates overlain by basalts and other lavas, the bowlders as well as 

 the matrix of these tuffs and conglomerates being largely of igneous 

 origin. The conglomerates were apparently only locally developed, 

 their formation being preceded and succeeded by volcanic activity.^ 

 The relative age of the dikes and the Tertiary sediments is not 

 known. 



3. The third theory involves shattering along the walls of the 

 dike, the fragments being rounded during their ascent in the dike- 

 rock. The explanation as to why the inclusions (so far as the 

 writer is able to judge from the specimens collected) consist largely 

 of anorthosite must lie in an unusual amount of shattering accom- 

 panying the formation of the dike-fissure in that very compact 

 rock, while little shattering occurred in the younger intrusives 

 which may still have been at a high temperature. It is certain 

 that the presence of several million inclusions under the conditions 

 described calls for extraordinary conditions. 



The material which has been removed from the inclusions 

 appears in part in the hand specimen of the kersantite as white xeno- 

 crysts of varying size. In the field these xenocrysts are not noticed, 

 on account of the weathered condition of the rock. It is also 

 probable that many of the feldspar phenocrysts — -and perhaps even 

 some of the pyroxene phenocrysts — -of the kersantite are really 

 xenocrysts of the anorthosite. 



Little Belt Mountains, Montana: In the Little Belt Mountains 

 Pirrson has described some minette dikes which have brought up 

 masses of a plutonic rock from some depth below the surface.^ 

 One of the inclusions is a mica syenite, 2 to 3 feet in diameter. The 

 minette does not show the least amount of endomorphic modifica- 

 tion, but retains its normal minerals and structure to the contact. 

 Pirrson writes: "From this we may infer that the mass was taken 

 up while the magma was extremely hot, and that it had acquired 

 very nearly the temperature of the fluid mass before the latter 



^ R. W. Brock, "Preliminary Report on the Rossland District," Can. Geol. Surv. 

 (1906). 



^ U.S. Geol. Surv., 20th Ann. Kept., Part 3, p. 536. 



