288 Pirsson and Rice — Geology of Tripyramid Mountain. 



with the anomaly that when the mass is deeply penetrated the 

 grain becomes finer, which would be contrary to experience. 



It has been sometimes suggested that the zonal arrangement 

 of different rock-types in an intruded mass may be due to the 

 absorption and assimilation of the surrounding country rock. 

 In the present case the enclosing rocks were of acidic types, 

 granite below, mica-schists and perhaps gneisses above, while 

 the border facies of the intrusion is the basic gabbro. We 

 should, therefore, have to imagine that in some manner a body 

 of gabbro was intruded which was not affected at its border, 

 but which assimilated more and more foreign material towards 

 its center, so that the gabbro became a monzonite and the 

 monzonite syenite. Apart from the mechanical difficulty of 

 such an operation it is to be noted that the gabbro, as shown by 

 analysis, contains 11*5 per cent of lime, the syenite 2 - 5 ; there- 

 fore, even if we make the most favorable assumption that the 

 granite, or schist, absorbed contained no lime, it would require 

 that the gabbro should make a thorough absorption and mixing 

 of between four and five times its own weight of country rock 

 to produce the syenite. In the upper (and outer) parts of the 

 syenite we have observed in several places small areas from a 

 few feet across down to several inches where the rock differs 

 from the main type in finer grain, more ferromagnesian 

 minerals and a streaky appearance, and it is possible that these 

 may represent blocks from the roof which sank into the mass 

 and have been mostly resorbed and charged with syenite 

 magma ; but the view that the whole body of syenite could 

 have been made from the gabbro by such a process, the above 

 considerations, it seems to us, are sufficient to refute. 



After a review of these alternative hypotheses it is now in 

 order to seek one which will best fit all of the varied facts which 

 have been presented concerning the geology of Tripyramid. 

 Briefly stated the facts are as follows: First, the concentric 

 arrangement with the most basic rock at the margin and the 

 most acidic inside ; second, the abrupt transition of one type 

 into another, with angular broken contact line in places, and 

 yet with very slight though perceptible evidences of contact 

 metamorphic effects; third, probable intrusion of the gabbro by 

 thenorite and positive intrusion of the monzonite norite by the 

 syenite as shown by the dikes above the contact on Slide 

 Brook* ; fourth, the common sheet jointing which passes 

 through contacts as in a unit mass ; fifth, the final intrusion of 

 aplite dikes. 



The concentric arrangement naturally suggests a differentia- 

 tion of a body of magma in place, as at the Square Butte and 



* A bowlder of gabbro on Slide Brook was noticed to be cut apparently by 

 a dike of fine-grained monzonite, about 10 inches wide. Not being in place 

 it cannot be regai'ded as conclusive evidence but tends to confirm the view 

 that the gabbro solidified first. 



