Plrsson and Rice — Geology of Tripyramid Mountain. 289 



Shonkin Sag laccoliths, but this is negatived by the contacts 

 and the syenite dike. The latter suggest a series of successive 

 intrusions ; but the zonal arrangement, the common sheet 

 jointing and the small amount of endomorphic effect show that 

 this could not have been of the ordinary character. 



Between the differentiation of a unitary body of injected 

 magma at one extreme and a series of successive intrusions, as 

 one ordinarily finds such a process evinced, at the other, which 

 might give rise to an igneous complex, it is easily possible to 

 imagine a whole chain of gradations. It appears to us that what 

 best explains Tripyramid is a process of intermediate nature, 

 in which both differentiation and repeated intrusions, separated 

 by only short intervals, took place. We might imagine this to 

 be somewhat as follows : First, the intrusion of a body of 

 monzonitic magma through the granite and between it and a 

 cover of schists ; second, differentiation in the body with pro- 

 duction of basic border masses, either by diffusion of the femic 

 molecules to the margin, or by their crystallization at the 

 edge, or both, with production of the gabbro and cooling in 

 this region until the latter had acquired a certain solidity ; 

 third, an upward movement of magma from below with 

 ruptures along the inner border of the gabbro and the bringing 

 of material of a somewhat different composition against it ; 

 fourth, differentiation continuing with formation of the inner 

 body of syenite and progressive solidification of the main 

 monzonite body from the margins and from above inwardly ; 

 fifth, further upward movement of magma from below, ruptur- 

 ing along the edges of the solidified monzonite, bringing 

 syenite magma against it with the injection in places of 

 syenite dikes into it ; sixth, cooling and solidification of the 

 syenite, and, lastly, injection of the final, most highly differen- 

 tiated and acid product from within and below, upward and 

 outward into the consolidated parts with formation of aplite 

 dikes. 



We therefore think that the two upward movements here 

 indicated were not of sufficient amount, or volume, to destroy 

 the zonal effects of differentiation, but sufficient to modify it in 

 the manner indicated, and they must have produced a further 

 doming of the upper surface of the intruded mass. We sug- 

 gest that they occurred while the already solidified parts were 

 still hot, and before jointing from contraction had taken place ; 

 this would explain the small amount of endomorphism seen 

 and the fact that the subsequent jointing took place as in a unit 

 rock body. It is possible that still another uplift of magma 

 may be indicated by a contact between the norite and monzonite 

 on Slide Brook, or they may grade into one another, but upon 

 this point, for reasons previously stated, the evidence is 

 wanting. 



