290 Pirsson and Rice — Geology of Tripyramid Mountain. 



The lamprophyric dikes may be regarded as complementary 

 to the syenite-aplite and their peripheral situation in the gab- 

 bro and contiguous granite is the customary one. 



In regard to the roof of the intrusion, it is known that a few 

 miles to the south and to the west the grauite batholith gives 

 place to mica schists beneath which it may reasonably be sup- 

 posed to descend, as previously stated. We suggest that this 

 roof had once a greater extension to the northward over the 

 now exposed granite, and that the Tripyramid mass was, in 

 large part, intruded between the uneven surface of the granite 

 and the mica schists, doming up the latter, as indicated in 

 fig. 6, in a purely diagrammatic way. 



Fig. 6. 



Fig. 6. Diagrammatic section to illustrate the probable origin and struc- 

 ture of Tripyramid Mt. 



This conception makes the intrusion laccolithic in its nature. 

 We do not desire to insist too strongly on this point because 

 we know so little of the relation of the igneous mass to the 

 surrounding granite beyond the fact that at three different 

 points on its edges the gabbro begins at about the same eleva- 

 vation. And we are also uncertain concerning the boundary 

 on the southeastern side. But it seems rather strongly indi- 

 cated, from the present shape of the mass and its sheet jointing, 

 that its upper surface had the domed form seen in laccoliths; 

 and, as this view of its structure seems the one best fitted 

 to bring into harmonious relation the varied parts and corre- 

 late it with similar occurrences elsewhere, it seems natural to 

 adopt it. According to this conception the sheeted jointing 

 merely represents the effects of contraction, as the planes of 

 cooling parallel to the domed surface descended into the solidi- 

 fied but still heated mass, as has been already discussed. 



We are quite conscious, in provisionally adopting this hypothe- 

 sis and in fact endeavoring to elucidate the structure of Tri- 

 pyramid, that more and better evidence would be desirable, but, 

 considering the facts that we have been able to obtain, this on 

 the whole seems the most accordant with them. 



Summary. 



Tripyramid Mountain is an igneous intrusion consisting of 

 several rock types concentrically arranged. On the outer bor- 



