Pirsson and Rice — Geology of Tripyramid Mountain. 283 



this dike is quite similar to the main mass of syenite. On the 

 south wall, against the monzonite, there is no special evidence 

 of endomorphic result from the contact, but on the north side 

 there is a definite fine-grained band, about two feet wide, 

 appearing like aplite. The two rocks seem to grade into each 

 other. This dike is of importance in considering the genesis 

 of the Tripyramid complex, as will be seen later. 



Narrow dikes of aplite consisting of alkalic feldspar and 

 quartz, with a small quantity of biotite, are very common and 

 cut all the other rocks of the complex. When of some thick- 

 ness, that is from a few inches up to two feet, they have the 

 sugar-granular texture of ordinary aplites ; when less than this 

 they may become quite dense and felsitic. The coarser ones 

 may be grayish or brownish white, but usually they are a 

 strong flesh-pink. One of these, about 6 inches thick, cuts both 

 gabbro and norite at the contact on Slide Brook, and another 

 of two feet in width traverses the monzonite in the rock bed 

 of the stream just above the "V." Narrow stringers and 

 dikelets are very numerous and may be seen traversing the 

 surface of the North Slide. 



No general system of trend has been observed among the 

 dikes, only in limited areas the parallel and reticulated arrange- 

 ment of aplitic stringers shows quite clearly that they fill joint 

 systems, which have been cemented and healed by them, as in 

 the norite and gabbro exposed at the contact on Slide Brook. 



Jointing. — The jointing of the rock masses composing the 

 complex of Tripyramid has been alluded to in several places. It 

 is a matter of importance because, when considered in general, 

 it is seen that, except locally near the Black Cascade, it follows 

 the surface of an oval dome, considerably eroded, of which the 

 Pyramids may be considered the present highest points. This 

 ovate sheeting, or onion-like structure, is one not infrequently 

 seen in granitic stocks and massifs, and is one often referred 

 to initial stages of weathering, that is to the sheai'ing effect of 

 alternate contraction and expansion produced by exposure 

 of bare rock surfaces to the cold of winter and the heat of 

 summer. We do not, in the present case, believe it has been 

 produced in this way, because it is too regular and massive, 

 and because it is just as well developed under heavy coatings 

 of glacial till as on exposed surfaces, as may be seen on Ava- 

 lanche and Slide brooks. During the period of the ice invasion 

 all the superficial effects of pre-glacial weathering must have 

 been swept away, and the rock surfaces bitten into rather 

 deeply, and at its close certain areas were left exposed to 

 weathering, while others were protected by heavy mantles of 

 till left upon them. It is suggested that the upper part of 

 the mountain was comparatively bare and the glacial deposits 



