Pirsson and Rice — Geology of Tripyramid Mountain. 279 



Acuity. As traced along the slope it shows little or no endo- 

 niovpliic effect in either type of rock, except that in the syenite 

 there is a narrow zone of two or three inches in thickness in 

 which the grain is a little finer and in which it is enriched in 

 spots with hornblende. Apart from this there is no change in 

 texture, nor anything that would indicate a contact except the 

 abrupt change in mineral composition. Another interesting 

 feature in this connection is that the great mass exposed in the 

 Slide shows everywhere a sheet jointing, similar to that seen in 

 the gabbro previously described, and this sheeting, sometimes 

 thick and heavy, sometimes thin and shelly, passes through both 

 rock varieties and across the contact, as if it were in a unit mass 

 of one rock type. The apparent significance of this is discussed 

 later. This sheeting, which is seen in fig. 2, conditions the 

 naked rock slopes seen on the Slide, which for long distances 

 are parallel to it, with a dip of about 25°-30°. It is also of 

 interest that the monzonite in the higher exposures in the head 

 of the Ravine of Avalanches and on the south slopes of 

 Fourth Pyramid, where it appears in massive outcrops, is quite 

 fine-textured. 



Above the monzonite, the syenite, as already mentioned, 

 extends to the top of the pyramid. Along the surface of the 

 Slide the rock is fairly fresh and solid, but the top consists of 

 broken blocks and debris in place, and is more altered. 

 The three pyramids and the whole top of the mountain are 

 composed of this flesh-colored syenite, always in this state of 

 broken blocks and debris, and everywhere covered with a more 

 or less dense thicket of spruce. These conditions make its 

 lower limit uncertain, but it pretty surely extends down as far 

 as the 3500 contour line and is thus shown on the map. 



South Slide and Brook. — On Slide Brook and above its 

 junction with Avalanche Brook the first rock in place is found 

 at the Black Cascade, a very short distance above the mouth 

 of Cold Brook. Here the stream pours over a low cliff of 

 gabbro, forming a fall some 15-20 feet high. The gabbro 

 is a black, heavy, very coarse-grained rock, quite similar to 

 that seen on Avalanche Brook, but of somewhat coarser tex- 

 ture. This is the original " ossipite " of Hitchcock,* which 

 because of its containing labradorite was placed in the " Labra- 

 dor System " in the New Hampshire report (according to 

 Sterry Hunt's classification of the Azoic formations, which 

 was followed by Hitchcock at that time, but later considered 

 by him as doubtful). A chemical study of the rock was made 

 by E. S. Dana,f who deduced that it consisted chiefly of 

 labradorite, with some olivine, and a little il men ite ; and as it 



* Op. cit., vol. i, pp. 37-40, vol. ii, pp. 213 et seq. 

 •f This Journal, 3d serips, vol. iii, pp. 48-50, 1872. 



