282 Pvrsson and Rice — Geology of Tripyramid Mountain. 



Eastt rn Sidt of the Mountain. — The lower eastern slope 

 of Tripyramid and the valley of Sabba Day Brook were not 

 visited by ue for reasons stated, and we are here compelled to 

 fall back on the observations of Huntington and Hitchcock. 

 They state that at the falls on Sabba Day Brook, as previously 

 mentioned, the exposures are of granite of the Conway type, 

 which is the common rock of the country." It appears on the 

 brook, but abont a mile higher up it is replaced by an area of 

 "labrador" rock. This is stated to have the large cleavable 

 feldspars which show the opalescence characteristic of this 

 variety. As both Hitchcock and Huntington had visited 

 the gabbro (ossipite) locality on Slide Brook and were well 

 acquainted with that rock, there can be no reasonable doubt 

 but that this area is composed of gabbro similar to the areas 

 of it on the western side of the mountain. Apparently the 

 reason why Hitchcock did not name it ossipite was because, 

 as he states, chrysolite (olivine) was not visible in it (megascop- 

 ically of course). We have, therefore, shown an area of it on 

 the geological map similar in a general way to that given in 

 the atlas accompanying his report. According to his statement 

 this area is bounded to the southeast, south, and southwest by 

 the porphyritic granite, or gneiss, as he calls it ; as no exposures 

 are specified we have not attempted to show this on the map. 

 In the same place Hitchcock says that on the North Tripyramid 

 the syenitic rocks appear in the same order as on Slide Brook, 

 "apparently cutting the labradorites." Though he does not 

 mention specific exposures, it seems clear from this that he 

 means that the monzonite (his gray syenite) occurs also on this 

 side of the mountain, and his section, previously given, is drawn 

 in accordance with this idea. 



Dikes. — No very large or important dikes have been seen 

 by us in the areas of exposed bed-rock. The largest are two 

 black dikes of altered camptonite, about 6-8 feet thick, which, 

 at right angles to one another, cut the porphyritic granite at 

 Norway Rapids, and whose presence in fact conditions the 

 small fall in Slide Brook to which this name has been given. 

 On the other hand, small dikes and dikelets are very common. 

 Thus the gabbro at the Black Cascade is cut by a number of 

 dikes of a gray-black, dense trap, varying from a few inches to 

 a foot in thickness. One of these is exposed for a hundred 

 feet or thereabouts in the rock bed of the stream. The petro- 

 graphic study shows these are micro-gabbros. A short distance 

 above the contact of the norite with the gabbro the bed-rock 

 in the brook is cut by a dike of syenite about 6 feet in thickness 

 with trend 1ST. 70° W. In minerals, texture, and appearance 

 *Op. cit., vol. ii, p. 217. 



