278 Pirason and Rice— Geology of Tripyramid Mountain. 



The actual contact of its rocks with the granite lias, however, 

 nowhere been seen by us. It is also notable that topographic- 

 ally the mass rises everywhere high above the granite imme- 

 diately about it, though elsewhere the granite, as in Mt. Osce- 

 ola a few miles to the northwest, rises to similar elevations. 



Avalanche Brook and the North Slide. — The narrow valley 

 of Avalanche Brook is in general cut in glacial drift and the 

 narrow stream bed is of transported blocks and pebbles. But 

 about a mile above its mouth there is exposed a very heavy 

 ledge of coarse black gabbro, sloping down stream, over which 

 the brook descends. The length of the exposed area is about 

 100 feet. It shows a thick heavy sheeting with strike approx- 

 imately N. 70° E. and a dip of 25° NW. The slope of the sur- 

 face is about parallel to the sheet jointing, which apparently 

 conditions it. The rock is identical in all respects with the 

 gabbro, or " ossipite " of Hitchcock, from the Black Cascade 

 on Slide Brook, mentioned later. It also appears as a ledge in 

 the wood road south of the stream on the bench above, but in 

 the woods no outcrops have been observed. 



Above this for a quarter of a mile, or less, there are no 

 exposures until the foot of the North Slide is reached. From 

 here almost to the very top of the North Pyramid is a contin- 

 uous surface of naked rock which extends far up to the head 

 of the gulch and also shows in massive outcrops on the lower 

 slopes of the opposite Fourth Pyramid. 



The rock forming the lower portion of the exposed mass is 

 a moderately coarse-grained, yellowish gray monzonite, which 

 is more or less altered ; the biotite which it contains resists 

 weathering somewhat better than the hornblende, the latter 

 being dull in luster, or even converted into earthy spots of 

 iron oxide. This rock persists up the Slide to an elevation of 

 about 400 feet above the stream, as determined by the aneroid, 

 where it gives place to a flesh-colored syenite which extends 

 from here to the top of the peak. 



The contact between the two rock types runs E. and W. 

 magnetic across the slope, and thus descends from a higher 

 point on the eastern side diagonally downward to a lower one 

 on the western, a fact whose significance will be considered 

 later. The contact plane appears nearly vertical, so far down 

 as it can be seen. In the years immediately following the 

 slide and the exposure of the fresh rock surface, this contact, 

 or rather the contrast in color of the red syenite above and 

 the gray monzonite below, as seen in mass at a distance, was 

 a very noticeable feature. Since then weathering has dulled 

 the colors and the growth of mosses and lichens upon the rocks 

 renders them much alike in appearance. When close at hand, 

 however, the contact was then, and is now, discovered with dif- 



