

Pirsson and Rice — Geology of Tri/pyramid Mountain. 281 



While one cannot pronounce with the certainty that is 

 desirahle which is the younger of the two rocks, the study 

 leaves a feeling and belief that the norite is, and this is con- 

 firmed in considerable measure by the micro-study of the 

 norite, given in detail later, which shows its feldspar ciwstals 

 bent and broken near the contact from movement under pres- 

 sure along the gabbro boundary, while crystallizing. If this is 

 so, then the gabbro must have been still very hot when the 

 norite came against it, and yet sufficiently stiff, or solidified, to 

 retain after the movement the broken angular border which 

 characterizes it. 



Ascending the stream bed from here the outcrops of norite 

 after a short distance give place to exposures of monzonite. 

 This monzonite is precisely similar to that found at the base 

 of the North Slide, as previously mentioned. It is usually 

 moderately coarse-grained but varies in places to rather fine 

 grain ; weathers brownish, but on a fresh fracture is rather 

 thickly mottled with ferromagnesian minerals which are dull 

 in luster and more or less altered. It resembles surprisingly a 

 specimen of the more feldspathic monzonite from Monzoni in 

 appearance and texture. It is composed of about equal parts 

 of orthoclase and plagioclase, with hornblende, some augite 

 and biotite, and a little quartz. 



No contact between the norite and monzonite is visible ; if 

 such exists it is covered with debris. We are uncertain, there- 

 fore, whether there is a sharp contact, as between the gabbro 

 and norite, or whether the norite grades into the monzonite. 



Outcrops of monzonite are seen at intervals following up the 

 stream, the best exposures being at the little gorge known as 

 the "V." As in the other types of rocks, it has a heavy sheet 

 jointing which dips away from the mountain mass. The 

 stream, following along the strike of this, has cut away the 

 rock so that one wall is made by the jointing planes, the other 

 by cross erosion, and this makes a steeply descending gorge. 

 Long surfaces of naked rock are exposed over which the stream 

 descends and into which it has cut pot-holes. 



Above this no rock in place is seen until the South Slide is 

 reached. The stream bed is in debris and drift material and 

 filled with bowlders of many rock types, the syenite and mon- 

 zonite of the mountain being mixed with glacial erratics. 

 Among them some blocks of black gabbro were noticed, and it 

 is thought that these may have been brought over the western 

 slope of the mountain from the direction of Avalanche Brook 

 by the southward movement of the ice. 



The lowest outcrops of rock on the South Slide are about 

 half-way up, and are of syenite similar to that of the North 

 Slide and it is all syenite from here to the top of the South 

 Pyramid. 



Am. Jour. Sci. —Fourth Series. Vol. XXXI, No. 184. — April, 1911. 

 20 



