280 Pirsson and Rice — Geology of Tripyramid Mountain. 



was thought to he a new rock kind the above name was given 

 to it. This was before the microscope had been used to study 

 thin rock sections in this country and the presence of pyroxene 

 in the rock remained undetected. Tims it is actually a gabhro, 

 though a very feldspathic one, as will be shown in the later 

 paper on the petrography of the area. 



The rock formed part of the " Norian system " of Hitchcock, 

 a term that was later supplanted by " Labrador system," and 

 from the name " Norian" the stream was called "Norway 

 Brook" by him. 



This black gabbro extends up stream, forming an exposure 

 about 500 feet long in the brook bed, where it is found, in a 

 massive, sloping, smooth-surfaced outcrop in contact with a 

 rock of very different appearance. This is brownish to reddish 

 gray in color, and, on a freshly fractured surface, is evenly 

 mottled light and dark with feldspathic and ferromagnesian 

 constituents, and is of medium granular texture. The petro- 

 graphical study shows it to consist chiefly of plagioclase, 

 hypersthene and iron ore, with some augite and biotite. It is, 

 therefore, a norite, and chemically is closely related to the 

 adjoining gabbro, but the contrast between the two in color 

 and appearance is striking. 



The contact of the two types is clearly shown for many yards 

 on a smooth surface, kept clean and bright by the stream 

 washing over it. Its general course is about N. and S. It is 

 not an even line but a broken one with offsets. The contact 

 appears at first sight irruptive and it seems that one of the 

 masses must have broken through the other. But on close 

 study it is found difficult to say definitely which is the older. 

 The contact is firmly welded, and both types come up to it 

 without change in texture or minerals, so no endomorphic 

 effects are visible. Nor does careful search show clearly 

 definite fragments of the one type enclosed in the other. In 

 one place the gabbro is penetrated by light-colored material in 

 a small dikelet which may be norite or aplite, its altered condi- 

 tion making it difficult to decide which. In addition the rock- 

 mass has a massive sheet-jointing which passes through both 

 and crosses the contact as if in a rock of uniform composition. 

 It has a northerly dip varying from N. 45° W. to IS. 25° E. 

 Both kinds have been much cracked and the cracks healed by a 

 later pneumatolytic deposit which shows as fine, whitish aplitic 

 stringers, often as thin as cardboard, but sometimes widening 

 out to narrow dikes and then in some cases assuming a pegmatitic 

 aspect. In one place for a number of feet a crack has occurred 

 along the contact, or closely adjacent, and this is filled like the 

 rest. 



