R. A. Daly — Mechanics of Igneous Intrusion. 123 



accessory augite. Here and tliere, with apparently gradual 

 transition, the dominant tonalite passes into a true basic to 

 medium-acidic hornblende-biotite granite. The triclinic feld- 

 spar ( again basic andesine, near Ab^ Ang ) here becomes quite 

 subordinate and orthoclase assumes its position as the chief 

 light-colored essential. Notwithstanding these and other vari- 

 ations in the character of the igneous body, it is to be regarded 

 as the crystallized product of a single intrusive magma. 



The shattered contact-zone which is the matter of interest 

 in the present connection, has been crossed in three places on 

 the line of contact running from the Columbia River to Violin 

 Lake, and has been mapped on the basis of the information 

 thus gained. The shatter-zone has not been mapped on the 

 western side of the batholith since the lack of exposures made 

 search for its limits unprofitable. It is highly probable, how- 

 ever, that the zone exists on that side, though it seems to be nar- 

 rower than on the southeastern boundary of the granitic tongue. 



An unusually fine exposure of the zone outcrops three miles 

 below Trail on the left bank of the Columbia. The local rocks 

 invaded by the magma are a coarse peridotite ( hornblendite ) 

 and an associated gabbro, both of which apparently belong to 

 the oldest series of rock in the area. For a distance of about 

 600 meters down the river and thus transverse to the molar 

 contact, the well-washed ledges are composed of a giant breccia. 

 Innumerable, sharply cut, angular masses of peridotite and 

 gabbro are enclosed in the granite and are transected by many 

 veins of the younger rock (fig. 3). The inclusions are of all sizes 

 up to blocks ten meters or more in diameter. Three kilometers 

 to the south westward the shatter-zone is more than a kilometer 

 in width ; the invaded formation is diorite which is markedly 

 schistose in many of the inclusions, becoming at times a true 

 amphibolite. It has proved impossible to separate a zone of 

 inclusions from the zone of apophyses because of theii' intimate 

 association and because of the massive character of the rocks 

 invaded ; hence the two have been combined in the map as a 

 zone of shattering. It is of rather extraordinary breadth in 

 this instance but there is illustrated the same kind of dynamic 

 action as that found along normal granitic contacts. 



At the granite-peridotite contact on the river bank, many of 

 the larger apophyses display an interesting case of differentia- 

 tion. At both walls of each apophysis the essential and 

 normally abundant bisilicates of the granite are absent or are 

 represented by rare shreds or plates of chloritized biotite. 

 The feldspars are here orthoclase, and microperthite with 

 accessory oligoclase-albite. Quartz is the remaining essential. 

 Other veins seemingly contemporaneous with these just de- 

 scribed, are composed entirely of the same aplite. The discovery 



Am. Jour. Sci.— Fourth Series, Vol. XVI, No. 92.— August, 1903. 

 9 



