164 



GEOLOGY. 



Observatory inlet has its whole course in the Coast 

 range and the rocks exposed along it consist mostly of the 

 greyish granitoid rocks characteristic of that range. 

 Schists outcrop along the lower portion, and at Granby 

 bay an important area of argillites, mineralized in places, 

 occurs as an inclusion in the granitic rocks. 



The argillaceous area at Granby bay has a maximum 

 width of nine miles (14-5 km.). It is surrounded on all 

 sides by granitic rocks and is considered to be an unde- 

 stroyed and deeply sunken portion of the old roof of the 

 Coast Range batholith. The basin is of great depth as the 

 rocks of the inclusion are exposed from base to summit of 

 mountains over 5,000 feet (1,524 ,m.) in height and they 

 must extend to a considerable depth below the present 

 surface. 



The argillites in the vicinity of Granby bay are coarsely 

 bedded, hard, compact rocks usually altered to some extent 

 and occasionally passing into mica and quartz mica schists. 

 The ordinary fine grained dark variety alternates in places 

 in thin bands with a lighter colored, coarser grained and 

 more felspathic type made up of tufaceous material. 

 Limestones, in small non-persistent beds, are occasionally 

 present, and near the southern boundary of the inclusion, 

 altered greenstones largely of pyroclastic origin are prom- 

 inent. 



The argillites are seldom and only over limited areas 

 cleaved into slates. They are folded into a number of 

 anticlines and synclines striking approximately east and 

 west or parallel to the long axis of the area. The dips, as 

 a rule, are regular and comparatively low, although iocally 

 the strata are greatly disturbed. No faulting on a large 

 scale has been detected. 



Dykes cutting the argillites are numerous throughout 

 the area. Two sets, one preceding, and the other subse- 

 quent to the mineralization of the region, have been dis- 

 tinguished. The former are genetically connected with 

 the enclosing granite rocks, and include a number of types 

 ranging from quartz porphyries and pegmatites to diorites. 

 The latter are usually lamporphyric in character. 



