FAULTS 319 



bluish gray Ordovician limestone outcrops. ISfot more tlian 250 yards 

 below it, on the same side of the river, Carboniferous shales outcrop. 

 These continue to oiitcrop for 2 miles farther down the river, dipping 

 to the westward. Carboniferous and Ordovician beds thus occur in adja- 

 cent outcrops, the entire Devonian and Silurian series being faulted down 

 out of sight. The fault is parallel with the trend of the Lower Eampart 

 fold and apparently cuts its western limb. The amount of throw is rep- 

 resented by the total thickness of the Devonian and Silurian. This can 

 hardly be less than 4,000 feet and may be much more. The effect of a 

 fault of this magnitude which parallels the principal axis of folding in 

 the region on the areal distribution of the rocks is considerable. To the 

 westward of it the rocks are all of Carboniferous age, while to the east- 

 ward lie the Silurian and Ordovician limestones. In the Carboniferous 

 rocks lying to the westward of the fault shales predominate, while mag- 

 nesian limestones comprise the bulk of the formation immediately to the 

 east. The marked differences which these two kinds of rock exhibit in 

 their powers of resistance to the agencies of erosion are very distinctly 

 represented in the two distinct types of topography seen on the two sides 

 of the fault. To the east rises abruptly the belt of hills and low moun- 

 tains standing from 400 to 2,000 feet above the Porcupine. On the west 

 lie the Yukon flats, showing no topographic relief except for a few low 

 hills generally not more than a hundred feet high near the eastern 

 margin. 



A few rods to the eastward of the fault just described a small fault 

 occurs on the north side of the river. The throw of this fault is very 

 small, rocks of the same formation appearing on both sides of it. Tt 

 seems quite possible that a fault may determine the eastern limit of the 

 Coleen basin. This basin is terminated abruptly on the east by a steep 

 ridge 500 feet high at the entrance to the Upper Eamparts. The sharp 

 topographic contrast between the flat plain of the Coleen basin and the 

 precipitous high north and south ridge which bounds it on the east is 

 very suggestive of a faultscarp. There is, however, no supplementary 

 evidence of the suggested fault available. 



In the Upper Eamparts, below Salmontrout river, the Porcupine 

 crosses a fault block 4 or 5 miles in width. Both physical and faunal 

 evidence indicates that a fault block has been dropped down here. The 

 outcrops below the Indian village half a mile show the limestones to Ijc 

 much broken up for 30 or 40 yards. Evidence of faulting is also seen 

 on the south side of the river 4 miles lower down. According to Mr 

 Girty's determinations, the Carboniferous limestones adjacent to the 

 Devonian beds are of Upper Carboniferous age, indicating that the Lower 



