TOPOGRAPHY AND DEAIXAGE 317 



Between the eastern and Avestern mountain belts described a to]io- 

 grai)hically distinguishable zone intervenes. This is the low, nearlv 

 level basin between the Ujjper and Lower Eamparts. This basin is 

 floored by Tertiary beds and Eecent silts. It is drained on the north side 

 of the Porcupine by the Coleen river, from which it has been named the 

 Coleen basin by Maddren.^ The north and south extent of this basin is 

 unknown, but it is probably considerable. Its east and west extent, as- 

 measured along the river, is about 25 miles. This old Tertiary basin 

 terminates abruptly on the east at the foot of the Upper Eamparts, where 

 walls of Paleozoic rocks 200 to 300 feet high confine the river to a narrow 

 gorge. On the west it extends a short distance beyond the Coleen river. 



Structure 



FOLDS 



It is not the intention to attempt here a complete interpretation of the 

 structural features involved. They include both folding and faulting 

 and are expressed by a perplexing amount of variation in strikes and 

 dips which have been interpreted only in part and in reference to some 

 of the more conspicuous structural features. 



The Paleozoic rocks of this region everywhere show the effects of ex- 

 tensive deformation. The strike of the beds, although highly variable, 

 has a general northerly and southerly trend. The very evident northerly 

 and southerly trend of some of the more conspicuous topographic feat- 

 ures, resulting in part from deformational agencies, confirms the opinion 

 that the northerly and southerly direction of the strike extends well be- 

 yond the river along which all of the direct observations on the structure 

 were made. Deformation of the rocks seems to have expressed itself in 

 two classes of folds, which may be distinguished as large and small, since 

 they differ very considerably in magnitude. The latter are sometimes 

 partially overturned, and where this has been observed the overturn is 

 always toward the east, thus indicating that the pressure came from the 

 west. Examples of partially overturned folds occur at the lowest out- 

 crops on the river and at a point 2i/2 miles above the mouth of Coleen 

 river. The older rocks are in part concealed over a considerable area by 

 Tertiary and Eecent silts above the Coleen river, and by lavas in the 

 Upper Eamparts, but the exposures observed suggest that the small type 

 of folding predominates in the belt extending from the Coleen to Camp- 



1 A. G. Maddren : Smithsonian exploration In Alaska in 1904 in search of mammoth 

 and other fossil remains. Smithsonian Miscellaneous Collections, vol. xlix, 1905, p. 12. 



