Geology and Mineralogy. 309 



The best-known exposures of these rocks occur along the north- 

 eastern flanks of a broad belt of fundamental granites and crys- 

 talline schists which apparently form the central nucleus or back- 

 bone upon which they rest. This belt is known in a general way 

 to extend up the Tanana River from near its mouth southeast- 

 ward across the White River below the Donjek. In the latter 

 region C. W. Hayes reports quartzites and limestones resembling 

 the Birch Creek and Fortymile series on the southern flanks of 

 the granite, but the width of the belt, and whether there is any 

 considerable extent of the gold-bearing formations along its 

 southern flanks, is as yet unknown. It may not improbably ex- 

 tend into the high range of the Tanana, of which Mount McKinley 

 is the culminating point and in which the Kuskokwim and Sus- 

 hitna Rivers of western Alaska take their rise, for from the 

 reports of Moravian missionaries and of the traveler Dickey it 

 appears that gold occurs in the sands of each of these streams. 

 To the westward the granite backbone appears to pitch gently 

 downward, as its surface area narrows, and no exposures are 

 known west of the Yukon River. It is probably not a continu- 

 ous mass of granite on the surface, but contains smaller areas of 

 the later rocks folded in with it. East of the international boun- 

 dary the area in which the granite occurs apparently widens, but 

 its exposures are less continuous, the overlying rocks not yet 

 having been worn away. One granitic axis appears to extend 

 eastward from the Fortymile district through the Klondike region 

 in a nearly east-west direction, which is that of the prevailing strike 

 of the sedimentary rocks. The Canadian geologists report a 

 second granite axis on the Dease River just below Dease Lake, 

 which may belong to the older granites, though they do not make 

 the same distinction that Spurr does between the older granites 

 and the later intrusive rocks. 



Rocks of the various gold-bearing series above the granite are 

 reported at the following localities: Their first appearance, to 

 one ascending the Yukon from the sea, is near the mouth of the 

 Nowikakat. From here up to the Tanana River, rocks of the 

 Birch Creek series outcrop frequently along the river, when not 

 concealed by Tertiary sandstones and conglomerates, and the 

 range of low mountains on the north side and parallel to the 

 river is probably formed of these and Fortymile rocks. About 

 3 miles above the mouth of the Tanana, granite is exposed on an 

 island in the Yukon, and 12 miles higher calcareous quartzitic 

 schists of the Fortymile series appear under the Tertiary con- 

 glomerates. From the mouth of the Tanana up to Fort Hamlin, 

 at the lower end of the Yukon Flats, the river runs in a canyon- 

 like channel, known as the Lower Ramparts, cut through a low 

 range of mountains which consist principally of the dark greenish 

 and reddish rocks of the Rampart series, except where these are 

 buried under Tertiary conglomerates. The latter rocks occur 

 immediately above the exposures of Fortymile rocks, and again 

 from My nook Creek up beyond the mouth of Hess Creek. Higher 



