ALASKA AND ITS MINERAL RESOURCES 159 



ican portion of the Yukon district, and the exposures of these 

 rocks as shown on the maps of his report have been indicated 

 in colors on the accompanying map. Data gathered by earlier 

 geologists, notably those of the Canadian Survey and of C. W. 

 Hayes and I. C. Russell, of the United States Geological Survey, 

 have provided suggestions as to the extent of these rocks in out- 

 side areas, but the reader need only bear in mind the enormous 

 area, the difficulties of exploration, and the want of accurate 

 maps of the region, to realize that generalization must as yet be 

 very tentative and liable to future change. 



As shown by the map, the belt in which these rocks have 

 been found extends about 500 miles in a general northwest- 

 southeast direction, but there are indications that the actual 

 extent of these exposures may be twice as great. 



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

 eastern flanks of abroad belt of fundamental granites and crys- 

 talline schists, which apparently form the central nucleus or 

 backbone upon which they rest. This belt is known in a gen- 

 eral way to extend up the Tanana river from near its mouth 

 southeastward across the White river below the Donjek. In the 

 latter region C. W. Hayes reports quartzites and limestones re- 

 sembling 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 im- 

 probably extend into the high range south of Tanana, of which 

 Mount McKinley is the culminating point and in which the 

 Kuskokwim and Sushitna 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 continuous mass of granite on the surface, but contains 

 smaller areas of the later rocks folded in with it. East of the 

 international boundary the area in which the granite occurs ap- 

 parently widens, but its exposures are less continuous, the over- 

 lying 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 



