304 BROOKS AND KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



ceeding strata on the Yukon are made up of littoral and in part at 

 least of fresh-water deposits, enil:)racing some very coarse material and 

 aggregating nearly 3,000 feet in thickness. This same epoch of deposi- 

 tion is probabl)' represented in the White-Copper Elver region and in the 

 Alaskan range, where, however, it appears to form the base of the Car- 

 boniferous, indicating that, if the older limestone had ever been present 

 in this area, it was removed by erosion before the coarse sediments were 

 laid down. The same horizon appears to be represented in southeastern 

 Alaska by argillites. The third epoch of deposition is one of calcareous 

 material, and here again there was an abrupt change, this time to deep- 

 sea conditions and marked by the appearance of a new fauna. This 

 Upper Carboniferous sea was probably widespread. Its thickest recog- 

 nized deposits are found in southeastern Alaska, but it seems quite prob- 

 able that the upper limestone member of the Cache Creek series was 

 deposited in it. Sedimentation may have continued unbroken into Tri- 

 assic times in the Yukon district, but in southeastern Alaska a period of 

 erosion intervened between the highest Paleozoic and the lowest Mesozoic. 



MESOZOIG 



On the evidence of the small collection of fossils alluded to above, 

 Mesozoic deposition in this area is believed to have begim with a Triassic 

 terrain. The fauna on which the provisional determination of the Trias- 

 sic was made by Dr T. W. Stanton was found at but a single locality — 

 between the mouth of Nation river and the extensive exposures of Upper 

 Carboniferous limestone above the latter on the north bank of the Yukon. 

 The fossils include a species of Halohia closety allied to if not identical 

 with a characteristic Triassic form. The other species of the fauna are 

 either new or without stratigraphic significance. This fauna occurs in a 

 limestone exposing about 15 feet at the outcrop. The limestone appar- 

 ently belongs in a shale and sandstone series immediately following the 

 Upper Carboniferous limestone, which is but slightly exposed where the 

 collection was made. Our very limited knowledge of this terrain makes 

 it imjjossible to offer any estimate of the thickness of the beds which 

 should be assigned to it. 



The first discovery of Triassic in the Yukon basin was made by Doctor 

 Hayes,^^^ who found Triassic fossils in some slates at the head of the 

 White river during his exploration from the Yukon to the Copper river. 



I 



^^ C. W. Hayes : An expedition through the Yukon district. The National Geological 

 Magazine, vol. iv, 1892, p. 140. 



