306 BROOKS AND KINDLE — PALEOZOIC ROCKS OF UPPER YUKON 



Tertiary beds included within this area which were overlooked in the 

 hasty examination. 



The Lower Cretaceous of the upper Yukon comprises a series of closely 

 folded rocks characterized by a large amount of silica. They included 

 primarily siliceous slates, slaty sandstones, and quartzites, with which are 

 associated some argillites and pyroclastics. One heavy bed (50-70 feet 

 thick) of massive tufaceous conglomerate was observed within the Meso- 

 zoic area about 5 miles below Washington creek, but may be an infolded 

 older or younger terrain. The pebbles of this conglomerate, which are 

 chiefly limestone, are well rounded, and some are 2 feet in diameter. 

 The dominating rock tj^pe of the Lower Cretaceous is a siliceous slate or 

 quartzite, sometimes interbedded with a clay slate. These rocks are 

 usually pyritiferous and iron-stained when weathered. Three miles 

 below Washington creek there is a series of beautifully banded slates and 

 quartzites. Here the brittle quartzite is broken by a series of fractures 

 at right angles to bedding, while the same movement has in large measure 

 been taken up in the cleavage of the slate. A quartz filling, sometimes 

 carrying pyrite, is not uncommon along these fractures. In at least one 

 instance it appears to be established that quartz veins cutting these Cre- 

 taceous rocks are auriferous. This conclusion is not without importance 

 in its bearing on the age of the mineralization which produced the aurif- 

 erous deposits of the Yukon. 



These rocks in a general way strike easterly and northeasterly, but 

 there are many local variations. They are usually closely folded, and no 

 determination of thicloiess, which probably does not exceed a few thou- 

 sand feet, could be made. On Washington creek they appear to rest un- 

 conformably on the Devonian and in turn are unconformably overlaid by 

 the Tertiary beds. Near Coal creek the Aucella-bearing beds seem to 

 underlie the Upper Carboniferous limestone, which has apparently been 

 thrust over them. 



Fossils were collected from three localities in this formation, which 

 were reported upon by Dr T. W. Stanton as follows : 



Lot 18. — South bank of the Yukon river, 400 yards below Glen creek : 



This lot includes many fragmentary specimens and impressions of an Ino- 

 ceramus, with a few imperfect specimens of Pecten, Pinna, two small speci- 

 mens doubtfully referred to Aucella, and a few other small undetermined 

 bivalve shells, together with very imperfect fragments of an ammonite possi- 

 bly belonging to the genus PerispMnectes, or some other genus with a similar 

 sculpture. The horizon of this lot is evidently either Jurassic or Lower Cre- 

 taceous, but the nature of the material does not permit a discrimination be- 

 tween these two periods. I judge that these fossils came from the same series 

 as the Aucella mentioned below. 



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