DESCRIPTION OF THE ROCKS 695 



Tropites sp. Arcestes (?) sp. 



Juvavites (?) sp. Undetermined ammonites. 



Ceratites (?) sp. 



The uppermost division of the Upper Triassic rocks of the Chitina 

 Valley is the McCarthy formation, 11 which directly overlies the thin 

 bedded limestones described above. The McCarthy formation consists of 

 a conformable succession of shales, cherts, and limestones (see plate 28, 

 figure 2), having an aggregate thickness of at least 1,500 feet and possibly 

 of 2,500 feet or more. The section in the type district is as follows : 



Section of part of McCarthy Formation on Nikolai Greek 



Feet 

 Black shale, with some chert beds (Halobia sp. and Arniotites (?) sp. 



near the top) 500 



Thin-bedded black chert 800 



Shales and thin-bedded shaly limestone 200 



The uppermost beds of this section are exposed in a mountain top, but 

 the neighboring mountains probably contain higher beds of the same for- 

 mation. The top of the Upper Triassic sequence in this region has not 

 been recognized. 



There is apparently an unconformity at the base of the McCarthy for- 

 mation, in the western part of the Chitina Valley, but it was not detected 

 in the type section. The presence of an important unconformity in this 

 position is also suggested by evidence from other districts (see page 711). 



The chert beds in the lower part of the above section on Nikolai Creek 

 are probably represented by a thick development of massive cherts in 

 other parts of the Chitina Valley, notably on Fohlin and Fourth of July 

 creeks. It is somewhat doubtful, however, as to whether the cherts of 

 Fohlin and Fourth of July creeks should be regarded as a local facies of 

 the basal member of the McCarthy formation, as a distinct formation 

 occurring between the McCarthy formation and the underlying Nizina 

 limestone, or as the local alteration product of limestone beds which 

 possibly included both those of the McCarthy formation and of the thin- 

 bedded Nizina limestone. 



These cherts resemble, and apparently hold the approximate strati- 

 graphic position of, the Upper Triassic cherts of the west coast of Cook 

 Inlet and of the Kenai and Alaska, peninsulas. 



The fauna of the McCarthy formation consists chiefly of a single 

 species, Pseudomonotis subcircularis (Gabb), which is characteristic of 



11 F. H. Moffit and S. R. Capps : Geology and mineral resources of the N'izina district, 

 Alaska. U. S. Geol. Survey Bull., No. 44S, 1911, pp. 28-30. 



