DESCRIPTION OF THE ROCKS 697 



A belt of Triassic rocks extends through the foothills south of the' 

 Alaska Eange from the headwaters of Gulkana Eiver to a point west of 

 Susitna Eiver. This belt lies in a geographic and structural position 

 similar to that of the Triassic rocks south of the Nutzotin Mountains, 

 and should probably be regarded as their western discontinuous extension. 

 The Triassic rocks south of the Alaska Eange include 14 fossiliferous 

 Upper Triassic limestone and a group of slates, tuffs, arkose, calcareous 

 sandstones, and limestones that are closely associated with, and probably 

 overlie, the fossiliferous limestone. The latter is underlain by basic 

 lavas and tuffs that seem to rest on a Carboniferous ( ?) limestone, and 

 that consequently correspond in strati graphic position, as well as in 

 lithologic character, to the Nikolai greenstone of the Chitina Valley, and 

 should probably be assigned to the Permian or early Triassic. 



The limestone, which occurs in a series of small detached areas, whose 

 discontinuity is probably due to structural disturbances rather than to 

 original lenticularity, is apparently only a few hundred feet thick, and 

 contains Halobia cf. superba Mojsisovics, Tropites sp., Discotropites (?) 

 sp., and Arcestes sp., which suggest a fauna closely similar to that of the 

 Chitistone limestone. 



The slate, tuff, arkose, and calcareous beds which apparently overlie 

 the limestone just described have a thickness of many hundred, if not 

 several thousand, feet. These beds contain Pseudomonotis subcircularis 

 (Gabb), which, together with the position of the rocks relative to the 

 underlying limestone, indicates that they belong in the same general posi- 

 tion as the McCarthy formation. In lithologic character they correspond 

 more closely to the Triassic volcanic beds of Skolai Pass and to the Tri- 

 assic limestones and tuffs of Port Graham, both of which contain this 

 same fossil. 



Cook Inlet and southwestern Alaska. — The Triassic rocks of the west 

 coast of Cook Inlet 10 include two fossiliferous Upper Triassic forma- 

 tions — the Kamishak chert and an older unnamed limestone. The local 

 section also includes greenstones which are older than the fossiliferous 

 Triassic rocks and which possibly belong in the Triassic. Beneath the 

 greenstones are slates which the writer believes are probably Paleozoic, 

 and above the Triassic rocks arc porphyries and tuffs that are probably 

 Lower Jurassic. 



The Upper Triassic limestone of the west coast of Cook Inlet has been 

 observed only at places of complex structure and generally in close asso- 



14 F. H. Moffit : Headwater regions of Gnlkana and Susitna rivers, Alaska. U. S. Geol. 

 Survey Bull., No. 498, 1912, pp. 22, 29-33. 



15 G. C. Martin and F. J. Katz-: A geologic reconnaissance of the Iliamna region, 

 Alaska. U. S. Geol. Survey Bull., No. 485, 1912, pp. 41-50. 



