DESCRIPTION OF THE ROCKS 693 



Hinnites ? Pseudomelania t 



Hinnites ? cf. Halobia occidentalis Natica ? 



Whiteaves Tropites ? 



Pleuromya Arcestes 



Turbo ? Arcestes ? 



The Nizina limestone, here described under that name for the first 

 time, includes the rocks described by Schrader and Spencer 7 as the un- 

 named lower member of the "Triassic series" or "Triassic shales and 

 limestones." These rocks were described by Moffit and Capps 8 as the 

 upper part of the Chitistone limestone, but all other descriptions of the 

 Chitistone limestone have distinctly excluded these beds, The name 

 "Nizina limestone" appears a few times 9 in Eohn's account of the geology 

 of the Chitina Valley, being applied to the same beds that Eohn formally 

 described as the Chitistone limestone. This use of "Mzina limestone" 

 appears only incidentally in the descriptions of other formations and of 

 the structure, and obviously was unintentional. The limestone to which 

 it was applied has otherwise, without exception, been called the Chitistone 

 limestone. For these reasons it is believed that this earlier use of the 

 name can be ignored, and should not be regarded as invalidating its appli- 

 cation to the formation here described. 



The jSTizina limestone consists of a succession of thin-bedded limestones 

 with a minor amount of interstratified shale. Massive limestone beds 

 occur rarely throughout the formation, and a few thick beds of shale may 

 be seen. There is a more or less gradual progressive change in the char- 

 acter of the formation from the base toward the top, the number and 

 thickness of the limestone beds decreasing upward. The lower part of 

 the formation consists, at many places, of practically all limestone, broken 

 only by very thin shaly partings. The greater part of the formation con- 

 sists of a fairly regular alternation of limestone and shale beds in about 

 the proportion of 5 or 10 parts of limestone to one of shale, the limestone 

 beds being from 4 to 18 inches thick and the shale beds from 1 to 3 inches 

 thick. Toward the top of the formation the proportion of shale increases 

 still more, there being a more or less gradual transition into the shales 

 and thin limestones of the overlying McCarthy formation. The thick- 

 ness of the beds here described as the Nizina limestone was estimated by 

 Schrader and Spencer as approximately 1,000 feet, and by Moffit and 



7 F. C. Schrader and A. C. Spencer : The geology and mineral resources of a portion 

 of the Copper River district, Alaska. U. S. Geol. Survey, special publication, 1901, pp. 

 32, 33, 46-47. 



8 F. H. Moffit and S. R. Capps : Geology and mineral resources of the Nizina district, 

 Alaska. U. S. Geol. Survey Bull., No. 448, 1911, pp. 21-27. 



6 Oscar Rohn : A reconnaissance of the Chitina River and the Skolai Mountains, 

 Alaska. U. S. Geol. Survey, 20th Ann. Rept, pt. 2, 1900, pp. 427, 429, 431, 435. 



L— Bull. Geol. Soc. Am., Vol. 27, 1915 



