MIDDLE ,7 (TRAj^isic 329 



amounting to nearly 90 degrees, is strong evidence of tliis nnconforniity 

 in portions of the field where direct contacts are not observable. 



The overlying younger series forms a continuous stratigraphic succes- 

 sion of younger Middle Jurassic and of Upper -Jurassic rocks. They con- 

 sist largely of soft blue shales locally carrying fossiliferous limestone 

 nodules. With the shales are associated varioiTs sandstones, often of loose 

 coherent texture and containing great multitudes of Aucella. Inter- 

 stratified beds of andesitic tuff, containing Belemnite remains, and 

 hornblende-biotite arkose are also found. In the eastern part of the field 

 stream bluffs reveal a curious interdigitation of lenses of cross-bedded 

 sandstone carr\dng Aucella, of shale beds, and of coarse conglomerate 

 lenses. Solitary water-worn boulders are found embedded in the shale 

 lenses. These various characteristics are held to belong to an ancient 

 delta deposit. In harmony with this interpretation is the fact that the 

 Aucella characteristic of these rocks is often found in soft sandstones 

 interstratified with sandstones inclosing abundant carbonaceous frag- 

 ments. 



At the head of Matanuska river the upper horizons of the shale become 

 strongly calcareous and there show pronounced ripple-marking. 



The fauna of the above succession of rocks is as follows : 



Cadoceras sp. Aucella cf. broiini Rouiller. 



Pleuromya. 



The fossils indicate faunas of both the upper part of the Middle Juras- 

 sic and of the Upper Jurassic. The former is the Cadoceras zone of the 

 west shore of Cook inlet, while the latter is the Aucella bronni zone of the 

 same region. 



It is interesting to note that the Upper Jurassic rocks of the Mata- 

 nuska region, which are faunally allied to those of the Nalmek formation 

 on the west coast of Cook inlet, show a certain lithologic similarity in the 

 presence of interstratified tuffs and arkose.* 



On the headwaters of Sushitna river is a notable accumulation of con- 

 glomerate. Perpendicular cirque walls cut in it reveal a thiclmess of not 

 less than 1,100 feet. Sandstone layers show it to be lying in horizontal 

 attitude. The conglomerate, especially in its upper horizons, consists ex- 

 clusively of large, well-rounded boulders of augite-andesite and very fresh 

 hornblende-biotite-quartz monzonite imbedded in a tuffaceous matrix. 

 Sheets of lava are intercalated in the conglomerate. The stratigraphic 

 relations of this deposit were not seen, but on lithologic reasons it is corre- 



* T. W. Stanton and G. C. Martin : Mesozoic section on Cook inlet and iilaska penin- 

 sula. Bull. Geol. Soc. Am., vol. 16, pp. 391-410. 



