1918] Davis: The Radiolarian Cherts of the Franciscan Group 315 



southern Alaska, there are numerous exposures of radiolarian chert. 

 These cherts are of varied colors, Mack, gray and green, being the most 

 common varieties. Brown and red varieties are also found. They are 

 thin and rather evenly bedded rocks, consisting of hard siliceous lay- 

 ers from half an inch to two inches, and rarely over three inches in 

 thickness. These are separated by thin partings of soft shaly ma- 

 terial. They are intensely crumpled and contorted and are often dis- 

 placed by small faults. Photographs of the cherts at Bear Bay and 

 Halibut Cove shown in the references enumerated are exactly like 

 photographs obtained from Franciscan radiolarian cherts. 



In thin section the cherts are seen to be very fine grained and show 

 no clastic material. Poorly preserved radiolaria are seen, in the thin 

 sections, where they appear as clearer spots in the stained and clouded 

 matrix of the rock. Occasionally, the remnants of broken spines or 

 residuals of the lattice structure may be seen. The age of the cherts 

 has been definitely determined at Kamishak Bay. 57 Here the cherts 

 occur in rather massive beds interstratified with thinner beds of lime- 

 stone, shale and sandstone. The shales and limestones, interbedded 

 with the chert, contain fossils — mainly Pseudomonitis subcircularis. 

 This species, which is very abundant in these rocks, is characteristic 

 of the Upper Triassic. Associated with the bedded cherts is a lime- 

 stone containing Haliobia superba, also a characteristic Upper Tri- 

 assic form. At Bear Bay and Cold Bay on Cook Inlet 58 Pseudo- 

 monitis subcircularis is again found in limestones interbedded with 

 cherts. In other places, where there are no fossils, the relations of 

 the cherts to other fossiliferous rocks are such as to indicate that 

 they are probably Upper Triassic. These relations and the peculiar 

 lithology of the formation have been the basis for the correlation of 

 various areas of cherts with the known Upper Triassic cherts of 

 Kamishak Bay and other points on Cook Inlet. Beside the lime- 

 stones, the cherts are associated with arkose sandstones and black 

 shales and with small amounts of conglomerates. They are also asso- 

 ciated with ellipsoidal basalts. These appear from the descriptions 

 to be often in the form of lava flows with tuffs, though much of the 

 basalt appears to be intrusive. The relation between the cherts and 

 the ellipsoidal basalts is very close ; in some places the areas of the 

 two rocks are so intermingled that they cannot be separated accurately 

 in the mapping. The basalts commonly show a very well developed 

 pillow structure. 



"U. S. Geo]. Surv. Bull. 485, p. 48, 1912. 

 ss Bull. Geol. Soc. Am., vol. 16, p. 394, 1905. 



