DESCRIPTION OF THE ROCKS 689 



Permian ( f) strata, — The youngest rocks of known age which underlie 

 the Triassic beds of Alaska are early Permian (?) limestones. These 

 limestones carry a fauna closely related to that of the Artinskian of Eus- 

 sia, which is regarded by some as pre-Permian, but which is more gener- 

 ally considered as the lowest division of the Permian. These Artinskian 

 limestones are very widely distributed in Alaska, occurring in nearly all 

 the larger geographic regions, and show that toward the close of the 

 Paleozoic (at about the beginning of Permian time) limestone-forming 

 seas extended over the larger part, if not all, of the area that is now 

 Alaska. Marine Permian deposits younger than the Artinskian are not 

 known and probably are not present in Alaska. 



Pre-Triassic ( ?) slates of undetermined age. — Throughout most of the 

 mountains on the Pacific coast of Alaska are large areas of slaty rocks of 

 very uncertain age. The evidence on the age of these rocks is scanty and 

 conflicting, and all that can be safely said concerning it is that they may 

 be as old as early Paleozoic or as young as Upper Cretaceous. It is prac- 

 tically certain that these rocks are, at least in part, older than Upper Tri- 

 assic, and the writer believes that they are, at least for the most part, 

 Paleozoic. They clearly underlie the lavas that are beneath the Upper 

 Triassic limestones and tuffs of the Kenai Peninsula (see page 697). 



Early Triassic (?) volcanic rocks. — The Upper Triassic sedimentary 

 rocks described below are underlain in many places throughout the Pa- 

 cific Mountain belt by volcanic rocks that include both lavas and tuffs and 

 that have in some places been described as greenstones. These rocks in- 

 clude the Nikolai greenstone of the Chitina Valley (see plate 26), .-the 

 basic lavas and tuffs of the upper Susitna Valley, the ellipsoidal lavas of 

 Kenai Peninsula and Kodiak Island, some of the greenstones of the 

 Iliamna-Clark Lake district, and the ellipsoidal lavas of Hamilton Bay, 

 in southeastern Alaska. In all tbese districts they clearly underlie the 

 marine Upper Triassic strata without recognizable unconformity. It is 

 only in the Chitina Valley that the basal contact of these rocks with 

 underlying beds of known age has been observed. Here they rest on Car- 

 honiferous tuffs, cherts, and slates. In the Kenai Peninsula, on Kodiak 

 Island, and prohably on the west shore of Cook Inlet, they overlie the 

 slaty rocks of unknown age that are described above. On Hamilton Bay 

 they are probably underlain by lower Permian ? (Artinskian) limestones. 

 These volcanic rocks may include also the greenstone of the Orca group of 

 Prince William Sound, as well as part of the greenstones of southeastern 

 Alaska. They may possibly be correlated with the lavas and tuffs beneath 

 the Permian (?) limestone of White Kiver, but it is more probable that 

 they are either Permian or early Triassic. 



