STRATIGRAPHY . 261 



consist of conglomerates, sandstones, and shales, usually slightly indu- 

 rated and only gently folded. They have been studied in some detail 

 by Dall * who determined them as belonging to the Kenai division of the 

 Oligocene. At Lituya bay the Kenai beds are overlain by Astoria beds 

 (Miocene). 



At Sitka some highly feldspathic sandstones were observed, whose 

 stratigraphic position was not determined. These were described by 

 Becker f as pyroclastic diorites. In thin-section they show many min- 

 erals derived from crystalline rocks. In the field they are sometimes 

 massive, but more often plainly bedded. Their stratigraphic position 

 has not been determined, but they may provisionally be assigned to the 

 Tertiary. 



In the northern part of the Alexander archipelago there are some 

 lavas, probably of Pleistocene age, but which may be in part Tertiary. 



Greenstones are the most widely distributed of the igneous rocks in 

 the province. This term is made to include various igneous rocks of a 

 rather basic character and of varied composition. The oldest intrusion 

 seems to have taken place in early Paleozoic times, and was of a diabasic 

 nature. These older greenstones are usually schistose. Diorites and 

 quartz-diorites occur both massive and schistose. Among the less com- 

 mon types are gabbros, pyroxenites, and amphibolites. The older green- 

 stones are usually much altered and made up chiefly of secondary min- 

 erals. In the Ketchikan district the diabases form the latest intrusives. 

 Syenites have been found at a number of localities, notably at the 

 Treadwell mine near Juneau. 



SUMMARY 



In the province under discussion Paleozoic terranes, ranging from 

 Silurian or older to the Carboniferous, have an extensive development. 

 Large masses of greenstones are intruded in the lowest members of the 

 Paleozoic succession. In part of the region, at least, a stratigraphic 

 break is known to occur somewhere in the Devonian. A series of 

 argillites occur which seem to belong to the Upper Paleozoic horizons 

 and Lower Mesozbic, but whose stratigraphic position was not deter- 

 mined. Mesozoic time is represented in one part of the region by sedi- 

 mentary strata, whose basal member is a conglomerate, overlying the 

 Paleozoic rocks unconformably, and in another part by large extrusions 

 of volcanic rocks. Very large injections of granite took place along the 

 Coast Range axis, probably during middle or latter Mesozoic times, and 

 in smaller masses elsewhere in the region. The Tertiary is represented 



* Coals and lignites of Alaska. 18th Ann. Rept. U. S. Geol. Survey. 



t George F. Becker : Gold fields of southern Alaska. 18th Ann. Rept., part iii, p. 43. 



XXXIX— Bull. Geol. Soc. Am., Vol. 13, laoi 



