SEDIMENTARY ROCK FORMATIONS. 55 



The later of these disturbances is that suggested by the vigorous cutting of the 

 left or west bank of Colville River into high bluffs (PL VIII), while the east bank 

 consists only of the edge of an expansive waste of low flats abandoned by the river in 

 its lateral migration during its down cutting. This shifting or lateral migration of 

 the river was evidently brought about by a tilt, which is apparently rnoi'e than a local 

 disturbance, for other examples indicate that there has been a genei'al tilt throughout 

 northern Alaska, which has caused and is still causing the larger rivers, notably the 

 Yukon, the Porcupine, the Koyukuk, the Kowak, the Colville, and the Anaktuvuk, 

 to cut their western banks." This differential upward movement of the region proba- 

 bly extends eastward to the international boundary or farther, and, judging from the 

 lateral migration of the Colville into the Pleistocene as well as the Tertiary and 

 Mesozoic terranes, it took place, in part at least, in the Pleistocene, and is probably 

 still in progress. 



During Lower Cretaceous time, deposition on the south of the range, in what is 

 now the Koyukuk Basin, was apparently accompanied by igneous extrusions and 

 followed by intrusions, which latter continued into post-Cretaceous time, 6 while the 

 range itself and the Arctic slope, so far as observed along the route of traverse, are 

 relatively free from igneous rocks. It was supposed by the writer that the northeast- 

 southwest trend of the Koyukuk Valley, from the Yukon to the sixt} r -eighth 

 parallel, probably represented in a general way a line of weakness in the earth's 

 crust, along which igneous phenomena are especially manifest. More recent work, 

 however, along Kanuti and Kowak c rivers and on Kotzebue Sound, shows that the 

 igneous rocks are distributed over a much wider area in this northern country than 

 in that occupied by the Koyukuk Valley. 



SEDIMENTARY ROCK FORMATIONS. 

 PALEOZOIC ROCKS. 



STKUCTUEE. 



Beginning with apparently the oldest, the several formations or rock series will 

 be briefly described. As the field is new and the investigations have not been detailed, 

 the formation names introduced are proposed provisionally. 



To afford a more comprehensive view of the relations of the several series and 

 to avoid repetition in referring to them individually, it will be well to note at the outset 

 some features of structure which are common to nearly all the Paleozoic rocks, and 

 which apply to the range as a whole, namely, that the series all strike approximately 



a The fact that the Yukon in the lower part of its course is cutting its right or western bank has been noted by Dall. 

 Russell, Spun-, and others. 



fcgehrader, F. C, Reconnaissance along the Chandlar and KojTikuk rivers, Alaska: Twenty-first Ann. Rept. U. S. Geol. 

 Survey, pt. 2, 1900, p. 481. 



Olendenhall, W. C, Reconnaissance from Fort Hamlin to Kotzebue Sound, Alaska: Prof. Paper, U. S. Geol. Survey, 

 No. 10, 1902, map, p. 30. 



