58 



for 70 miles (no km.) below Dawson, as well as all the other 

 Tertiary areas of lignite-bearing beds belong to this period 

 of deposition. Developments of similar sediments associ- 

 ated with diabases and andesitic and rhyolitic volcanics 

 occur in the vicinity of Indian and Fortymile rivers, within 

 a few miles of Dawson. These are the most southerly of 

 these Tertiary sediments developed in Yukon or in the 

 district between Dawson and Skagway. 



In Eocene or Miocene time, a gradual uplift occurred 

 which, though of an orographic character, was accompanied 

 by volcanic activity and by a considerable local disturbance 

 of Eocene beds. The exact date of this orogenic movement 

 is somewhat in doubt. Dawson [30, p. 79] refers the 

 uplift to the Eocene, but Brooks [6, pp. 292,293] has pro- 

 duced considerable evidence to show that the dynamic 

 revolution occurred during late Eocene or early Miocene 

 time. A long period of crustal stability ensued, during 

 which what is now the Yukon plateau as well as, in the 

 opinion of some geologists, the Coast range and other 

 adjoining tracts [67, pp. 117, 132] were reduced to a nearly 

 featureless plain which was subsequently elevated. Daw- 

 son [29, pp. n-17] maintains that the planation was 

 accomplished during the Eocene epoch, and lhat the 

 Miocene was a period of vulcanism, deposition, and 

 accumulation, and agrees with Brooks [6, pp. 290, 292, 293] 

 in considering that the subsequent uplift occurred in 

 Pliocene or early Pleistocene time. Spurr, however, 

 shows that the erosion of the Yukon plateau was contem- 

 poraneous with the deposition of the Miocene strata in 

 the lower valley of Yukon river and, therefore, urges that 

 the Yukon plateau was planated in Miocene time and 

 subsequently uplifted in late Miocene or early Pliocene 

 time. [70, pp. 260, 262, 263]. From the information 

 available, however, it seems probable that the Jura- 

 Cretaceous sediments were largely deformed by the 

 Eocene or Miocene (post-Laramie) dynamic movements; 

 that the district was peneplanated during Eocene or pre- 

 Pliocene post-Eocene time; and that this planated tract 

 was uplifted to practically its present position during the 

 Pliocene epoch. 



During the long period of crustal stability previous 

 to this last important uplift the topography was reduced 

 to the form of a broad and gently undulating plain, and 

 only occasional unreduced hills and ridges remained 

 projecting above the general level. This lowland surface 



