57 



This Jura-Cretaceous period was also characterized by 

 intense volcanic activity, the evidence of which is recorded 

 in the great amount of ash and volcanic breccia intercalated 

 with the normal sediments and in places even exceeding 

 them in amount. In places, dykes are numerous and flows 

 are extensive, and everywhere the volcanics of this period 

 appear to be andesitic in character. Vulcanism persisted 

 until after sedimentation had ceased, and along Norden- 

 skiold river great masses of these andesites occur overlying 

 eroded surfaces and edges of the Jura-Cretaceous sediments. 



A widespread deformation terminated the Jura-Creta- 

 ceous period of sedimentation, at the close of which a 

 considerable land mass, comprising the greater part of 

 southern Yukon at least, was above the sea. Degrading 

 action followed, and from that time to the present there is 

 no evidence to show that any portion of the district 

 between Skagway and Dawson has been submerged 

 beneath the sea. 



Following this Jura-Cretaceous disturbance, and mainly, 

 it is thought, during Tertiary, but possibly extending into 

 Pleistocene time, the district was subjected to several 

 volcanic invasions. As a result of what appears to be the 

 oldest of these invasions, basalts pierced the older formations 

 and flowed over the land surface, and in places, hundreds 

 of feet of basalt-tuffs accumulated. The basalts exposed 

 in Miles canyon, along Lewes river below Tantalus, and 

 along the river above and below Selkirk, all belong to this 

 period. Along Nordenskiold river near Carmack, the 

 basalt-tuffs have their greatest known development. 

 About this time, dykes of granite-porphyry, syenite- 

 porphyry, and rhyolites invaded the older formations, and 

 rhyolites also flowed over the land surface, generally in 

 thin sheets, and in places were accompanied by great 

 amounts of related tuffs and breccias. To the north and 

 especially in the vicinity of Indian river, diabases and ande- 

 sitic rocks occur intimately associated with sediments 

 considered to be of Eocene age. 



In upper Cretaceous time a transgression of the sea took 

 place along the present Yukon basin and also probably 

 extended to other portions of Alaska and Northern Yukon. 

 Deposition continued well into the Eocene, although in the 

 upper Yukon basin the Eocene is represented only by fresh 

 water beds which seem to have been laid down in isolated 

 basins. The Kenai lignite-bearing beds of the Rock Creek 

 coal area, which extend along the east side of Yukon river 



