56 



ever, the records are very indistinct from the period at 

 which the older schistose rocks were formed until late 

 Silurian or Devonian times when a great portion of the 

 district was involved in a widespread dynamic revolution, 

 which caused extensive deformation and metamorphism 

 and was accompanied by considerable volcanic activity. 

 At the close of this disturbance a considerable area was 

 above the sea and a long erosion interval ensued. Some 

 time before the middle Devonian, however, a great part 

 of Yukon sank beneath the sea, and at about that time 

 vulcanism became active at a number of points. The 

 older pyroxenites and andesitic members of the Perkins 

 group are thought to have been intruded at that time. 



This sea invasion prevailed at least well into the Car- 

 boniferous, and several thousand feet of calcareous, 

 siliceous, and argillaceous sediments, now represented by 

 quartzites, cherts, slates, and limestones, were deposited. 

 The limestone hills, ridges and ranges, that are now 

 so prominent along Tagish lake, Lewes river, Lake Laberge, 

 and elsewhere, are the result of this period of deposition. 

 Sedimentation was brought to a close by a widespread 

 deformation, and at about this time vulcanism became 

 active, and andesitic rocks invaded the district and buried 

 extensive areas under flows and tufaceous accumulations. 



In Jurassic — apparently early Jurassic — time an exten- 

 sive crustal movement occurred which was accompanied 

 by the injection of vast amounts of igneous materials, 

 including the earlier of the great batholiths of the Coast 

 range. These batholiths constitute probably the largest 

 exposed post- Paleozoic intrusive masses in the world, and 

 afford unexcelled opportunities for the study of batholithic 

 intrusions on a tremendous scale. 



A considerable area was above the sea at the close of this 

 disturbance, and what was probably a short period of 

 erosion ensued. This was followed by a gradual sinking 

 of the land in Jura-Cretaceous time, which continued until 

 an extensive land mass was submerged. 



The materials accumulated in this Jura-Cretaceous sea 

 were chiefly such as have produced upon consolidation, 

 arkoses, conglomerates, sandstones, shales, and coal seams. 

 These rocks have an aggregate thickness in places of over 

 6,000 feet (1,800 m.) and nowhere has the original top of 

 the series been discovered, the uppermost beds having now 

 been removed by erosion. All the bituminous and anthra- 

 citic coals of Yukon were deposited during this period. 



