STRUCTURE 31J 



near the month of the Tanana, the prevailing strikes are north GO to TO 

 degrees east. Passing eastward, the strnctures gradually swing, nntil at 

 the 146th meridian they are about east and west, and from here to the 

 International boundary they gradually swing to the southeast. Not only 

 do the prevailing strike lines of the bedding planes conform to this gen- 

 eral system, but the major joint planes also mark a crescent opening 

 toward the south. The dips are varied, but the majority are steep and 

 northerly. 



The structures above described are those of the Mesozoic and older 

 rocks. Folding since Eocene Tertiary times has been rather local and 

 not very intense, and the structures of terrains of that period of deposi- 

 tion do not always conform to the above system. There is, however, a 

 general parallelism between the belt of Tertiary which stretches westward 

 from the International boundary and the structures of the older rocks. 

 This may be explained by the fact that these rocks represent the deposits 

 of a Tertiary river whose drainage lines were determined by the bedrock 

 structi;res of the older terrains. 



The general distribution of the terrains (see map, page 265) indicates 

 that pre-Ordovician metamorphic sediments are exposed along a broad 

 truncated anticlinal uplift or anticlinorium. Such an interpretation was 

 first suggested by Spurr, on the basis of his studies made in 1896. "Within 

 this broad uplift there are an infinite number of smaller folds, for the 

 deformation has been exceedingly intense. In fact, so crumpled are the 

 old metamorphic schists that it appears hopeless to determine their de- 

 tailed structures. The Devonian and Carboniferous rocks are far less 

 disturbed than the older sediments. This may be in part because they 

 were not so near the axis of maximum movement, but is due largely to 

 the fact that they were deposited after the earlier crustal movements 

 which affected the Ordovician rocks. jSTortheast of the Yi:kon valley 

 two anticlinal uplifts cross the Porcupine river, exposing rocks older than 

 any of those traversed by the Yukon below the International boundary. 

 These rocks along the Porcupine are but little altered, as compared with 

 the older terrains to the southwest of the Yukon. 



OUTLINE OF GEOLOGIC BISTORT 



It is intended here to present in brief review the succession of geologic 

 events which have affected the province under discussion. It will be 

 evident from the foregoing that the fragmentary nature of the data does 

 not permit of a close definition or analysis of the various epochs of depo- 

 sition and earth movements which have produced the terrains here dis- 



XXX— Bull. Geol. Soc. Am., Vol. 19, 1907 



