264 BROOKS AND KINDLE PALEOZOIC ROCKS OF TIPPER YUKON 



Such a generalization of the stratigraphic sequence in a region as little 

 known as Alaska of necessit)^ implies correlations which are by no niea^is 

 definitelj' established. For example, all the elevated silt and gravel de- 

 posits are here placed in the Pleistocene, while it is by no means impossible 

 that some of them may not be older. Again, the lignite-bearing beds are 

 all included in the Eocene, yet some of these may be older. In considering 

 the pre-Carboniferous horizons, many of which are both unfossiliferous 

 and intricately folded, the problem becomes still more difficult; therefore 

 the lower part of the stratigraphic column, as here presented, must be 

 considered only as suggestive, except for specific sections where the age is 

 definitely established as shown in succeeding pages. 



Attention has already been directed to the general strike lines, which, 

 paralleling the dominant topographic features of Alaska, trend north- 

 westerly to about the 146th meridian, then bend southwesterly. While 

 locally there are many variations from these trends, nevertheless the 

 northwest-southeast and northeast-southwest structural lines dominate. 

 It will he shown that these two structures have dominated all the crustal 

 movements since pre-Ordovician times. 



The general sequence presented in the above table involves, as has been 

 stated, certain correlations based on very imperfect data. The evidence 

 relating to this matter will be presented in some detail, taking up each 

 period separately. The Mesozoic and later terrains will, however, only be 

 briefly considered in this paper. 



PRE-ORDOVICIAN 



The terrains here assigned to the pre-Ordovician embrace two very dif- 

 ferent groups of rocks, believed to be of synchronous age. In the one are 

 included the quartzites and associated sediments of the Porcupine basin, 

 and in the other the mica quartzites, mica schists, and crystalline lime- 

 stones, which are widely distributed in the Yukon basin. 



A series of thin-bedded quartzites with intercalated beds of shale and 

 some limestones, which outcrop on the Porcupine river near the Inter- 

 national boundary, are described in the accompanying paper by Mr Kin- 

 dle. These rocks are indurated, but not metamorphosed, and, so far as 

 known, are not cut by any igneous rocks, with the exception of a very 

 few small dikes. Mr Kindle assigns this series to the pre-Ordovician on 

 stratigraphic grounds. 



The metamorphic rocks, assigned to pre-Ordovician age, occur in two 

 areas within this province. The smaller (see map, figure 1) forms a 

 part of a belt stretching across northern Alaska and is divided by Schra- 



