Complex geologic Structure. 139 



at the contacts. The basic eruptives are confined to a narrow 

 strip less than a quarter of the width of tlie plateau belt and 

 lying along its western side. The sequence of these rocks, as 

 well as their relation to the Coast Range granite, is extremely 

 involved, and much further study will be required in order fully 

 to determine these relations. Their age is probably upper 

 Paleozoic and Mesozoic, though very few fossils were found and 

 none except in the less altered western portion of the belt. 



East of Ahklen valley there is another belt of granite, quite 

 distinct in character from that of the Coast range. It is free 

 from hornblende and contains a large amount of pink feldspar, ' 

 giving a decided red color to the rock in mass. The granite has 

 in some places a well developed gneissoid structure, the cleavage 

 being approximately parallel with the direction of the lake. 

 Teslin river flows in a valley deeply filled with silt and gravel, 

 so that not more than two or three rock exposures occur through- 

 out its whole length ; but so far as could be determined at a dis- 

 tance the escarpments on both sides of the valley are composed 

 of rocks similar to those forming the plateau west of the lake. 

 About thirty miles above the mouth of the river the hills toward 

 the northeast are com230sed of bright red sandstones with yellow 

 and gray shales, probably less altered and perhaps younger than 

 any of the sandstones above described. 



The extensive plateau region between the Yukon river and the 

 northern base of the St Elias mountains is composed of various 

 kinds of crystalline rocks with small areas of highly altered sedi- 

 ments. Gray hornblende granite similar to that forming the 

 Coast range of southern Alaska occurs in a somewhat narrow 

 belt just north of the St Elias mountains. The prevailing rock 

 of the greater part of the region north of this belt is a reddish 

 granite quite free from hornblende and frequently containing 

 large porphyritic crystals of feldspar. Both kinds of granite are 

 jcut by numerous dikes or covered by sheets of eruptive rocks, 

 from the most recent vesicular basaltic lavas to highly altered 

 diabase. The red granites, at least, appear to be Archean, de- 

 posited upon which are small areas of sedimentary rocks that 

 have been infolded with the granite and penetrated by the basic 

 dikes and thus so completely changed from their original con- 

 dition that no clue is afforded as to their age. They consist of 

 arkose-conglomerates, slates and marbles. North of the Kluantu 

 valle}^ the only clastic rocks seen were a few exposures of con- 



