308 Scientific Intelligence. 



which were probably produced by the rock movements that 

 crushed the veins of the older series, and assays of their ores 

 have as yet shown but insignificant amounts of gold and silver. 

 These veins, like those in the granite, are much less abundant than 

 those in the Birch Creek and Fortymile series ; hence it is 

 thought that the latter are probably the principal source of gold 

 in the placers. 



The younger rock series noted are, briefly, the following: 



Tahkandit series. — This is a temporary division made for pur- 

 poses of mapping and consists of limestones, sometimes white and 

 crystalline, generally -green or black, alternating with shales. In 

 certain localities, notably on the Tahkandit River, it has con- 

 glomerates carrying greenish pebbles supposed to be derived from 

 the rocks of the Rampart series; thus an unconformity or erosion 

 interval is suspected. In the beds of this series have been found 

 fossils of Carboniferous age and plants of Devonian aspect. 



Mission Greek series. — Later than the Tahkandit series, but, 

 like it, not very well defined, is the Mission Creek series, consist- 

 ing of shales and thin-bedded limestones with gray sandstones. 

 Locally there are thin beds of impure lignite and at the base a 

 conglomerate containing pebbles not completely rounded derived 

 from older rocks in the neighborhood, which sometimes carries 

 gold. The beds of this series are sometimes altered and sharply 

 upturned and folded, but generally have a rather fresh appear- 

 ance. In the neighborhood of shear zones they are impregnated 

 with pyrite and carry small quartz veins. The limited explora- 

 tion of these rocks has developed no important deposits of min- 

 eral. The age of the beds is as yet uncertain, but they are in 

 part as late as Cretaceous. They sometimes overtop the Tah- 

 kandit beds and rest directly on rocks of the Rampart series. 



Kenai series. — Next above the Mission Creek rocks, and not 

 always readily distinguishable from them, is a great thickness of 

 rather loosely consolidated conglomerates, shales, and sandstones, 

 generally greenish in color, which are the coal-bearing rocks of 

 the region ; they everywhere contain plant remains, and rest 

 unconformably upon the older rocks. They have, however, been 

 folded to a certain extent, and stand upturned at angles of 20° to 

 60°. They are supposed to be of Eocene Tertiary age. 



Later Tertiary beds. — Other and more recent Tertiary beds 

 have been observed, generally in the more open country of the 

 Lower Yukon, which have little economic importance, though 

 they sometimes contain thin lignitic seams. They are variously 

 known from the localities where they have been observed, as the 

 Nulato sandstones and Twelvemile and Porcupine beds, the two 

 last named being assumed to belong to the same series. . . . 



As regards the occurrence of the gold-bearing formations, the 

 Birch Creek, Fortymile, and Rampart series described above, 

 they constitute a belt which extends about 500 miles in a general 

 northwest-southeast direction, but there are indications that the 

 actual extent of these exposures may be twice as great. 



