158 ALASKA AND ITS MINERAL RESOURCES 



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. In the beds of this series 

 have been found fossils of Carboniferous age and plants of De- 

 vonian aspect. 



Mission Creek series. — Later than the Tahkandit series, but. like 

 it, not very well defined, is the Mission creek series, consisting 

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

 cally there are thin beds of impure lignite and at the base a con- 

 glomerate (" cement rock " of the miners) containing pebbles not 

 completely rounded derived from older rocks in the neighbor- 

 hood, which sometimes carries gold. The beds of this series are 

 sometimes altered and sharply upturned and folded, but gener- 

 ally have a rather fresh appearance. In the neighborhood of 

 shear zones they are impregnated with pyrite and carry small 

 quartz veins. The limited exploration of these rocks has devel- 

 oped no important deposits of mineral. The age of the beds is 

 as yet uncertain, but they are in part as late as Cretaceous. 



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

 always readily distinguishable from them, is a great thickness 

 of rather loosel} 7 consolidated conglomerates, shales, and sand- 

 stones, 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 the Twelvemile and Porcupine beds, the two 

 last named being assumed to belong to the same series. 



The more recent formations, silts and gravels, will be consid- 

 ered under the heading " Detrital or placer deposits." 



DISTRIBUTION OF GOLD-BEARING ROCK FORMATIONS 



The most definite facts with regard to the occurrence of the 

 gold-bearing formations, the Birch creek, Fortymile, and Ram- 

 part series described above, were obtained by the reconnaissance 

 made by members of the United States Geological Survey in the 

 summer of 1896, under the charge of J. E. Spurr, in the Amer- 



