78 RECONNAISSANCE IN NORTHERN ALASKA IN 1901. 



The Bergman series consists essentially of thin- or medium-bedded, impure, gray 

 or brownish sandstone and dark slate, with some dark shale and occasionally con- 

 glomerate, but along the north it is bordered by a belt of conglomerate about L0 

 miles wide which is apparently the basal member. The sediments of (he series have 

 been derived very largely from igneous rocks, as is shown by the generally feldspathic 

 constituents of the sandstones and the presence of basaltic, diabasie. and granitic 

 pebbles or finer detritus in the conglomerate on the lower part of Alatna River 

 and at Lookout Mountain. The supposed basal belt of conglomerate along the north- 

 ern border, however, so far as it was observed in the John River region, is com- 

 posed essentially of the debris of limestone and mica-schist derived from the Skajit 

 formation and the Totsen series, on which latter it unconformably lies. This con- 

 glomerate is normally coarse, and in some instances contains bowlders. 



Owing to the wide separation of the outcrops visited, data for forming an ade- 

 quate estimate of the thickness of the Bergman series have not been obtained. From 

 a general impression, however, it seems safe to suggest that it is probably at least 

 2,000 feet. 



Structure. — The Bergman series, like the Koyukuk series to the south, has been 

 considerably folded and somewhat faulted, but apparently to a less degree than the 

 latter. Faulting has been observed on Koyukuk River just below Bergman and 

 farther downstream, below Kanuti Rirer. Though local folding has been pronounced, 

 and the rocks are sometimes highly tilted and stand on edge, the prevailing dip of. 

 the series seems to be gently southward. On the whole the series does not seem to 

 have received from the geologically late crustal movement the widespread tilt exhib- 

 ited by both the Anaktuvuk series on the north of the range and the Koyukuk series 

 on the south. Lookout Mountain apparentlj r represents part of an anticlinal fold 

 occurring in the series. Here the beds strike a little north of west and dip south 

 at an angle of about 80°. A pronounced jointing trends northwest and dips S0° 

 NE. A minor jointing trends north and dips east at an angle of S0°. On the north, 

 where the beds apparently rest against the Totsen series, the dip is south at an angle 

 of about 40°. 



Age. — In 1S99, owing to the presence of coal, lignite, and fragmentary plant 

 remains in the beds, notably above Tramwa}' Bar, and the resemblance of the beds in 

 many instances to the Kenai formation occurring elsewhere in Alaska, it was con- 

 jectured that the rocks might be Tertiary and in part Kenai. The more extended 

 observations made during the last season, though a large area of the series was 

 traversed, have thrown no more definite light on the subject, for determining fossils 

 were not found. It has been shown, however, that between Bergmau and John 

 River the series bears a marked resemblance lithologically to the Anaktuvuk series, 

 or Lower Cretaceous, and that its rocks, on the whole, are more indurated than the 



