BERGMAN AND NANUSHUK SERIES 247 



extent of about 60 or 70 miles. On the north it rests unconformably on 

 tlie schists of the Totsen series at the base of the mountains, while on 

 the south it is apparently infolded with the Koyukuk series, which it 

 is supposed to closely succeed in geologic age. The series consists essen- 

 tially of thin-bedded or mediura-bedded impure graj' or brownish sand- 

 stones and dark slates, with some dark shale and occasional conglom- 

 erates ; but along the north it is bordered by a belt of conglomerate from 

 several to 10 miles in width, which apparently represents the basal 

 member of the series. The series is undoubtedly of sedimentary origin, 

 but the sediments have been largely derived from igneous rocks, as 

 shown by the generally feldspathic constituents of the sandstone and 

 by the presence of basaltic or diabasic and granitic pebbles in the con- 

 glomerate on the Alatna river and at Lookout mountain. The sup- 

 posed basal conglomerate on the north is, however, cornposed essentially 

 of limestone and mica--schist materials de.rived from the Skajit formation 

 and the Totsen series. An accurate estimate of the thickness of the 

 series cannot be given. From a general impression, however, it seems 

 safe to indicate that it will probably amount to 2,000 feet. 



Structure. — The series has been considerably folded and somewhat 

 faulted, but to a much less degree than the Koyukuk series. A pro- 

 nounced jointing trends northwest and southeast and dips 80 degrees 

 northeast. A minor jointing trends north and south and dips east at 

 an angle of 80 degrees. On the north, where the series apparently rests 

 against the Totsen series, the dip is about 45 degrees south. 



Age. — No fossils beyond undeterminable lignitic plant remains have 

 thus far been found in the Bergman series. From its apparent close 

 relations, however, to the Koyukuk series it seems that the Bergman 

 series is probably Cretaceous. Lithologically it bears a strong resem- 

 blance to the Anaktoovuk series to the north of the range. 



NANVSHUK SERIES (UPPER CRETACEOUS) 



Character and occurrence. — On the north, the Nanushuk series succeeds 

 and seems to unconformably overlie the Anaktoovuk series, while north- 

 ward it disappears beneath the Tertiary rocks of the coastal plain, with 

 which its relations are also apparently unconformable. Its width in a 

 north and south direction is about 30 miles, while its east and west dis- 

 tribution is probably somewhat similar to that of the Anaktoovuk series. 

 The rocks are mainly thin bedded gray and brown sandstone, generally 

 fine grained and sometimes friable, slate-colored arenaceous and impure 

 fossiliferous limestone, dark shale or mud rock, soft uncleaved slate, 

 fine grained gray quartzite, drab-colored chert, and bituminous coal. 



