344 proceedings: geological society 



Slope. The former, composed chiefly of pre-Mesozoic rocks, extend 

 east and west across Arctic Alaska, and are regarded as a continuation 

 of the Rocky Mountain System of the United States and Canada. The 

 Arctic Slope is divided into the Anaktovuk Plateau and the Coastal 

 Plain. The former is a rolling upland forming the northern front of 

 the mountains and is composed chiefly of Mesozoic and Tertiary rocks. 

 The latter is a nearly featureless plain of Pleistocene rocks. The drain- 

 age is characterized by a series of closely spaced rivers flowing nearly 

 north from the mountains to the Arctic Ocean. 



Schists of probable pre-Cambrian age were deposited and metamor- 

 phosed early in the history of the region. The earliest sedimentation 

 of definite age took place in Carboniferous time, when the European 

 section was closely followed. Only 300 feet intervene between the 

 Geschellian and upper Triassic. A series of marine post-Triassic rocks, 

 whose faunas have not yet been studied, probably represent the Jurassic 

 and perhaps extend into the Cretaceous. Evidence from neighboring 

 areas indicates that the Arctic Mountains were deformed in early Ter- 

 tiary times. Their general accord of summits may indicate base-level- 

 ing, but the lack of definite evidence leaves it an open question. The 

 tilted and leveled Pliocene rocks in the Anaktovuk Plateau indicate 

 that this area was uplifted and base-leveled in late Pliocene or early 

 Pleistocene times. The Coastal Plain was uplifted in the Pleistocene, 

 probably before the Wisconsin stage of glaciation. Since then there 

 have been minor oscillations probably less than 25 feet in amount. 



Ores of the Mount Lyell copper district of Tasmania (illustrated) : 

 C. C. Gilbert and J. E. Pogue. Two types of deposits are repre- 

 sented, typified in the two largest mines of the district. The Mount 

 Lyell mine opens up a pyritic mass of immense size, low in copper con- 

 tent but with values in gold and silver. The ore is densely granular 

 pyrite, bearing thruout chalcopyrite in minute stringers and filaments 

 and including localized areas of tetrahedrite and of enargite. Bornite, 

 chalcopyrite, sphalerite, and galena are also present but in subordinate 

 amounts. At the North Mount Lyell mine the ore minerals, in order 

 of importance, are bornite, chalcopyrite, chalcocite, tetrahedrite, and 

 pyrite; these form mineralized zones in the schists and not a great 

 pyrite body as at Mount Lyell. Metallographic study of polished 

 sections indicates that both types of deposits were dominantly formed 

 thru replacement of the minerals of the schists, and that the deposition 

 took place during a distinct mineralizing epoch marked by solutions 

 progressively changing in composition and depositing a series of sulfide 

 minerals in sequential and transitional stages. The order of deposition 

 among the principal minerals is pyrite, chalcopyrite, bornite, chalco- 

 cite, corresponding to a decreasing iron and increasing copper content. 

 Enargite and tetrahedrite probably mark a late stage in the mineralizing 

 epoch; with these chalcopyrite of a second generation is associated. 

 The sections give no conclusive evidence of secondary sulfide enrich- 

 ment. The ores studied are the property of the U. S. National Museum. 



Ralph W. Richards, Secretary. 



