826 



SCIENCE. 



[N. S. Vol. XI. No. 282. 



Tanana and the Yukon. The next succeeding 

 formations which overlie the gneisses uncon- 

 formably are grouped together as the Older 

 Sedimentary Series, and include the gold bear- 

 ing horizons of the Fortymile region. It is 

 probably of Silurian and pre-Silurian age. A 

 second series of Paleozoic rocks are classed as 

 the Younger Sedimentary Series. These are 

 largely Devonian and Carboniferous as deter- 

 mined by fossil evidence. They include a broad 

 belt which was traced westward from Lynn 

 Canal to the Nabesna river. This younger series 

 is cut by large masses of intrusive rocks. The 

 largest belt of these intrusives is the Coast 

 Range granite which extends westward to the 

 White river. 



Along the northern base of the St. Elias 

 Range were found considerable areas of effusive 

 rocks which have often been tilted, forming 

 monoclinal uplifts dipping southward and 

 faulted on the north side. These eflfusives, to- 

 gether with those of the Mount Wrangell group, 

 are probably both Tertiary and recent. The 

 Pleistocene is represented by sands, silts and 

 gravels. The northern limit of glaciation is 

 traced westward as far as the Nabesna river. 

 During the maximum extension of the Cordil- 

 leran ice sheet the White River, Tanana, and 

 Nabesna vallej's were occupied by glaciers 

 which extended far north of the general limit 

 of glaciation. The white volcanic ash of the 

 upper White river was traced westward as far 

 as the Fortymile basin. It is plainly an feolian 

 deposit. 



The copper deposits are largelj"^ placer, and 

 are of native copper. Small veins of native 

 copper were found in dioritic rocks connecting 

 white Carboniferous limestone, and also in the 

 white limestone itself near the contact. The 

 gangue mineral is calcite. 



Beconnoissance along the Chandlar and Koyukuk 

 Rivers, Alaska : By Me. F. C. Scheadee. 



Geology of the Silver Peak District, Nevada : By 



Me. W. W. Tuenee. 



The Silver Peak District lies in western Ne- 

 vada near the California line. The scenery is 

 typical of the Great Basin ; isolated ranges 

 lying between broad valleys, most of which 

 are sinks. In the lowest part of most of the 



valleys are playas, while between the ridges 

 and the playas are detrital slopes of Pleistocene 

 age, often of vast extent. The configuration 

 of the country is in the main due to differential 

 uplift and subsidence, and the valleys are thus 

 chiefly of orographic origin. Such a series of 

 displacements must have been accompanied by 

 normal faulting, and scarps, originating in this 

 way, are to be seen in the region. In general 

 the main faults trend north and south and east 

 and west. 



Subsequent erosion has greatly modified the 

 shapes of the ridges, and partly filled the val- 

 leys with detritus. 



In Miocene time much of the Silver Peak 

 Range, which attains an elevation of 9500 feet, 

 did not exist. Over a portion of its present 

 site was a broad basin occupied by Lake Es- 

 meralda. The deposits of this lake underlie 

 the valleys and form foothill areas and arch 

 up over the central part of the Silver Peak 

 Range, showing that these mountains are in 

 part late Miocene or post-Miocene origin. 



With the exception of certain gneisses of 

 doubtful age the oldest rocks of this district are 

 Lower Cambrian, the Middle Cambrian and 

 Silurian being also represented. All of these 

 paleozoic rocks are rich in fossils. The late 

 Eocene or early Bliocene beds of Lake Esmer- 

 alda contain an abundance of fossil fish, dicoty- 

 ledonous and other leaves, silicified wood, and 

 fresh-water mollusks. According to Professor 

 Knowlton the dicotyledonous leaves are repe- 

 sented by holly, oak, sumach, and bayberry, 

 showing that the climate has undergone a great 

 change since Miocene time. From a well 

 watered region it has become an arid one in 

 which there are no running streams. 



Volcanic activity began in this region in 

 early Paleozoic time but after these first rhyo- 

 litic flows the volcanic forces appear to have 

 been inactive for a very long period. During 

 and subsequent to the deposition of the lake 

 beds rhyolitic and andesitic eruptions occurred 

 in great volume, followed near the beginning 

 of the Pleistocene by eruptions of pumice and 

 basalt, one crater being clearly of Pleistocene 

 age. F. L. Ransome, 



David White, 



Secretaries. 



