December 31, 1909] 



SCIENCE - 



973 



net, chalcopyrite, pyrrhotite, sphalerite and pyrite 

 are common to both localities. Axinite, fluorite, 

 epidote and specularite occur in the contact zone 

 at Coppereid, but were not noted at Adelaide. 

 At the latter place the altered limestone contains 

 vesuvianite, diopside and orthoclase. 



The antimony and quicksilver deposits, with the 

 exception of some stibnite at Seven Trouglis, are 

 all, so far as is known, in Triassic or Jurassic 

 rocks, and are supposedly of the same age as the 

 antimonial silver-gold ores. Xo facts are known, 

 however, that rule out a Tertiary age for some of 

 these deposits. 



The nickel and cobalt deposits in Cottonwood 

 Canyon consist of sulpharsenites of nickel (gers- 

 dorffite in part), tetrahedrite and some compound 

 of cobalt with sulphur, arsenic or antimony, with 

 the various oxidation products of the.se minerals. 

 The ores fill small fissures in much altered an- 

 desite or andesite breccia cut by diorite, and may 

 be genetically related to the intrusion of the latter 

 rock. 



The southern portion of Humboldt County is 

 part of a metallogenetic province characterized 

 chiefly by the prevalence of antimonial ores of 

 silver with numerous and widely scattered de- 

 posits of stibnite and cinnabar. There are in addi- 

 tion some deposits of gold-silver, copper and 

 nickel-cobalt ores. Ore deposition probably began 

 immediately after the intrusion of the Triassic 

 and Jurassic sediments in late Mesozoic time by 

 a granodioritic magma, comparable with that 

 which invaded the rocks of the Sierra Jfevada at 

 the same period and continued into the Tertiary. 

 The known Tertiary deposits are essentially gold- 

 silver ores and copper ores, but it is possible that 

 some of the other types are also Tertiary. 

 Refractive Index of Canada Balsam: F. C. Cal- 

 kins. 



A very convenient and constantly utilized aid 

 to the determination of minerals in thin section 

 being a comparison of their refractive indices with 

 that of Canada balsam, it is obviously important 

 to know as definitely as possible how widely the 

 refringence of balsam in good slides is likely to 

 vary. The published statements regarding this 

 matter, however, are meager and contradictory, 

 and their experimental basis appears in no case to 

 have been recorded. The following experiments 

 were carried out for the purpose of determining 

 the approximate mean and extremes of the re- 

 fractive index of the balsam in the slides made 

 for the U. S. Geological Sur\-ey. 



First, the refractive index of balsam (ij) was 



compared with u of quartz (1.544) in 300 slides 

 from one to eight years old. It was found that 

 j; exceeded a in only one case out of one hundred, 

 except where the cover-glass was sprung away; 

 where v was greater than 1.544 the excess was 

 extremely small and the balsam was noticeably 

 yellow. 



The lowest value observed was between y and 

 j3 of nearly pure albite, about 1.535 ± .002. 



Mr. W. T. Sehaller supplemented these observa- 

 tions by measurements with an Abbe rcfractometer 

 on blank preparations representing the condition 

 of the balsam in normal, in undercooked and in 

 overcooked preparations. The extremes found by 

 Mr. Sehaller were 1.535 and 1.543, the mean of 

 eleven measurements 1.5393. The refractive index 

 of one sample of highly fluid uncooked balsam 

 was found to be 1.524. 



It therefore appears that the mean refractive 

 index of Canada balsam in good petrographic 

 slides is about 1.54, and that it rarely is less than 

 1.535 or more than 1.545. 



Paleozoic Erosion Channels: E. O. Ulrich. 



Fossil erosion channels and caverns afford a 

 valuable proof of the repeated emergence of the 

 sea bottom, they being mostly of sub-aerial origin. 



Channels and caverns of Pennsylvanian, Missis- 

 sippian and late Devonian age have been described 

 and figured, but earlier examples, though abun- 

 dant and of unmistakable origin, have remained 

 but imperfectly known. 



Erosion channels may be divided into three 

 classes: superficial, submarine and subterranean. 

 The first class embraces all sub-aerial channels 

 formed by running water, including tidal over- 

 flows. The second class embraces all channels 

 produced by currents scouring the sea bottom; 

 these are very rare as strong currents manifestly 

 seldom occur in the shallower epicontinental seas. 

 The third class includes solution cavities and 

 caverns formed in limestones and dolomites by 

 the action of acidulated surface waters. 



As illustrating superficial erosion, may be men- 

 tioned channels in the Trenton at Trenton Falls, 

 Xew York, which were probably in the nature of 

 ■' guts " on ancient tidal flats. Concomitant with 

 the formation of these channels gravitational 

 slumping occurred, resulting in their partial filling 

 with much distorted strata. A doubtful instance 

 of submarine erosion is found in the Fern Glen 

 (Kinderhook) in northern Arkansas, with an over- 

 lap of Boone chert. To this same class probably 

 belongs an intraformational erosion surface exhib- 

 ited by the Lowville near Watertown, N. Y. Most 



