578 E. T. Wherry — New Occurrence of Carnotite. 



frequently met with in the vicinity of the masses of black 

 shale, and in fact small nodules of it are occasionally found in 

 the shale itself. 



That it has been deposited in its present form by circulating 

 surface waters is clear. It is often locally concentrated along 

 the more porous beds, and streaks may be observed beneath 

 cracks in the rock from which water trickles out during wet 

 weather. But its source is apparently to be sought in these 

 lenses of black shale. 



These dark shales do not owe their color to carbon, as might 

 be supposed at first sight, but, as shown by microscopic exam- 

 ination, chiefly to the presence of hornblende, biotite, and other 

 silicates in a fine state of subdivision, yet, in part, in fairly 

 fresh condition. They can best be accounted for as a result of 

 the peculiar climatic conditions which, as pointed out by 

 Barrell,* prevailed here during late Mississippian times. 



Under ordinary conditions of weathering silicates are decom- 

 posed, and only silica, kaolin, and ferric oxide remain to enter 

 into the formation of sedimentary deposits. But in arid cli- 

 mates disintegration in general exceeds decomposition, so that 

 heaps of greatly broken up, but comparatively unaltered, min- 

 erals will accumulate, and, when a more or less sudden change 

 to conditions of heavier rainfall sets in, considerable quantities 

 of these undecomposed minerals may be carried down into 

 places of accumulation of sediments. These dark shales are 

 believed, then, to represent " black sand " streaks, which, by 

 reason of their distinctive specific gravity, have accumulated at 

 points where the flow of the streams was favorable. 



The materials of this portion of the Pottsville conglomerate 

 were evidently derived from the crystalline Highlands of 

 Pennsylvania, New Jersey, and New York ; for while white 

 quartz makes up the majority of the pebbles, occasional pieces 

 of banded gneiss, crystalline limestone, and serpentine, of types 

 familiar in that region can be seen in the carnotite-bearing 

 beds. It has been found by Hillebrand \ that vanadium is an 

 almost universal constituent of igneous rocks, including the 

 metamorphics of this Highland region, and is largely contained 

 in their dark silicates. Under ordinary conditions of weath- 

 ering much of this vanadium is carried away in solution, so 

 that the average sediments contain only one-fifth to one-tenth 

 as much as the rocks from which they were derived. In the 

 present instance, however, it would seem reasonable to expect 

 the concentration of practically all of the vanadium in the 

 black shales as a purely mechanical process. Analysis of sev- 

 eral samples of these rocks showed amounts varying from 0*08 



*Bull. Geol. Soc. Amer., vol. xviii, pp. 449-476, 1907. 

 f Bull. U. S. Geol. Survey No. 167, pp. 49-55, 1900. 



