Geology and. Natural History. 291 



computation was 507, in tlie second it was 765, including those 

 of tlie first. Tlie final values of the equatorial radius of the 

 earth compared to previous determinations are as follows : 



Clark, 1866 6,378-206 meters 



C. and G. Survey, 1906 6,378-283 ±34 " 



C, and G. Survey, 1909 6,378-389 ±18 " 



The values for the depth at which isostatic compensation becomes 

 complete are eight per cent greater than in the determinations 

 from the first paper. Some discussion and several charts are also 

 given of areas of excessive and deficient density in the United 

 Slates. The geologic importance of the geodetic study of these 

 local departures from isostatic equilibrium is obvious, but the 

 work has as yet hardly advanced to sufficient completeness to 

 enable comprehensive conclusions on this subject to be drawn. 



J. B, 



4. The Gold Hill Mining District of JSForth Carolina; by 

 F. B. Lanby. Bull. No. 2J, North Carolina Geol. andEcon. Surv. 

 Pp. 119, 23 plates, 5 figs.— The Gold Hill Mining District lies in 

 the south-central portion of the state in Rowan, Cabarrus and 

 Stanley counties. The principal surface features are a long, low- 

 lying, flat-topped ridge, the Gold Hill Ridge and a series of hills, 

 the Beaver Hills. The rocks of the district are about equally 

 divided between the sedimentary and igneous groups. The sedi- 

 mentary series, consisting of slates, tuffs and breccias, interca- 

 lated with which are minor flows of rhyolite and andesite, makes 

 up its southeastern portion. The igneous rocks, which lie in the 

 northwestern half, consist chiefly ot a much metamorphosed basic 

 effusive rock, called a greenstone, into which diurite and granite 

 have been intruded. These two rock series are separated from 

 each other by a great fault, the Gold Hill fault, which extends 

 across the district in a northeast direction. This fault is marked 

 by a zone of highlj'^ crushed and schistose rocks, much minor and 

 local faulting and numerous closely spaced joints. There is much 

 folding of the sedimentary rocks, particularly near their contact 

 with the igneous area. 



The ore-bearing veins are developed in the zone of minor fault- 

 ing to the east of the Gold Hill fault. They appear for the most 

 part to follow the trend of the fault, but are often found follow- 

 ing the strike of the schistosity. They are of two types, one of 

 which is marked by an extensive silicification of the wall rock and 

 is largely gold-bearing with only small amounts of copper. The 

 other presents a minimum of silicification and consists of a zone of 

 narrow rifts in the slate parallel with the schistosity. This series 

 carries chiefly copper with only small amounts of gold. The ores 

 are chiefly auriferous pyrite and chalcopyrite. In general the 

 pyrite is the older mineral, and after deposition was more or less 

 shattered by further movements. This shattering was followed 

 on the one hand by the deposition of chalcopyrite and more 

 pyrite, and on the other by the introduction of pyrite and gold, 



