MARYLAND GEOLOGICAL SURVEY 129 



These types do not admit of separate mapping. The pyroxenites 

 and peridotites are facies of the gabbro and the serpentine is a local 

 alteration product. 



The gabbro of this belt is both medium grained and coarse grained; 

 grey when fresh, green when uralitized, and a rusty brown on the 

 weather surface. A nodular weathering is also very marked in 

 the gabbro boulders strewing the meadows between Grays Hill and 

 Iron Hill station. The nodules are somewhat ellipsoidal in form, 

 3 x 3^-5 inches in diameter. The uralitization of the pyroxene on 

 the periphery of these nodules assists in their easy separation from the 

 rock. The nodules do not possess a concentric or radiate structure, 

 but are a product of spheroidal weathering on a small scale. 



The essential constituents are feldspar, pyroxene and olivine; the 

 accessories are magnetite and apatite; and the secondary constituents 

 are hornblende, tremolite, epidote and serpentine. The feldspathic 

 constituent is both a basic bytownite (Abi An 5 ) and an anorthite (Abi 

 An 12 ). 



The feldspar of the Iron Hill gabbro, a continuation into Delaware 

 of the Grays Hill areas, was isolated and analyzed for Professor 

 Chester 1 by Mr. Kiggs. The specific gravity ran as high as 2.749. 

 The anaysis is as follows: 



IRON HILL FELDSPAR. THEORETICAL COMP. 



of At^An^. 



Si0 2 44.09 44.87 



A1 2 3 35.41 35.66 



Fe 2 3 51 



(FeO not det.) 



MnO trace 



CaO 18-47 18.61 



MgO none 



Na 2 99 .86 



K 2 19 



Loss on ignition 35 



Total 100.01 100.00 



The feldspar, as a rule, is very fresh. Where alteration has taken 

 place the product is epidote. The proportion of feldspar varies from 

 nil to somewhat more than half the rock. 



i Bull. U. S. Geol. Survey, No. 59, Washington, 1890, p. 28. 

 9 



