ZINC ORES OF THE EDWARDS DISTRICT 9 



quartzite, are classed as of Grenville age) there is an extensive area 

 of granite which is younger than, and intrusive in, the sedimentary 

 rocks. 



All these rocks are of Precambrian age and, while the sediments 

 were, of course, deposited at the surface, they were subsequently 

 buried to great depth, folded and metamorphosed from ordinary 

 limestones, shales and sandstones into crystalline limestones, or 

 marbles, schists, gneisses and quartzites. This metamorphism in 

 part preceded, to a large degree was contemporaneous with and, 

 to a relatively slight degree was subsequent to, the intrusion of the 

 great mass of granite and the injection of granite and pegmatite 

 into the sediments. 



Later erosion exposed, at the surface, the whole series of rocks 

 whose mineral composition and physical character had been 

 determined by agents and conditions prevailing at considerable 

 depth, doubtless some thousands of feet. Any attempt to interpret 

 the history of the rocks must necessarily be based upon this deep- 

 seated origin of their dominant features. 



As the denuding agents which exposed these rocks at the 

 surface act with extreme slowness, it is obvious that the period of 

 erosion must have been an exceedingly long one. It was finally 

 terminated by the submergence of the denuded area beneath the 

 sea of early Paleozoic time ; and the surface of the Precambrian 

 rocks, rugged, but with low relief, was buried beneath Paleozoic 

 sediments, beginning, in this vicinity, with the Potsdam sandstone. 



Later the region was again raised above the sea and erosion wore 

 away the veneer of Paleozoic rocks, excepting scattered remnants, 

 and once more exposed the rugged surface of the Precambrian. 



Apparently there has been no subsequent marine submergence of 

 the region, the conditions of denudation persisting to the present 

 time. 



Thus, in the vicinity of the ore deposits, the rocks show a clear 

 record : First, marine conditions, with the deposition of thick masses 

 of sediment; second, a deep-seated folding, with accompanying 

 intrusion and injection of granite and intense metamorphism; 

 third, long-continued erosion, exposing at the surface the meta- 

 morphosed sediments and igneous rocks of deep-seated origin ; 

 fourth, the marine submergence of the region and consequent 

 burial of the Precambrian rocks to a moderate depth beneath 

 Paleozoic sediments, but with neither metamorphism nor igneous 

 intrusion of any moment; fifth, the reelevation of the region above 

 sea level and the erosion of the Paleozoic sediments. 



