MINERAL RESOURCES OF THE STATE OF NEW YORK 299 



form rather of a vein or a band than the usual lens characteristic 

 of the district. This would also be a favorable condition for mining. 

 In the shoot or lens type of deposit, the levels are short and there 

 is need for constant development to keep the ore supply ahead of 

 the progress of extraction. 



The Edwards district represents a type of ore occurrence that 

 is not at all common among the productive mining districts of the 

 country and for that reason the progress of operations has been 

 attended with more than usual interest. In its broader features 

 of geology it finds some analogy in the remarkable occurrence at 

 Franklin Furnace, N. J., inasmch as the country rocks in both 

 instances are Precambrian metamorphic limestones and the ores 

 themselves seem to have been accumulated in the earliest times. 

 The ores and their immediate mineral associates, however, differ 

 very markedly in the two occurrences and there are other points 

 of contrast that make the comparison of remote value. 



The district proper comprises a belt of crystalline limestone which 

 is partly included within the Gouverneur sheet recently published 

 by the United States Geological Survey, one of the several areas of 

 similar limestones that border the Adirondack highland on the 

 northwest side. The limestone is of early Precambrian age, belonging 

 near the base of this oldest of geological groups and has been thor- 

 oughly recrystallized as a result of regional and igneous metamorphism 

 within Precambrian time. The zinc ore is accompanied by many 

 silicate minerals like tremolite, diopside, talc and serpentine, which 

 are directly or indirectly the products of the recrystallization of the 

 calcareous beds and largely formed before the zinc blende itself was 

 introduced. A study of the mineral characters and of the phenomena 

 of the ore deposition has recently been completed by C. H. Smyth jr, 

 and published as part of the investigation of the district conducted 

 by the State Museum (see list of references appended to this article). 



It would appear that the explanation of the derivation of the 

 ores most in accord with the observed data involves replacement 

 rather than deposition in open spaces and that this replacement 

 was effected during a period when the limestones were deeply 

 covered, probably by several thousands of feet overlying rock. The 

 agency operative in the process was hot water which carried iron 

 and zinc in solution and as they attacked the limestone and the 

 silicate minerals deposited the metallic sulphides in their place. 

 An invading body of granite, the last of the deep seated magmas 

 that forced its way into the Grenville formations, doubtless had 

 an effective part in stimulating the ore-forming process if it did not 



