Bulletin - United States Geological Survey, Issue 455

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Page 75 - The principal points developed in this study may be summarized as follows : 1. The rocks of the area here described have been greatly altered through pressure and chemical metamorphism, as indicated in the prevailing secondary schistose structure and the abundant development of the secondary minerals — chlorite, epidote, and hornblende — and small amounts of others. The alteration has advanced sufficiently far in the schistose phases to destroy in most cases the original structure and minerals...
Page 75 - ... 2. From structural, petrographic, and chemical evidences the rocks are shown to have been derived from an original andesite, but in their present much altered state they are, according to present usage, more properly designated meta-andesites ; that these are intimately associated with the corresponding volcanic elastics. Furthermore, the popular name greenstone...
Page 71 - Rocks. The rocks vary in texture from dense aphanitic to medium fine-grained, with the porphyritic structure shown usually in the massive types. The original minerals are entirely altered to secondary minerals in many of the sections, but, with few exceptions, some trace of the original outline of the feldspar constituent is shown and more or less of the original rock texture preserved. While this is true for the feldspar constituent, the original bisilicate constituent is completely altered in every...
Page 140 - Weed, WH, Types of copper deposits In the southern United States : Trans.
Page 72 - Clements f has shown that in some of the basic volcanics of the Hemlock formation the great abundance of chlorite in some sections is more than could result from the alteration of that amount of the original bisilicate present, and points out that it is derived in part from the altered glassy base. This explanation is likely applicable to some of the sections of the Virgilina rocks, since the amount of chlorite is in excess of the original bisilicate, and is probably a derived product in part from...
Page 73 - ... altered specimens. To what extent the ore-bearing solutions have aided in the alteration it is not possible to say, but that the change has resulted in part from such action is doubtless shown in the metalliferous veins of the district. When I, II, and III, analyses of the least altered rock, are compared with analyses of recognized andesites occurring elsewhere, no marked differences in the essential constituents are shown. Higher SiO, and...
Page 93 - A curious feature in the composition of some of these rocks is the presence of the green carbonate and the sulphuret of copper. This fact, which appears to have been observed at a very early period after the settlement of the country, has unhappily kept alive the hope of discovering in these shales veins of a workable copper ore, and has led to repeated enterprises in mining, from which no really profitable result appears ever to have been derived. Most of the openings, either ancient or modern,...
Page 70 - World mine, 6^ miles south of Virgilina, in the Carolina portion of the belt, a partially loose-textured, fine-grained, purple rock is mixed with the surrounding green schists. The material bears every resemblance to a tuff/ and is streaked in places by the characteristic actinolite shade of green due to alteration, and contains inclosures of a darkcolored massive material, usually of small but varying dimensions and partially rounded in outline. The fragmental or clastic nature of the mass is plainly...
Page 75 - Virgilina district. 3. The rocks are pre-Cambrian in age and represent an area of ancient volcanics similar to others described as occurring along the Atlantic Coast region from eastern Canada to Georgia and Alabama and in the Lake Superior region.
Page 71 - ... for the lath-shaped feldspars, and the twinning is not at all or only slightly Indicated. Sometimes the feldspar grouping is suggestive of the sheaflike arrangement described by Clements" in similar volcanic rocks of the Hemlock, formation of Lake Superior. Poikilitic texture is well developed in many of the larger feldspar laths. Feldspar is the only porphyritically developed mineral, and it consists of fairly large stout laths of broadly striated plagioclase. with maximum extinction angles...

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