. Cecil county. Geology. MARYLAND GEOLOGICAL SURVEY 103 The pegmatites may be composed almost wholly of a white alkali feldspar, as is the case in the vein just east of Rock Spring, where the vein is enclosed in steatite-serpentine rock. The pegmatites, offering as they do in many cases so sharp a contrast in chemical and mineral constitution to the bounding rock, can hardly be produced by lateral secretion. Percolating waters descending through a weathered zone of mater- ial, which has subsequently been removed (e. g. Triassic shales and sandstones), and ascending waters, which in their long
. Cecil county. Geology. MARYLAND GEOLOGICAL SURVEY 103 The pegmatites may be composed almost wholly of a white alkali feldspar, as is the case in the vein just east of Rock Spring, where the vein is enclosed in steatite-serpentine rock. The pegmatites, offering as they do in many cases so sharp a contrast in chemical and mineral constitution to the bounding rock, can hardly be produced by lateral secretion. Percolating waters descending through a weathered zone of mater- ial, which has subsequently been removed (e. g. Triassic shales and sandstones), and ascending waters, which in their long circuitous route have acquired the acid silicates, may produce the pegmatites by their deposits in these fissures. Such an origin seems highly probable for the most silicious or the most feldspathic of the peg- matite veins. It does not necessarily explain all the pegmatites. There are some of a more granitic character to which an aqueo- igneous origin may be ascribed. Structural Relations and Age of the Crystalline Formations. the mica-gneiss. The Susquehanna section of the mica-gneiss shows a formation which, while finely gneissic at Bald Friar, passes northward into a conglomeritic phase. This, in turn, becomes a fine-grained sericitic or sometimes chloritic quartz-schist with more or less feldspar Fig. 5.—Diagram showing type of unsymmetrical overturned folding. Somewhat less than three miles north of the Mason and Dixon Line, the quartzose beds give place to a dark blue-black slate, which forms a continuation of the well-known Peach Bottom slate belt on the west side of the Susquehanna. The strike varies from N 30° E to ~N 70° E. Where cleavage is developed it dips steeply southeast. The stratification varies from horizontality with an undulating sur- face to verticality. The average inclination is ± 35° and is uni- formly to the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readab
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