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 present. 



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 southeast. 



