102 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



veins, which are concealed by soil, is attested by the innumerable frag- 

 ments of this material which are strewn throughout the plateau. The 

 much more ready disintegration of the bounding rock, compared with 

 that of the vein material, results in a soil quite free from boulders 

 of the underlying rock, but full of " flint stones." 



The quartz often contains tourmaline crystals and is coated with 

 a red or black stain. 



The quartz veins are plainly precipitations in cracks and fissures 

 from heated solutions and may be the work of descending, ascending 

 or laterally moving water. The nature of the veins and the sur- 

 rounding rock are such as to lead us to infer that descending silica- 

 charged water and lateral secretion will explain their formation. 

 The fissuring of the rock is due to diastrophic movements. 



The pegmatites bear evidence of a similar origin. They differ 

 from the quartz vein only in the addition of an alkali feldspar and 

 muscovite or biotite. This material may line both walls of a fis- 

 sure, while the central mass is pure quartz. Such a pegmatite 

 occurs half a mile above Conowingo Station. It has a width of some 

 three hundred feet, with a central vein of quartz some fifty feet 

 wide. It strikes N 40° to 55° E, and is probably a part of the vein 

 which has been so long quarried for " flint " at Castleton, on the 

 Harford county side of the Susquehanna. 



A quarter of a mile south of Conowingo Station, also on the river, 

 there is a pegmatite dike possessing a width of eighty-four feet. 



On Octoraro Creek, between the paper-mill at the fork in the road 

 and the State line, there are three pegmatites of a granitic character. 

 The constituents are quartz, microcline, muscovite and a little biotite. 

 The first possesses a width of ninety feet. The second of these 

 bodies lies about midway between the first and the State line, while 

 the third and largest (100 ft.) is close upon the State line. 



The wall-rock is norite and meta-norite. This pegmatite is quar- 

 ried on the west side of the creek, on Taylor's farm, for feldspar. 

 It is said to have yielded some 10,000 tons of feldspar. Other flint 

 veins also occur on the same farm. The " spar quarries " in the 

 serpentines have already been mentioned and lie for the most part 

 north of the State line. 



