1920.] 



NATURAL SCIENCES OF PHILADELPHIA. 



355 



Quartz Diabase. The quartz diabases form minor wooded ridges, 

 rising above the country occupied by the softer shales in which they 

 occur as sills and dikes. The exposures consist chiefly of enormous 

 boulders, many of which have travelled slowly down the hillsides, 

 and have been collected from the fields by the farmers to form stone 



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fences. When the need for road metal arose, these rocks, locally 

 known as " u*on-stone, " have been crushed. 



The quartz diabase (Fig, 2.) is typically a fine-grained, dark 

 greenish black or mottled black and gray rock, which may become 



