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A. \V. GRABAU — SILURO—DKVONIC CONTACT IN NEW Y<)KK 



examination at a subsequent date showed this so-called pocket to be a 

 residuary dike of sandstone, formed by the filling of a fissure which 

 penetrated the entire thickness of the Bullhead limestone and entered 

 the Rondout Waterlime from 2 to 3 feet. The total depth of the fissure, 

 as now exposed with its filling of sandstone, is in the neighborhood of 

 10 feet. The dike is squarely cut off at the top. where the Onondaga 

 limestone rests on its truncated end and on the limestones flanking it. 

 The Onondaga limestone is entirely unaffected by the dike, being evi- 

 dently deposited after the formation and truncation of this remarkable 

 mass of sandstone. The width of the fissure is scarcely anywhere over 

 2 feet, but lateral offshoots extend for many feet into the walls of Bull- 

 head limestone. These offshoots or rootlets of the dike are irregular, 

 commonly narrow, and often appear as isolated quartz masses in the 

 Bullhead or the Waterlime, the connection with the main dike not being 



FlGTJ re ti. — Fragment of Rock from Margin of Sandstone Dike, 



Show in- angular pieces of limestone included in the sandstone and injected by tongues of the 



sandstone (natural size). 



always observable. Such masses of quartz sandstone have been noted 

 at a distance of 20 or 30 feet from the main dike. They are always small. 

 The dike itself has been traced for more than 30 feet in an east-and-west 

 direction in the sloping walls of the quarry. The walls of this ancient 

 fissure are very irregular, angular masses of the limestone projecting 

 into the quartz rock, while narrow tongues of sandstone everywhere 

 enter the limestone. Extensive brecciation of the limestone has occurred 

 along the margin, and the sandstone there is filled with angular frag- 

 ments of the limestone, which show no traces of solution or wear by 

 running water. These limestone fragments are themselves frequently 

 injected with tongues of the quartz sand, as shown in figure 6. 



Microscopic examination of the sandstone of the dike shows the grains 

 to be well rounded and closely united by a deposit of secondary silica. 

 Each grain is enveloped by this secondary quartz, which is in optical 



