THE SILURIAN SECTIONS 549 



Feet 



Upper Medina gray sandstone 90* 



Iron-stained sandy shales 480* 



Lower Medina white sandstone 60 



Iron-stained sandy shales 600 



Oneida white sandstone and gravel beds 200 



1,430 



* A part of this thickness is to be regarded as of Clinton age. 



The stratigraphic succession here appears to be very much as in the 

 Lehigh Gap, only that the Upper Shawangunk is even more thin- 

 bedded ; near the top recur the Medina red beds that are probably of 

 Clinton age, though no Clinton fossils were seen. The Lower Shawan- 

 gunk consists of grayish white, heavy and cross bedded quartzites. 

 The conglomerate is restricted to the lower 8 feet, where the pebbles 

 range in size up to .75 inch. Higher in the formation they are re- 

 stricted to the bedding planes. 



Arthrophycus alleghaniense was seen at 4 feet and at 90 feet above the 

 base of the Shawangunk. 



Ordovician. 



Great break. All of Richmondian absent. 



Hudson River shales. Well exposed here, but the contact with the Sha- 

 wangunk is faulted. 



Susquehanna Gap at Bockville (= Fort Hunter), Pennsylvania. — 

 North of Harrisburg and 50 miles southwest of Port Clinton. See Sec- 

 ond Geological Survey of Pennsylvania, Final Keport, volume I, 1892, 

 pages 637, 643, 669-673; volume II, 1892, page 735. 



Silurian. The Silurian is here overturned and thrusted on itself to the east- 

 ward over the Hudson River series, and the thicknesses given are 

 therefore not at all reliable. Regarding this, Lesley writes as follows : 

 "Following the mountain only a few miles eastward from the Susque- 

 hanna to where the beds of number IV lean in their natural attitude 

 (dipping north), the formation becomes of its usual thickness; and 

 following the mountain westward from the Susquehanna not more 

 than 15 miles, the usual thickness of the formation is again resumed. 

 We have, therefore, some right to ascribe its abnormal thickness at 

 the Susquehanna to the overturn" (639). 



Shawangunk formation. The upper shaly and thin-bedded wbite Shawan- 

 gunk with greenish shale partings is said by the Second Pennsylvania 

 Survey to have a thickness of between 300 and 400 feet, and consists 

 of greenish-grained quartzite devoid of pebbles. The lower sandy 

 thin-bedded and conglomeratic Shawangunk appears to vary between 

 60 and 70 feet. The basal beds resting on the Hudson River shales 

 consist of conglomerate ; the upper 10 feet with the pebbles ranging 

 up to .75 inch and the basal 5 feet consist of quartzite pebbles averag- 

 ing between 2 and 3 inches. The probable thickness of the entire 

 Shawangunk here may be 800 feet. 

 XL — Bull. Geol. Soc. Am., Vol. 27, 1915 



