GEOLOGY OF THE NEW YORK CITY AQUEi)UCT 45 



from drill cores is seldom highly colored. The protected beds are 

 more commonly greenish in color and contain much iron sulphide. 

 Occasional thin limestone beds occur in the upper portion at High 

 falls — one of 4 feet forms the lip of the lower fall. The High 

 Falls shale is confined to the Rondout valley and on the line of 

 the aqueduct is 67-100 feet thick. 



(21) Shawangiink conglomerate. The Shawangunk is a con- 

 glomerate and sandstone. The constituent pebbles are almost wholly 

 quartz, well worn, and varying in size from that of sand to pebbles 

 of several inches diameter. But for the most part the pebbles are 

 small, abundantly mixed with sand, bound together by a silicious ce- 

 ment. Rarely a true quartzite is developed and still more rarely a 

 shaly f acies. The rock is therefore very hard, brittle, and in the un- 

 disturbed portions fairly impervious and resistant. But it suffers 

 from crushing along zones of disturbance in folding and faulting 

 and these zones are very imperfectly recemented. It is a durable 

 rock, very resistant to ordinary decay, but forms great talus slopes. 

 It is used for buhrstones (millstones), etc. It varies in thickness 

 on the lines of the aqueduct from 280-400 feet. The rock is lim- 

 ited in its northward extension to this district — southwestward 

 it is much more broadly exposed in the continuation of the Shaw- 

 angunk range. 



The Shawangunk completes the conformable Siluro-Devonic 

 series down to the erosion interval at the close of the Ordovicic. 

 The series of conglomerates, sandstones, limestones, and shales 

 make an imposing column approximating 3000 feet of strata differ- 

 entiated with more or less ease into 15 separate and mapable 

 formations and a possible 5 or 6 more with careful paleontologic 

 work. The series begins with the capping beds of the Shawangunk 

 range and its northward extension toward the Hudson river at 

 Rondout and Kingston, and thence westward constitutes the rock 

 floor while its structures control the surface configurations far be- 

 yond the limits of the region under consideration. Immediately 

 to the north and partly within the area here treated is the famous 

 Rosendale cement region, the pioneer cement district of America 

 and for many years the best producer. The strata used 

 are almost exclusively the upper members of the Siluric 

 (^''cement beds") closely associated with the Cobleskill between 

 the Manlius proper and the Binnewater sandstone. Rarely the Be- 

 craft from the Devonic series furnishes some cement rock. 



/ Cambro-Ordovicic formations. Between the Precambric 

 metamorphics of the Highlands beneath and the Siluro-Devonic 



