GEOLOGY OF THE NEW YORK CITY AQUEDUCT 99 



the whole length of pressure tunnels whose position would be modi- 

 fied by a shifting of river crossing. This is because the aqueduct 

 will approach the Hudson with nearly 400 feet head — i. e. 400 

 feet above river level or with an equivalent pressure. For this 

 reason it is considered necessary to plan a rock pressure tunnel 

 beneath the river which can deliver the water at nearly the sam^e 

 elevation again on the east side. 



Thus any one of the " New Hamburg group " involves a contin- 

 uous pressure tunnnel reaching from the margin of Marlboro moun- 

 tain to Fishkill range, a distance of approximately seven miles, 

 while any of the '' Highlands group " permits the substitution of 

 two more or less separate siphon tunnels (Moodna creek and Hud- 

 son river) of considerably less combined total length. 



A reliable conclusion as to the choice of crossing is probably best 

 reached through a comprehensive understanding of the geologic de- 

 velopment of the region together with a consideration of specific 

 local conditions. With this end in view a condensed outline of 

 geologic history, so far as it bears upon the questions at issue, is 

 inserted. But for a more comprehensive discussion of these matters 

 the reader is referred to the explanatory chapter of part i. 



Geology 



This particular locality, including as it does the Highlands of 

 the Hudson and the district lying along its northern border, is one 

 of the most complicated stratigraphically and structurally to be 

 found in the entire region. The strata represented include more 

 than half the total geologic scale reaching from the oldest sedi- 

 ments following the Archean up to and including a part of the 

 Devonic series [see pt i]. The rock types include granites, diorites, 

 gneisses, schists, marbles, serpentines, slates, quartzites, sandstones, 

 limestones, shales, and, less extensively, other varieties. And the 

 region bears the evidence of no less than three periods of mountain- 

 making disturbances, each in its turn adding to the succession of 

 foldings, faultings and unconformities. 



The oldest formation is a crystalline gneiss — a characteristic 

 rock of the Highlands. It represents an ancient sediment that has 

 been completely recrystallized during some of the earlier mountain- 

 making period. It is older than the .Cambric. Interbedded with 

 it to a limited extent are quartzite beds, ancient limestones (now 

 usually serpentinous in character) and schistose beds ; and in it are 

 many igneous injections, mostly granites of various types. All 



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