SIXTH REPORT OF THE DIRECTOR I9O9 1 5 



hattan island. The task has proven a difficult one. It has been found 

 necessary to summarize the data and give results rather than 

 to present the whole mass of facts. The final outcome will be 

 the solution of a series of problems very largely in the form they 

 have presented themselves to the practical engineer and others 

 working on them. The bulletin therefore falls readily into the 

 line of applied geology and it is believed will have distinctive educa- 

 tional use in that field. 



Detailed study of old drill borings in New York city has led 

 to the opinion that the present conception of the areal and struc- 

 tural geology of Manhattan island and the East river needs con- 

 siderable correction. This, however, applies only to the southern 

 portion under heavy drift cover. The tunnel developments of the 

 city make details of this sort of much importance. Instead of the 

 island being wholly Manhattan schist, the data seemed to require 

 that at least two narrow belts of the Inwood limestone and 

 Fordham gneiss should extend through the lower east side. Ex- 

 plorations designed to< test this theory have proven it to 

 be true. Limestone does not follow the East river continuously. 

 A large area of grano-diorite cuts through the gneisses in Long 

 Island City and Brooklyn and crosses the East river into Man- 

 hattan. A corrected map of this section will be published in the 

 bulletin. 



The exceptional advantages offered by tunnels now being con- 

 strue :ed throughout the Highlands region along the aqueduct 

 makes it possible to see the prominence of some structural 

 features that are otherwise obscure. Faults and crush zones 

 and other effects of movement are surprisingly numerous. They 

 are in all stages between complete recementation and complete 

 decay. Some of the older are in as good physical condition as 

 the original wall rock, others of later origin are reduced to clay 

 or similar incoherent matter to great depth. In at least one such 

 case on Manhattan island, complete decay extends to a depth of 

 over 300 feet. 



Additional exploration on the Hudson river indicates clearly 

 that there has been glacial widening of the preglacial gorge at 

 the Storm King entrance to the Highlands and that there is 

 probably also glacial overdeepening. Borings in the channel 

 have penetrated silts, sands and drift to a depth of - 674 feet 

 at which point it is now believed rock bottom has been en- 

 countered. 



