CHAPTER XVIII 



AREAL AND STRUCTURAL GEOLOGY SOUTH OF 59TH 



STREET 



The necessity for exploration in certain sections of this area can 

 not be appreciated without a statement of the local geology and 

 especially of the revision of both areal and structural geology that 

 the writer has based upon an exhaustive study of all the available 

 drill cores and other data to be found in southern Manhattan, East 

 river and Brooklyn. 



Below Central Park there is now little geology to be gathered 

 from a study of the present surface. But as far south as 31st 

 street the bed rock geology is pretty well known from earlier re- 

 ports and from recent improvements that have exposed the under- 

 lying rock. All of this portion is mapped as Manhattan schist ex- 

 cept one small area of serpentine at 59th street between loth and 

 nth avenues. There is no reason to modify this usage. A careful 

 study of a great number of rock borings from the Pennsylvania 

 Railroad tunnel across Manhattan at 32d street proves beyond 

 question that bed rock is jManhattan schist, including almost all 

 known variations and accompaniments, for the whole width of the 

 island along that line. 



Still farther southward the points that have yielded exact in- 

 formation about bed rock are less numerous, and below 14th street 

 are confined to deep borings or an occasional very deep excavation 

 for foundations. Even these sources of information are lacking 

 over large areas. The greater number of borings available are 

 along the water front. Their distribution is such as to indicate that 

 the west side and central portion and southerly extremity of the 

 island are all underlain by Manhattan schist. This is true eastward 

 to the East river at 27th street, and as far eastward as Tompkins 

 square at loth street and almost to the ^Manhattan tower of Brook- 

 lyn bridge in that vicinity. 



To the eastward of these limits, i. e. to the eastward of the line 

 projected from Blackwell's Island to the ^Manhattan tower of 

 Brooklyn bridge, there is a more complicated geolog}^ The borings 

 of the East river water front are decidedly variable. They are 

 certainly not all Manhattan schist of the usual types. Those most 

 unlike the Manhattan are at the same time most like some varieties 



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