232 NEW YORK STATE MUSEUM 



of the Fordham, and indicate that these formations both occur. 

 The lack of any data in the beginning of this investigation except 

 on the water front made it impossible to draw more than very gen- 

 eral lines. Drawn in this way, the lines of course are too straight, 

 but it is certain that they indicate more nearly the actual existing 

 areal distribution of formations than any of the maps now in exist- 

 ence.^ They indicate a southward extension of the Blackwell's 

 Island belt of Fordham gneiss toward the ^Manhattan tower of 

 Brooklyn bridge. How much of this anticlinal fold of Fordham 

 actually brings this formation to the surface it is impossible to say, 

 but that it may be expected to be encountered along this line is 

 evident. 



On the east side a parallel belt of Inwood limestone is indi- 

 cated and this again is succeeded by a Fordham gneiss area 

 which occupies the rest of the eastern margin. Explorations 

 made along the line of the gas tunnel across East river at 72d 

 street^ indicates comparatively narrow belts of limestone there 

 in both the east and west channels. The limited width of limestone 

 at these points, together with the occurrence of two strongly de- 

 veloped disintegration zones, seem to indicate rather extensive 

 squeezing out and faulting of this formation along fault planes 



1 In the summer of 1908 the writer was assigned the task of studying 

 in detail the evidences of geologic structure beneath the drift in southern 

 Manhattan. Before any drilling v/as attempted in the city by the Board 

 of Water Supply, a thorough canvass was made of all previous borings in 

 this district and the cores and records were personally inspected. More 

 than 300 such borings were found in which some of the core could be 

 secured for identification and classification as to formation and condition. 

 Most borings were given no weight at all in the final summary of this 

 evidence unless the rock core or at least fragments of it could be secured. 

 After all of these newly assembled data were tabulated and plotted on the 

 map, it was evident that if the identifications were correct the areal and 

 structural map of southern Manhattan needed extensive revision. A new 

 map therefore was made and presented to the chief engineer of the Board, 

 October 30, 1908. This has been used since as the basis for exploration of 

 the Lower East Side section. This original tabulation and map only 

 slightly modified was published under the Areal and Structural Geology of 

 Southern Manhattan Island [N. Y. Acad. Sci. Annals, April 1910, v. 19, no. 

 II, pt 2]. The extensive explorations of the board have made further revision 

 necessary [see accompanying map, pi. 34]. Exploratory' boring is still in 

 progress (October 1910) and some slight modifications of boundary lines 

 may yet be made. 



2 This is taken from Prof. J. F. Kemp's description of The Geologic Sec- 

 tion of the East River at Seventieth Street, Xew York [X. Y. Acad. Sci. 

 Trans. 1895- 14:273-76]. 



