266 NEW YORK STATE MUSEUM 



amount of displacement is in no case fully known, makes the struc- 

 tures in some minor details impossible to accurately interpret at this 

 stage of the work. 



Fault zones. As nearly as the material recovered can be 

 classified and accredited to the above three formations it has been 

 done. On this identification together with the location of points of 

 greater decay the chief fault zones are drawn. The chief ones are 

 judged to be thrust faults but it is possible that one is a normal 

 fault. Such a combination is comparatively rare where the zones 

 are so close together, but it seems to best explain the relations of 

 beds as interpreted from identification of the present borings. It is 

 not an unknown association though in this region. It probably in- 

 dicates faulting in two different periods. This is consistent with 

 the observation also that some of the fault breccia ground is not 

 much decayed while others are badly affected. Probably the later 

 movements have not allowed rehealing of the crevices and they are 

 then the lines of chief circulation and alteration. 



It is clear, upon examination of the section as now known,^ that 

 both the eastern and western belts of limestone are too thin and 

 narrow to accommodate the whole Inwood limestone. The Inwood 

 normally is a formation of about 750 feet or more in thickness. It 

 is therefore certain that a part of it has been cut out by squeezing 

 or faulting. If by faulting then there would be expected to be in 

 each case somewhat greater decay than usual along the fault zones. 

 The fact therefore that such decay zones are found along one mar- 

 gin of the limestone bed in each case leads to the conclusion that 

 faulting is the true cause. In some cases thrust faulting would be 

 required to produce the result and leave the beds standing in their 

 present relations [see pi. 38]. 



INTERBEDDED LIMESTONES OLDER THAN THE INWOOD 



The finding of limestone beds within the Fordham gneiss forma- 

 tion so persistently in the Lower East Side borings is one of the 

 geologically interesting and rather surprising results of recent ex- 

 ploration. All of the borings in the Fordham gneiss area in this 

 particular district except those near the East river have shown some 

 limestone. 



The individual beds vary greatly in thickness, ranging from only 

 a few inches to many feet. Because of the steepness of the dip of 

 the beds and the obscurity of this factor in many borings it is sel- 



^ October 1910. 



