250 NEW YORK STATE MUSEUM 



1 It would appear that at least one of these borings (no. 7) is 

 near the schist-limestone contact. ^lay they all lie then in the 

 weakened contact zone? 



2 It is true that at least one core (also from no. 7) shows a 

 badly broken condition. Alay they all lie in a fault zone? 



3 There is no reasonable doubt but that the geologic structure at 

 the south end of Morningside Park is that of a pitching anticline 

 carrying the limestone beneath the schist in its southward extension. 



May the excessive decay be due to this relation ? 



The evidence on these various possibilities' is not complete enough 

 to make a conclusion very reliable. But tliere are two or three 

 factors that have a bearing and they unite pretty well in supporting 

 one view. 



These factors are : (a) the exact alinement of these three holes, 

 (b) the crushed core of hole no. 7, (c) the overturned position of 

 the formations 10 blocks farther north, (hole no. 33), together 

 with the apparently normal position in hole no. 16. 



All of these points are consistent with the opinion that we have to 

 do here with the crush zone of a fault, one that runs rather straight 

 and one that follows not far from the contact of the schist and lime- 

 stone at this point. And it is probable that the weakness follows 

 the west margin or limb of the limestone anticline as it plunges be- 

 neath the schist. Such evidence as there is favors this view. 



If that is true, then one may expect that the worst ground is not 

 very wide, but that one probably can not go entirely around it. The 

 best line would run south far enough to get above the limestone, 

 and then cut across the weak zone nearly at right angles. It is cer- 

 tain that the ground improves southward. 



Later borings are all confirmatory of the conclusion that the weak- 

 ness is narrow and dies out rapidly southward as soon as the lime- 

 stone passes well beneath the schist. No bad ground has yet been 

 found on io6th street where the tunnel will probably be located. 



4 The East river section 



Preliminary studies of southern ^Manhattan and the East river 

 led originally to the conclusion that the portion of the East river 

 forming the great eastward bend from 32d street to Brooklyn bridge 

 probably has a simpler geologic structure than those portions farther 

 north or south. It was long known that the structure at Black- 

 v^^ells Island is very complex and involves all of the local formations 

 in close folding and considerable faulting. But there seemed to the 



