GEOLOGY OF THE NEW YORK CITY AQUEDUCT 207 



tion of the rock a little farther north (at hole no. 40) shows that 

 these limestone beds are actually much weaker than the gneiss. 

 Therefore the deepest portion of the buried channel is to be expected 

 between holes no. 40 and no. 45, and probably nearest to hole no. 40. 



4 Depth of old channel. How deep the buried channel may 

 be can not be accurately estimated. But if the same dip slope as 

 is shown by the rock surface from hole no. 46 to no. 45 prevails 

 northward toward hole no. 40, a depth somewhat below -100 feet 

 may reasonably be expected. In the absence of data bearing upon 

 the depth of other portions of this ancient channel or of the lower 

 Bronx river with which it must have been connected, it is impossible 

 to estimate more closely. 



5 Interpretation of hole no. 40. There is so little rock actually 

 saved from the more than 200 feet of possible core on this hole 

 that its real character is very obscure. 



There are three possible explanations for the condition found in 

 the last 100 feet. 



a The drill may have followed a large mud seam. 



b The material may be only residuary rotten limestone still wholly 

 above the gneiss. 



c The actual contact may have been penetrated and a part of 

 this rotten material may be decayed gneiss within a crush zone. 



The difficulty in drawing absolute conclusions is increased by the 

 fact that matter falling in from above has been a continued source 

 of trouble and is more or less mixed with the rock material of 

 lower points. Therefore, the fact that the sand taken from the 

 lowest points, 335 feet, is silicious instead of calcareous, may not 

 prove satisfactorily that the rock at that point is wholly silicious. 



It is worth noting, however, that the harder rock in the upper 

 portion of the hole was in places much crushed and that mud seams 

 were encountered before reaching this last 100 feet. 



It is also worth noting that the same dip slope of rock surface 

 as prevails between holes no. 46 and no. 45 if continued northward 

 to hole no. 40, would cut that hole a considerable distance (75 feet) 

 above its bottom. 



In view of all the conditions, therefore, it is judged that there i'S 

 a crush zone here, that hole no. 40 penetrates it, that it is badly de- 

 cayed, that the plane of the crush zone dips steeply northward and 

 cuts both limestone and gneiss, that a tunnel at about -300 feet 

 would cut this zone south of station 640 and north of station 642, 

 and that all other portions of the line are in comparatively satisfac- 



