﻿I3§ NEW YORK STATE MUSEUM 



stream to such extent that the stream normally at present has be- 

 come subterranean. It is the writer's opinion that the case en- 

 countered by the drill boring is similar. The apparent cavern is 

 probably a slightly enlarged joint along a line of somewhat abun- 

 dant underground circulation and perhaps associated with some 

 crush zone developed by the small faulting known to occur in this 

 immediate vicinity. It is probably not entirely empty but contains 

 residuary clay, and in all likelihood is very narrow and not exactly 

 vertical, so that the drill rods were bent out of their normal course 

 and wedged into the lower part of the crevice. Smaller spaces 

 of this sort were encountered at a few other points. 1 



These occurrences seem to indicate that the limestone beds yield 

 rather readily to solution by underground water, and that this cir- 

 culation has been at one time active to at least 50 feet below pres- 

 ent sea level. With present ground water level nearly 200 feet 

 above sea level it is extremely unlikely that any such action is 

 going on at so great depth. The occurrence is therefore strongly 

 corroborative of former greater continental elevation when the 

 deep stream gorges, now buried, were being made. These deeper 

 caverns or solution joints probably date from that epoch. 



Imperviousness and insolubility. The question of impervious- 

 ness and closely associated with it that of solubility, is of great 

 practical importance in this particular work. The immense pres- 

 sure under which the tunnel will be placed in crossing this valley 

 makes it impossible to construct a water-tight lining. Everywhere 

 much depends upon the rock walls to help hold the water from 

 sserious iloss. Wherever the rock is fairly impervious except 

 occasional crevices or joints they can be grouted and safeguarded 

 satisfactorily. But where a formation is of general porosity this 

 can not be so successfully done. Even more difficult to handle is 

 the rock wall which is soluble and which therefore with enforced 

 seepage may tend to become progressively more porous. That this 

 consideration is not wholly theoretical is shown very forcibly by the 

 Thirlmere aqueduct of the Manchester (England) Waterworks. 

 In that case a 3 mile section was built through limestone country 

 using the same local limestone for concrete aggregate. Although 



1 In constructing the tunnel several clay-filled spaces have been discovered 

 in the same vicinity at elevation — 100. One of these extended vertically with 

 a width of 1 to 2 feet and from it a great mass of mud ran into the tunnel. 

 At one point it was connected with a horizontal space of the same kind 

 extending 15 feet. It can be seen that the original crevices have been en- 

 larged by water and that they were originally formed during faulting. 



