GEOLOGY OF THE NEW YORK CITY AQUEDUCT IO3 



hole, leads to the conclusion that they are more intimately depend- 

 ent upon the limestone structure for their existence than upon the 

 glacial drift or any superficial factor. Their abundant flow, some- 

 times on high ground, indicates rather extensive structural con- 

 nections and this is ibelieved to be the limestone bed itself and that 

 such flows would be encountered also in depth. The occurrence of 

 sinkholes suggest also possible solution channels and cavities and 

 distant outlets. The types of rock to be encountered on the lines 

 represented by this group are easily workable. Among them all 

 the Hudson River slates is probably the most satisfactory from any 

 standpoint. It is generally easy to penetrate and has a capacity for 

 healing its own fractures. For this reason it can be considered good 

 ground, tight and safe. But a considerable distance of the tunnel 

 can not be kept in slate — perhaps even more of it than can be 

 proven from surface observations. The other formations are con- 

 siderably less satisfactory. The limestones are in places shattered 

 and are liable to abundant flow of water. The quartzite is ex- 

 tremely hard, as difficult to penetrate as granite, and where crossed 

 by the faults is probably not healed at all, while the gneiss is doubt- 

 less of similar character to that of the Highlands crossings to be 

 discussed later. 



Only minor modifications result from a choice of the individual 

 crossing, whether " Peggs point," " New Hamburg," or " Danskam- 

 mer." In one of them. New Hamburg, it would appear possible to 

 cross the actual river section wholly in slates. This seems to be the 

 reasonable conclusion from the diamond drill boring at Cedar Cliff. 

 But even that line necessitates crossing at least two fault contact 

 lines immediately at the east bank and beneath Wappinger creek at 

 depths not immensely less than that below the river itself and both 

 wholly within the range of influence of the river waters. It would 

 appear therefore that the situation is not materially altered in the 

 present discussion, no matter which particular crossing of this 

 group is considered. 



2 The Highlands group [see cross section]. In this group of 

 crossings there are two separate features to consider, (a) the 

 Moodna creek valley which these lines all cross, and {h) the Hud- 

 son river itself. Their characteristics are as follows : 



a Moodna creek [see separate Moodna creek discussion]. So 

 far as known Moodna creek can be crossed almost wholly in slate. 

 It is possible that the underlying limestone may come near enough 

 to the rock floor of the valley to be penetrated but there is little 



