102 NEW YORK STATE MUSEUM 



fingers. The whole section of the tunnel had to be timbered right 

 up to the very face of the workings, and the ground gave so much 

 trouble that the work was slowed down and became more than 

 usually expensive. It took the greatest ingenuity and care to carry 

 on the work. Certain patches or streaks were hard, but these were 

 not of sufficient prominence to dominate and materially influence the 

 behavior for the first 500 feet. Substantial rock was entered at 

 about that point and gave little trouble except at two or three 

 crush zones where a badly decayed condition was encountered even 

 at greater distance from the surface. 



The special geologic interest which attaches to this occurrence is 

 involved in the origin and distribution of this decayed rock. There 

 is no good reason to conclude that such decay as this is postglacial 

 simply from the fact that glacial drift lies immediately above it. 

 As a matter of fact there must have been a very great deal of this 

 kind of residuary material previous to the glacial erosion, most of 

 which has been removed, and, after mixing with other materials, 

 has become the drift soil. There is no reason, however, why cer- 

 tain protected places should not still preserve patches of such 

 residuary matter. They are seldom seen, of course, because of 

 the almost universal cover of drift, but in such a piece of work 

 as the aqueduct where continuous trenching or tunneling was car- 

 ried on across the whole district, numerous places were fovmd where 

 such decayed material is still to be seen in its natural position. It 

 is common in crush or fault zones along which underground cir- 

 culation is encouraged, and in a few places borings have discovered 

 such a condition to a depth of several hundred feet. One such 

 occurrence became of practical significance at Foundry brook, as 

 noted under text head. Superficial material of the same sort is 

 more seldom seen, and this exhibit at the north end of the Garrison 

 tunnel is considered to be the best illustration yet discovered in 

 this region. 



The fact that exactly the same type of rock is perfectly fresh at 

 other places both where it is fully exposed and also under drift 

 cover is complete proof, all other conditions being essentially the 

 same, that the decay is not postglacial. Otherwise decay of this 

 sort would be much more general than it is. But if it is of preglacial 

 origin, then there is no possibility of foretelling its development and 

 extent, as it is directly related to the irregularities of quality and 

 physical condition of the rock along which decay has been easy or 

 difficult. 



