. Annual report. New York State Museum; Science; Science. GEOLOGY OF THE NEW YORK CITY AQUEDUCT 211. Sc h i s t" Crush -a^orvc^ Fig. 37 Sketch of the geologic structure at shaft 13 on the New Croton aqueduct. Interpreted from field observations 5 It would appear probable that this zone was penetrated at the worst possible level, i. e. near enough to its Wholly decayed upper part to furnish no resistance at all to the overlying sand and gravel, and not deep enough to reach the more substantial (although prob- ably crushed) rock that may reasonably be expected to prevail at no very much gre
. Annual report. New York State Museum; Science; Science. GEOLOGY OF THE NEW YORK CITY AQUEDUCT 211. Sc h i s t" Crush -a^orvc^ Fig. 37 Sketch of the geologic structure at shaft 13 on the New Croton aqueduct. Interpreted from field observations 5 It would appear probable that this zone was penetrated at the worst possible level, i. e. near enough to its Wholly decayed upper part to furnish no resistance at all to the overlying sand and gravel, and not deep enough to reach the more substantial (although prob- ably crushed) rock that may reasonably be expected to prevail at no very much greater depth. The chief point is that the weak spot has a reason and is not an accidental thing that might be expected just anywhere. But it must be admitted, in spite of this fact, that a casual examination of the locality would not make one suspicious of its existence, and it is surprising that the spot could have caused so much trouble. From the above it will be seen that in several respects the Bryn Mawr case is somewhat similar to this. They both indicate fault- ing; they are in the same type of rock; they both show or indicate caving tendencies. On the other hand, there are certain elements of difference some of which are capable of very materially modifying any conclusion that might be based upon the simple facts of likeness. For exan> ple — it should be expected (i) that the fault movement at shaft 13 would be the greater because of lying in the more prominent lines of such displacement of the region, (2) being a thrust movement, the crush effect is probably more prominent at shaft 13 than at Bryn Mawr, (3) occurring at greater elevation above probable cir- culation outlet, the opportunity at shaft 13 for extensive and rather deep decay is the greater, (4) being cut so near the surface (160 feet), its condition there is not necessarily a reliable guide to the seriousness of decay at a greater Please note that these images are extracted from scanned page image
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