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Ch. 2: Manhattan Island

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44               GEOLOGY OF NEW YORK CITY
on hard pan, which varies in thickness from o to 18 to 20 feet. In such cases where the hard pan was very thick we put the concrete bed on this hard pan after removing- about 4 feet of it. Of course, where we encountered rock without an over­lying layer of hard pan we founded it on this rock." (Hard pan is here understood as a very consolidated mixture of clay and boulder rock.)
The foundations of the Hudson Terminal Building rest on the Manhattan schists or gneiss 85 to 110 feet below the street curb (J. V. Davies, C. E.), passing through, above the gneiss, 10 to 15 feet of " hard pan," above which were sand beds, mud, and filling. Manhattan schist was taken 390 feet below curb at Cortlandt and Church Streets (A. S. Coffin), and a flake of granite from granite vein at 240 feet below curb at Cortlandt and Broadway (A. S. Coffin).
General conditions respecting the foundations of buildings at the lower end of Manhattan Island have been described in the following language, by Engineer T. Kennard Thomson:
" Under the New York quick-sand, which is from thirty to sixty feet thick, we find from two to thirty feet of hard pan. Sometimes this is directly on the rock, and sometimes we find masses of sand and boulders under the hard pan. Good hard pan, if on bedrock, will hold up any building that ever will be raised in New York, but nothing except personal examina­tion of each individual caisson foundation on the job is suf­ficient to decide whether the hard pan is really good, and even then it is necessary to determine if the good hard pan extends to the rock or not. This opinion is the result of having been down in the air chamber more than two thousand times.
The deepest caisson foundation in New York is under the Mutual Life building, where the depth is one hundred feet below the curb and eighty-five feet below the surface water, through thirty feet of quick-sand, twenty-three feet of hard pan, and then thirty-two feet of sand, boulders and decomposed rock, which we took out with a shovel."
Fossil evidences as to age and past conditions of the Man-
Ch. 2: Manhattan Island Page of 281 Ch. 2: Manhattan Island
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