GEOLOGY OF THE XEW YORK CITY AQUEDUCT 26/ 



dom possible to compute their thickness closely. It is probable that 

 most of them are not over 5 to 10 feet thick, although rarely a 

 thickness of 25 or 30 feet may be represented. It is certain also 

 that a considerable number of separate beds are penetrated. All 

 attempts to correlate the limestone cores from different adjacent 

 holes have so far met with little success. Xo doubt some of those 

 cut at great depth in one hole correspond to others cut higher in 

 an adjacent hole. But the differences in thickness are notable even 

 in the best cases, and it is evident that little dependence can be put 

 upon uniformity of thickness as a factor in correlation. The 

 foldings and crumplings, and shearing have probably affected the 

 limestone members of the series more than any others. Limestones 

 in comparatively thin beds are, under such conditions, especially 

 liable to excessive thinning and thickening through recrystalliza- 

 tion and rock flowage. It is not at all likely that any single bed at 

 present preserves much uniformity of thickness. In some places 

 they are pinched out entirely while in others they may attain a 

 thickness much greater than the original. It is possible also that 

 some of them are repeated by folding, '\^'hether or not this is true 

 in the Lower East Side section no one can tell. On the whole there 

 is no direct evidence of repetition in this way. After making al- 

 lowance for all possible duplication there is still a surprisingly large 

 number of limestone interbeds represented — probably 10 — a 

 larger number in succession than is known anywhere else in south- 

 eastern Xew York [see pi. 38]. 



In petrographic character these so called limestones are all es- 

 sentially very coarsely crystalline dolomitic marbles or silicated dolo- 

 mites of still more complex constitution. Occasionally a very pure 

 carbonate rock is represented that corresponds in appearance very 

 closely indeed to the best grades of the Inwood, but there is no doubt 

 whatever of the true interbedded relation of these limestones. Their 

 similarity of appearance to the Inwood in certain facies is so great 

 that from the petrographic evidence alone one could not dift'eren- 

 tiate them. Their fixed relation however is unmistakable and they 

 belong unquestionably to an entirely different geologic formation 

 from, the Inwood — a much older one, in fact the oldest known 

 formation in southeastern Xew York — equivalent to the Grenville 

 series of the Adirondacks and Canada. The silicated facies con- 

 tains many of the common products of metamorphic processes. 

 Recrystallization has produced micaceous minerals such as phlogo- 

 pite and chlorite in abundance. Original and secondary quartz is 



