GEOLOGY OF THE NEW YORK CITY AQUEDUCT 59 



(6) Unconformity between the Poughquag (Cambric) quartzite 

 and the underlying crystallines. An interval in all observable cases 

 of great length and profound changes involving mountain folding, 

 metamorphism of the profoundest sort, and extensive erosion. 



(7) Among the crystallines of the south side of the Highlands 

 there is one break of similar importance, between the Inwood lime- 

 stone and the underlying gneisses. Whether or not it is the same 

 as no. 7 above is not clear, but even if it represents the same break 

 the relations are somewhat different in degree and character because 

 of the lack of quartzite in almost all cases. 



Within the gneisses of the Grenville series and their associates 

 of all kinds there are no breaks of the unconformity type known. 

 The contacts are eruptive in character, or are displacements 

 instead. 



c Folds and mountain-forming movements. All of the forma- 

 tions from the oldest up to and including the Lower Devonic strata 

 are folded. Many of the smaller (minor) folds exhibit complete 

 form in the stream gorges of the district, but all of the larger ones, 

 the main folds, have in earlier time been eroded to such extent 

 that the series is beveled off and only the truncated edges are to 

 be seen, exhibiting strata standing more or less perfectly on edge, 

 and making restoration of the form a very difficult or impossible 

 task. This is only partially accomplished in the Siluro-Devonic 

 margin along the Shawangunk range ; it is more complete in the 

 Cambro-Ordovicic north of the Highlands, and it reaches its most 

 perfect development in the crystallines of the Highlands and New 

 York and Westchester counties. These dift'erences correspond 

 roughly to the dift'erences in age of the strata, and, taken together 

 with the evidence of the profound unconformities, indicate that 

 mountain-forming movements of far-reaching importance visited 

 the region no less than three times. Each time of such disturbance, 

 of course, the underlying older series was affected by the move- 

 ments of that epoch in addition to any previous ones, and as a con- 

 sequence the older is to be expected to show more complexity of 

 such structures. Each succeeding series separated by such activity 

 is therefore one degree simpler in structure. 



Of these three epochs of great disturbance, one is (i) Precambric 

 and corresponds to the time interval marked by the unconformity 

 betAveen the Poughquag quartzite and the gneisses; a second (2) is 

 Postordovicic and corresponds to the time interval marked by the 

 unconformity between the Hudson River slates and the Shawan- 



