GEOLOGY OF THE NEW YORK CITY AQUEDUCT 217 



2 Manhattan schist, the most abundant formation, chiefly mica 

 schist with very subordinate hornblende schists, and usuaUy with 

 abundant pegmatite lenses and veins. 



3. The Inwood limestone, a white, dolomitic marble when fresh, 

 which shades into impure, micaceous varieties. 



4 The Fordham gneiss, varying from a thinly schistose or 

 quartzose rock to a strongly banded or a very massive and much 

 contorted gneiss. The oldest formation of the district. 



5 The Yonkers gneiss, an original intrusive granite, now 

 squeezed into a gneiss. Younger than the original Fordham. 



6 The Ravenswood grano-diorite or as it might be called in 

 engineering practice, granite ; an original, intrusive rock now some- 

 what gneissoid from pressure. Younger than the original Fordham. 



The Manhattan schist, the Inwood limestone and the Fordham 

 gueiss are cut by veins or dikes of coarsely crystalline granite, 

 technically called pegmatite. They are of irregular distribution and 

 do not affect the tunneling operations one way or another. 



All the formations older than the glacial drift have been com- 

 pressed into a series of northeast and southwest folds, and all have 

 as a rule a steep or almost vertical dip. The axes of the folds are 

 not horizontal, but usually pitch downward to the south at low 

 angles. Erosion has developed a series of ridges trending north- 

 east and southwest. The limestone being a softer and more easily 

 eroded rock, almost always underUes the valleys or flats and the 

 river channels. It is certain also that there is some faulting. 



Rock at depth 



The distribution of geological formations along the proposed 

 lines has been shown on the accompanying map [pi. 32]. In gen- 

 eral the kind of rock at tunnel depth will be the same as at the 

 surface as indicated on the map for each point. Such error as there 

 is, arises from two causes : (a) Uncertainty as to the exact location 

 of some of the contact lines between two formations (usually due to 

 drift cover), and (b) dip and pitdh of the strata. 



In the first case (a) where the drift is particularly heavy, it is 

 sometimes impossible to fix a contact Hne accurately from surface 

 features alone. 



In the second case (b) it must be appreciated that nearly all of 

 the formations dip eastward at a very steep angle, so that a form- 

 ation would usually be found to extend a Httle further east at depth 

 than at. the surface. And also all formations pitch southward, so 



