50 NEW YORK STATE MUSEUM 



success and in some places the residue after this solvent action is 

 of the consistency of mud. A nearly vertical attitude of the beds 

 accentuates the opportunity. The most troublesome piece of ground 

 encountered on the whole line of the New Croton aqueduct, con- 

 structed in 1885, was in a weak zone and crevice in the Inwood near 

 the village of Woodland on the margin of the Sawmill valley [see 

 discussions of Bryn Mawr siphon and New York city distributions 

 in part 2] . 



The thickness probably varies but in many places where there is 

 only a narrow limestone belt it is due more to shearing or faulting 

 out than to original thinning. The most satisfactory estimates are 

 based on the explorations at Kensico dam and the field observations 

 at I52d street. They indicate an approximate thickness of 700 feet. 

 But in all cases either the margins are obscured or there is possibility 

 of faulting to modify measurements. There are no fossils. Weath- 

 ering and erosion has almost everywhere developed valleys or de- 

 pressions especially small tributary valleys in all formations, but as 

 pointed out years ago by Professor Dana the principal valleys pre- 

 vailingly coincide with the limestone belts. 



(3) Lowerre quart zite. At Hastings-on-Hudson and again 

 near Croton lake, there is a quartzite that appears to be 

 conformable with the Inwood above. There is possibly more than 

 50 feet. It is a simple, clean quartzite. The other quartzites of 

 Westchester and New York county have a more distinct relation- 

 ship to the underlying gneisses with which they are conformable. 

 The Lowerre of the type locality is of this second class. In the 

 great majority of places where this bed would be expected to occur 

 there is not a trace of it. 



h Older metamorphic crystallines (Grenville series).^ "The 

 lowest and oldest, as well as the most complex in structure and rock 

 variety, of all the formations of the Highlands region of south- 

 eastern New York is essentially a series of gneisses." Cutting 

 these gneisses as intrusions of various forms are a great number 

 and variety of more or less distinctly igneous types. In form they 

 vary from small dikes or stringers to great batholithic masses; in 

 composition, from the extremely basic peridotites or p yroxinites of 



iThis interpretation of the larger relations of the complex gneisses 

 constituting the basis of the series, lying below the Manhattan-Inwood- 

 Lowerre series, was presented by the writer under the title: Structural 

 and Stratigraphic Features of the Basal Gneisses of the Highlands. N. Y. 

 State Mus. Bui. 107 (1907). P- 361-/8. The accompanying description is 

 largely an abstract of this paper. 



