28 NEW YORK STATE MUSEUM 



Its general elevation at the quarry is about 260 feet A. T. If the 

 base of the trap kept to the same horizon which it has where it 

 turns so abruptly eastward at Verdrietege Hook, the rise of the 

 underlying sandstones up the dip would carry it from 620 to 660 1 

 feet higher above sea level than it actually is at the quarry. The 

 evidence seems conclusive therefore that the abrupt turn at Hook 

 mountain north of Nyack, is due to the descent of the base of the 

 sheet to a geologic horizon 600 or 700 feet lower than that occu- 

 pied where it crosses the Rockland lake-Nyack road. 



From the quarry at Hook mountain for several miles north- 

 ward the position of basal contact can generally be determined 

 best from a boat. A mile north of the Manhattan trap rock 

 quarry, near the powder house landing, the contact is beneath the 

 water level for 150 yards, a large part of the descent from the 

 quarry level (260 feet) being caused by its breaking across the 

 sandstone beds. Just south of where it reaches the river, a ledge 

 of sandstone 30 feet thick is exposed along the shore, and the trap 

 must be at least 50 feet above the water. 



Northward the base of the trap rises again across the beds of 

 sandstone as represented in figure 3 to a hight of about 50 feet 

 above the river. ,From this point it lies conformably on some- 

 what indurated shales, the outcrop of which slowly descends, so 

 that within a half-mile the base of the trap sheet is again below 

 the water level for a distance of 75 yards. This exposure is 

 directly beneath the southern end of the Rockland Lake trap rock 

 company's quarry (a, fig. 4). Immediately to the north the base 

 of the trap must ascend across the beds of metamorphosed shale, 

 since the latter are next exposed along the river with the base of 

 the trap several yards above them. After this ascent the trap 

 again becomes conformable with the shales, the trend of which 

 once more brings the contact beneath the water level 100 yards 



Ht must be understood that these figures and many others given in this 

 report are subject to some variation, owing chiefly to possible variations 

 in the angle of dip. In this calculation a variation of 1° makes a differ- 

 ence of 40 to 50 feet in the result, and it is not always easy to determine 

 the angle of dip within lo with the small clinometer the geologist uses. 

 This is particularly the case if the rock exposure is obscure. 



