36 NEW YORK STATE MUSEUM 



it is higher in the series than in New Jersey, and therefore it 

 cooled nearer the surface and probably more rapidly. Its own 

 temperature at the time of intrusion may also have been less. 

 West of Haverstraw the trap occupies a much higher horizon than 

 farther south. The cases of more extensive alteration occur along 

 that part of the sheet which lies at the lowest horizons, whereas 

 the cases of no alteration occur where the trap is highest in the 

 series. The facts seem to indicate that the trap reached the 

 higher horizons in the sedimentary series with a somewhat lower 

 temperature and, being nearer to the surface, cooled faster than 

 at greater depths. Its effects therefore on the surrounding beds 

 were less marked. Other causes may have contributed to this- 

 result, but the two mentioned are believed to be the chief ones. 



Local examples of metamorphism. At Piermont indurated arkose 

 sandstone is exposed near the trap, below the railroad station, 

 and a mile north of the station a small exposure of hard straw- 

 colored sandstone and black hornfels was found in the bed of a 

 small stream, about 10 feet below the trap. 



At the Manhattan trap rock company's quarry, Verdrietege 

 Hook, the shale beneath the trap is dark blue or gray colored and 

 somewhat indurated. Where the trap cuts across sandstone, there 

 is but little change in the latter. The contact metamorphism 

 even where most marked, can hardly be detected in beds 30 or 40 

 feet away. The exposures here afford good opportunity for com- 

 paring the relative alteration of shale and sandstone under similar 

 conditions of heat, pressure, etc. 



Good examples of highly altered shale (hornfels) occur 100 yards 

 above Rockland lake landing, and less intensely altered shales 

 are exposed at the water's edge just below the southern end of 

 the quarry at the landing. At the Gosgriff quarry at Trough 

 Hollow, shales within 20 feet of the trap are slightly changed in 

 color, but sandstones are not visibly altered. 



At the southern entrance to the West Shore railroad tunnel 

 at Long Clove, the shales are indurated for not more than 20 feet 

 from the trap, and even at the contact, the metamorphism is not 

 intense. 



