NEWARK ROCKS OP ROCKLAND COUNTY, N. Y. 35 



Effect on the neighboring shales and sandstones. Both the under 

 and overlying beds have been more or less metamorphosed by con- 

 tact with the trap. The most marked macroscopic changes are 

 1) a greater induration, 2) a change in color, red shales in general 

 becoming, near the trap> purple and then a blue black, streaked 

 with gray or green, and 3) the development of secondary minerals, 

 commonly epidote and tourmalin. The rock sometimes has a 

 banded or mottled appearance, due to the formation of lime- 

 silicate hornfels. 



The alteration has been most pronounced in the case of the 

 shales. Beds of sandstone are less altered, particularly in color 

 and in minerals, though not infrequently somewhat indurated. 

 The red shale has sometimes been altered to a very hard, black, 

 flinty rock, sometimes mottled or banded and known as hornfels. 



In New Jersey the contact metamorphism has been more com- 

 plete, and has affected the beds for much greater distances from 

 the trap, than in New York state. The shales are often com- 

 pletely altered for over 100 feet from the contact, and traces of 

 metamorphism are discernible for still greater distances. In no 

 ease was such widespread and extensive alterations noted along 

 the Palisades in New York state. I know of no place where in- 

 tense alteration has affected the beds for more than 25 or 30 

 feet, and very often unaltered sandstones or shales are found much 

 closer than that to the trap. One or two localities have been 

 noted where the rock adjoining the trap is entirely unchanged, 

 a condition never seen farther south. 



The lesser metamorphism at the northern part of the Palisades 

 is, I believe, due chiefly to two causes. The first is the texture 

 and composition of the inclosing beds, and the second is the posi- 

 tion of the trap, i. e. the geologic horizon it occupies. Other 

 conditions being the same, the silicious sandstones have been less 

 altered than the argillaceous and calcareous shales. Since in 

 New York state the adjoining layers are more frequently sand- 

 stones than in New Jersey, the metamorphism is less marked. 

 But more important, I take it, than the texture or composition of 

 the beds is the horizon the trap occupies. In New York state 



