THE NEWARK ROCKS 39 



the intrusive masses are younger than the extrusive sheets, 

 although the evidence is not conclusive. From a priori consid- 

 erations it may be suggested that the lava formed intrusive 

 sheets after the formation became so thick that it could not 

 readily rise to the surface ; whereas, earlier in Newark time the 

 lava was able to break through the thinner beds and over- 

 flow. 



Meta7norphosed sliales. — The chief effect of the trap on the 

 shales is the contact metamorphism which has been produced by 

 the larger intrusive masses. The most marked macroscopical 

 changes are [a) a greater or less induration, (b) change in color 

 — red shales in general becoming purple and then a blue-black, 

 streaked with gray or green near the trap, and (c) the develop- 

 ment of secondary minerals, commonly epidote and tourmaline. 

 The rock often has a banded or mottled appearance, due to the 

 formation of lime-silicate hornfels. Of these three changes the 

 third is the most significant. Mere induration or change of 

 color does not necessarily signify "baking," but when all three 

 occur together, and only in layers in close proximity to certain 

 trap sheets, proved to be intrusive by their structural relations, 

 the changes can be safely ascribed to the igneous rock. Many 

 of the altered shales on weathering become a pale blue or ashy- 

 gray color, a tinge never taken by other layers. 



Detailed microscopic study of the altered shales has been 

 made by Messrs. Andreae and Osann 1 from specimens collected 

 at the base of the Palisades at Hoboken and Jersey City. Their 

 results, which were published in Germany, are inaccessible to 

 many readers in this country, and are therefore here briefly 

 summarized. They group the metamorphosed rocks into four 

 classes : 



1. Normal slate hornfels, not distinguishable from hornfels 

 formed by contact with intrusives which cooled at great depth. 



2. Hornfels containing numerous tourmaline crystals. 



1 Tiefencontact an den intrusiven Diabasen von New Jersey. Separat-abdruck 

 aus den Verhandlungen des Naturhist.-Med. Vereins. zu Heidelberg. A r . T. V., Bd. I. 

 Heft. 



