FEATURES OF TRAP EXTRUSIONS IN NEW JERSEY 311 



decomposed trap, and lumps of hardened mud. The whole is much 

 affected by weathering, but it is easy to distinguish between the igneous 

 and sedimentary material. This mixture forms an irregular band 

 about six inches in width. Below is red shale, not perceptibly altered, 

 extending downward for several feet. Above the mixed band, the 

 trap is vesicular for a few inches, and passes upward into fine-grained, 

 dense rock. The impression produced is that the first thin stream of 

 advancing lava flowed over a bed of rather damp mud and became 

 mixed with it, and that this in turn was soon covered by later streams. 

 In this quarry for one hundred feet or so along the quarry face, 

 about six feet above the contact, there is a band of trap, two or three 

 feet in width, having the structure 

 shown in Fig. 8. The lines of the 

 sketch represent cracks from one- 

 half inch wide down to a mere 



seam, which extend approximately horizontally along the cliff" face 

 and back into the rock, and permit it to be separated easily into slabs. 

 The cracks are filled with a loose, granular material, generally of a 

 dark color, but showing calcite grains in places. This same slab-like 

 or platy structure can be observed as of frequent occurrence in the 

 more massive portions of the trap sheet in various localities, though 

 seldom so well developed as here. It is a fair inference that these 

 plates represent rivulets of lava, and that the chilled surfaces have 

 been more easily weathered. The advance of the lava flow should 

 not be pictured as similar to that of a sheet of water. On account of 

 viscosity and partial congelation, the spreading out of a sheet of even 

 the most fluid lava is quite a different process. We should picture 

 the advance of this Watchung sheet as similar to the form of the Hawai- 

 ian lava flows described by Major Button: 



When these lavas are discharged they come up out of the ground in enormous 



volumes, are intensely heated, and are very liquid As they become cooler 



they become more viscous. The cooling takes place upon the surface of the mass 

 while the interior still remains hot and preserves a viscous liquidity. The super- 

 ficial crust of cooled lava undergoes rupture at numberless points, and little rivu- 

 lets of lava are shot out under pressure. Preserving their liquidity for a short 

 time, they spread out very thin and are quickly cooled, forming pahoehoe. 

 Scarcely is one of these little offshoots of lava cooled when it is overflowed by 

 another and similar one, and this process is repeated over and over again. In a 



