44 Geology of Hudson County, Neiv Jersey. 



compact and fiac-grained when obtained from the eastern side 

 of the hill, i. e., the lower surface of the trap-sheet. The west- 

 ern or upper surface shows a much coarser texture, and is more 

 difficult to break into regular blocks ; for this reason, the rock 

 along the eastern face of the hill is best suited for shaping into 

 paving and building-stones, though the very tine-grained stone 

 at the bottom of the hill is also more difficult to work than the 

 intermediate variety. This difference in the texture of the trap 

 can be readily seen in the cliifs at Weehawken ; the rock near 

 the base of the cliff, where it comes in contact with the sedi- 

 mentary beds below, is very compact, crypto-crystalline, and 

 breaks with a conchoidal fracture ; while in the same cliff, a 

 hundred feet above, the separate crystals of augite and feldspar 

 can be distinguished by the unaided eye. The rock that is in- 

 termediate in structure furnishes the best paving and building 

 stones. 



When a thin chip of the trap-rock is ground down on a lapi- 

 dary's wheel sufficiently thin to be translucent, it is found, upon 

 examination with a microscope, to consist principally of bladed 

 crystals of augite, sometimes hornblende, and a feldspar, usually 

 oligoclase ; these crystals are interlaced in every direction, and 

 frequently interspersed with dark masses of magnetite. It is 

 principally the size of the augite and feldspar crystals, that de- 

 termines the texture of the rock.* 



The crystalline structure of the trap furnishes one of the 

 proofs that it was at some time in a fused or semi-fused condi- 

 tion, and has become a crystalline solid upon cooling. That the 

 trap was forced in between the layers of sandstone and shale, 

 and also injected into fissures therein, forming dikes, we have al- 

 ready given abundant proof. As may be seen in the section 

 along the bank of the Hudson, the stratified rocks beneath the 

 main trap-sheet are always altered and more or less metamor- 

 phosed ; the junction of the main trap-sheet with the overlying 

 sedimentary beds is not exposed in Hudson County, though it 



* Analyses of the trap from the Erie Railway tunnel in Bergen Hill, are given in Prof. 

 Cook's "Report on the Geology of New Jersey," 1868, pp. 215 and 216, with remarks upon cer- 

 tain of the varieties. 



