FENNER, THE WATCHVSG BASALT 99 



In Northern Xew Jersey, the igneous rocks associated with the Triassic 

 (Newark) sediments include the intruded sheet of the Palisade diabase 

 and the three extruded flows of Watchung basalt. The Palisade diabase 

 appears to form a continuous sill, approximately parallel to the stratifica- 

 tion, stretching diagonally across the State from New York into Pennsyl- 

 vania but concealed in places by late sediments or glacial drift. The 

 Palisade ridge in the North and Eocky Hill and Sourland Mountain in 

 the South are considered portions of the same sheet. Its thickness is 

 everywhere several hundred feet and in places is believed to reach nearly 

 1,000 feet.^ At several places, dikes and apophyses thrown off from the 

 main mass reach the surface in small detached areas. 



The three surface flows form the "Watchung ridges and several isolated 

 knobs. A maximum development of 1,200 feet is believed to be attained 

 in the thickest parts of Second Mountain. In both First and Second 

 Mountains, certain slight differences in composition and appearance verti- 

 cally indicate a composite character of flow. An interbedded shale in the 

 southern portion of Second Mountain is held by Lewis® to be evidence of 

 differential downward movements within the area between successive 

 flows. Such movements would not be unexpected in a region experienc- 

 ing the history described.^" 



LOCAL PKCULIARITIES OF THE FIRST "WATCHUXG SHEET 



The basal layer of the First Watchung sheet apparently marks the 

 initiation of igneous activity. The flow spread over the surface of the 

 accumulating sediments and filled the depressions occupied by the playa 

 lakes. Over the dry areas, the lava came to rest quietly, and the normal 

 process of cooling and crystallization followed without interruption. The 

 rock formed under these conditions presents a dense, homogeneous mass, 

 which, under the microscope, is found to have the holocrystalline texture 

 normal to basalts. Over the lake beds, however, the lava, which appears 

 to have approached in composition the eutectic ratio of its component 

 minerals and to have been almost at the temperature of crystallization 

 thus determined, ^^ was quickly chilled from the effects of water and 

 vaporized steam. An exposure of the bottom layer in a quarr\^ in the 

 southwestern part of Paterson shows a mixture of lava and very fine mud 



8 .T. V. Lewis : Ann. Kept. State Geol. N. J. for 1907, p. 129. 



"Ann. Kept. State Geol. N. J. for 1906, pp. 110-115. 



1" Compare the geological history of the Rift Valley of East Africa, as described by 

 J. W. Gregory : "The Great Rift Valley," London. 1896, especially chapter 12. 



" C. X. Fenner : "The Crystallization of a Basaltic Magma from the Standpoint of 

 Physical Chemistry," Am. Jour. Sci., 4th sen., vol. 29. pp. 217-234. 1910. 



