The Watchung Basalt flows and the thick sedimentary sequences as now exposed in Bergen, Essex, Union, Passaic, Morris, and Somerset Counties, New Jersey, are part of the remnant of a former. graben. This graben has been largely destroyed by erosion. The graben is generally believed on the basis of paleontologic evidence to have been formed in the Late Triassic. About 21h to 1 million years before the end of Triassic sedimentation in the graben, lava flowed out onto the graben floor to form the large flood basalts of First Watchung Mountain. Approximately 600,000 to 250,000 years after the first volcanic episode a second period of basaltic eruptions produced the lavas of Second Watchung Mountain. Following a lapse of 11h million years to 600,000 years another flood of basalts formed the lavas of Third Watchung Mountain. As the lavas solidified they accumulated body stresses and most of this stress was relieved by jointing. Subsequently, the region was folded to form an arch. During the buckling of the arch subsidiary border synclines were formed along the contact with the granitic walls of the horsts. This tectonic event produced joints whose abundance and distribution are related to the geometry of the structure. Late in the history of the Newark basin in New Jersey, during the Pleistocene, a continental glacier covered a part of the area. After the disappearance of the glacier, decompression of the crust produced sheeting structures in the basalt. Thus the jointing systems observed today are the results of the combined geologic processes of volcanism, tectonism, and glaciation.The history of the events involved in the tectonic subsidence and volcanism in the Newark basin bears a parallel relationship to the order of events taking place in the North American plate in its location in the ancient continent of Pangaea. It is suggested here that ( 1) the rifting to form and develop the Newark graben, (2) the deep rifting associated with the volcanism, and (3) the downfaulting of the graben which produced the central arch and the border synclines are the result of the following events in the continent of Pangaea: (1) the development of a linear zone of tension, (2) deep rifting which separated the North American plate from the adjacent parts of Pangaea.Basalts form the three ridges known as First, Second, and Third Mountains. The feeder dikes from whence the lava welled forth are hidden by cover. A feeder dike was observed cutting through the lower flow unit of First Mountain, near West Orange, N.J. The areal extent of the Watchungs is now about 500 square miles (1,29•5 km 2 ) ; they probably covered an area of 2,000 square miles ( 6,180 km 2 ). The lavas are tholeiites. Supplementary topics include a discussion of the characteristics of fissure eruptions and of vesiculation in basaltic magma. "Watchung Mountains-The trap 'ridges extending from Bedminster and Somerville to Oakland and Darlington are known as the Watchungs or Orange mountains. They are sometimes called First, Second and Third Mountain, beginning with t...