155 



the river for several miles, being continuous, in fact, with that 

 already spoken of, which forms the northwestern flank and base 

 of the Sourland Mountain. About a mile southeast of Mount 

 Airy, the blue and purple shale contains an abundance of epidote 

 in oblate spheroidal concretions of very regular shape. The 

 average breadth of the belt is very considerable, amounting at 

 least to a quarter of a mile. 



West of Ringoes the shale in some places exhibits a considera- 

 ble degree of change from its proximity to a bold dike of trap, 

 which shows itself in a hill above the mouth of the Alexsockin 

 creek. In crystalline structure this trap departs even more 

 widely from the fine-grained basaltic variety seen in the hills 

 near ilie Passaic, than does that of Goat Hill and Belle Mount, 

 lower down the Delaware. It is coarsely granular and crystal- 

 line, consisting chiefly of a yellowish-white felspar, traversed by 

 bladed or elongated tabular crystals of green hornblende of con- 

 siderable size. Much of this rock bears indeed a closer resem- 

 blance to some varieties of sienile than to ordinary trap rock, 

 exhibiting the justness of that generalization which makes the 

 trappean family of rocks to include those sieniies which are 

 composed of the two minerals, felspar and hornblende. Between 

 this belt of trap and the Alexsockin creek we find a disturbance 

 in the direction of the dip, being the only irregularity of this kind 

 of any magnitude in the whole red sandstone region, which is 

 referable to the protrusive action of the igneous rock. On the 

 southeastern side of the dike the strata of shale and sandstone 

 are in some places much contorted and broken, but their prevail- 

 ing dip is towards the southeast, or directly opposite to the usual 

 quarter. This contrar}' order of the dip throughout the district 

 along the Raritan and its north branch, I attribute to the pecu- 

 liar direction taken by the currents which precipitated these 

 sediments in this portion of the basin, and not to any upheaving 

 force. 



1 have already when describing the several zones of the red 

 shale and sandstone formation along the Delaware, mentioned 

 the altered aspect of the strata over the wide tract called the 

 " Swamp," the general limits of which were at the same time 

 defined. With the exception of some occasional exhibitions of the 

 trap rock, particularly in the vicinity of the southeastern margin 



