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describe. Its essential constituent is felspar, which is united in 

 some cases with liornhlende. It then belongs to the class of rocks 

 denominated greenstones. In other instances the felspar is asso- 

 ciated with augite, a very common case along the Delaware ; 

 and this mixture is a true dolerite rock. Again we find it con- 

 stituted of felspar, augite, and the titaniferous oxide of iron, a 

 composition which entitles it to the name of basalt. This basalt, 

 however, often contains hornblende, as we may behold in the 

 rock of Goat Hill. A fourth variety, of more rare occurrence in 

 New Jersey, is the kind called toadstone, an amygdaloidal trap 

 composed of a vesicular cement or paste, usually of fine-grained, 

 rather earthy basalt, embracing small spherical or ellipsoidal 

 cavities, occupied by extraneous minerals. A trap rock of this 

 character occurs in the ridge north of Scotch Plains. 



The crystalline structure of these trappean rocks is no less 

 various than their composition, presenting every gradation, from 

 that of a homogeneous paste, in which all traces of a distinct 

 grain disappears, to a coarsely granular aspect, in which we may 

 plainly detect the several constituent minerals. The same ridge 

 or broad dike frequently shows all these shades of crystallization, 

 disclosing usually the more compact varieties near the out- 

 skirts of the belt, and the conspicuously granular kinds near 

 its centre. 



The structure of the trap is usually that of a massive or amor- 

 phous rock, destitute of any greater regularity in its divisional 

 joints than such as occasion a prevalence of the cubical and 

 trapezoidal form in the blocks into which it separates. But in 

 some instances is displays an approach to the bedded structure, 

 as if, while in the melted state, layer had flowed over layer in a 

 nearly parallel arrangement. This may be seen at several 

 localities on the Delaware. In other cases, for instance at the 

 Little Falls of the Passaic, we find it assuming the true basaltic 

 character, separating into regularly formed perpendicular pris- 

 matic columns. At the same locality the lower portion of the 

 mass of trap has the spheroidal structure, being a confused 

 assemblage of concentric nodules. 



Almost every relation which igneous rocks ever present to the 

 strata intersected by them, may be witnessed in these trappean 

 belts of the red sandstone region. But the usual modes of posi- 



