12 



The formations will therefore be treated in the ascending order, 

 as regards their superposition, or in the order of geological time, 

 as respects the date of their production. 



I shall commence with an account of the geology of the 

 Highlands — these ridges consisting of the oldest or primary for- 

 mations. 



CHAPTER I. 



Primary Rocks of the State. — Geology of the Highlands. 



Comjiosiiion and Structure. — The rocks which constitute the 

 chain of hills to which we give the general name of the High- 

 lands of New Jersey, are embraced, with a few exceptions, in the 

 group denominated by geologists the Gneiss System. They are 

 composed of the same assemblage of materials as the ordinary 

 varieties of granite, viz. quartz, felspar, mica, and hornblende 

 (and sometimes augite, magnetic oxide of iron, garnets, &c.), but 

 differ from the true granites by possessing a stratified structure. 



Their strata are, however, very frequently penetrated by veins 

 and dykes of granite, sienite, greenstone, and other rocks of 

 unequivocal igneous origin, a circumstance naturally calculated 

 to lead the inattentiv^e observer to infer that the o;ranitic or unstra- 

 tified primary rocks form an extensive portion of these hills. 

 This prevailing misconception is heightened by the granitoid 

 character of the gneiss, which is seldom comparatively of the 

 schistose kind, being far more commonly a massive rock in thick 

 beds, containing relatively few divisional planes. Its analogy to 

 common granite is still further increased by the relative defi- 

 ciency of its mica — the usual mixture being either felspar and 

 quartz, with a little mica, or felspar and quartz alone; or felspar 

 and quartz and an excess of hornblende ; and, not unfrequently, 

 felspar, quartz, hornblende and magnetic oxide of iron, which in 

 many places seems to take the place of the mica, giving to the 

 rock the speckled aspect of a micaceous gneiss. Magnetic oxide 

 of iron is in fact an abundant, we might almost say a characteristic 

 constituent in the rocks of this region, for it occurs not merely as 



