108 



gravel swept to this point from the valley of the Flatkill and from 

 the flank of the adjacent mountain. 



The obvious identity as to mode of origin between many rocky 

 conglomerates of the secondary periods, and masses such as this 

 of relatively modei'n date, in which we behold unequivocal evi- 

 dence of the short duration and violence of the action by which 

 the miscellaneous debris from the adjacent rocks was hurled to- 

 gether, will aid us, when adverted to again, to understand the 

 nature of the circumstances that gave rise to the patches of cal- 

 careous conglomerate which form the uppermost deposit of the 

 middle secondary strata, immediately at the southeastern base of 

 Ihe Highlands. 



ECONOMICAL GEOLOGY. 



The purer varieties of the liincstone of this formation pro- 

 duce, by burning, a lime in no respect inferior to that derived 

 from Formation II., either for building purposes or for agri- 

 culture. The numerous beds of magnesian limestone furnish a 

 source for hydraulic cement, which may, at any future day, 

 where circumstances shall warrant it, be manufactured along 

 the Delaware at a very small expense. It is this same rock, in 

 its range to the northeast, which has for some years past, at Ron- 

 dout, near the Hudson, yielded a superior cement at a low price, 

 and in large amount. 



Travertine occurs in two or three localities in Sussex, at the 

 base of the ridge formed of this limestone stratum. The water 

 percolating through the rock, carrying with it only the carbonate 

 of lime and leaving undissolved the oxide of iron, silica, and 

 other impurities of the stratum, must of course, in depositing its 

 calcareous particles, produce a material of great purit3\ This 

 deposit, called travertine, is usually in the condition of a yellowish, 

 porous, concretionary limestone, which burns into lime with great 

 facility, and yields a product of extreme purity and whiteness. It 

 usually collects near the base of limestone rocks, where copious 

 springs, highly charged with the calcareous matter of the stra- 

 tum, enter moist meadows or swampy grounds. It is of two 

 kinds, concretionary or stony, and pulverulent. The first is well 

 adapted for making into lime, or, when procurable in sufficiently 



