THE UPPER BARRIER WAS BROKEN. 353 



It is Constantly dropping off or wearing away. Its fallen 

 portions constitute a part of the loose gravel through 

 which the traveller labours, beneath. This substance 

 yields to mechanical, and chemical agency more readily 

 than the harder strata which it supports. It therefore un- 

 dergoes excavation, while the superior and firmer strata 

 of limestone project and overhang, until they break off 

 by their own weight. Owing to this abrasion or decay 

 of the brittle shistus, the calcarious rocks above, jut far 

 beyond their present base, and threaten him who takes 

 shelter below them. Masses of various sizes, from small 

 stones to rocks of many tons weight, have fallen from the 

 summit thus undermined, and now occupy the space at its 

 foot. As the excavating or undermining process goes on, 

 other pieces will be detached, and the chasm be propor- 

 tionally enlarged. This foundation of slate is of vast ex- 

 tent in these parts of North America. Shistus emerges 

 from the strata of granite on the banks of the Hudson, at 

 Newburgh and Fishkill, and underlays the limestone to 

 the northward of both; as well as the sandstone of the 

 Catskill mountains. Travelling north,' it shows itself 

 again at the water-falls in the neighbourhood of Albany, 

 and at the village of Waterford. The same kind of ar- 

 gillaceous slate prevails beyond Stillwater to the falls of 

 Fort Edward, and supports the limestone over which the 

 Hudson at Glen's falls is precipitated. Turning west- 

 ward, the s'histus, over which the Mohawk river falls at 

 the Cohoez, is covered with granitical stones and rocks 

 from Schenectady to Palatine ; and at the latter place, 

 by a limestone that is replete with petrifactions. At the 

 little falls where the lock-navigation has been opened, 

 huge strata of a compact, striated, dusky and ferruginous 

 quartz conceal it. At Utica a coarse, granulated, sili- 

 ceous sandstone overspreads it. But at Oriscany the 

 siate again makes its appearance, and continues until the 



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