UPON ROCKS OF THE NEW YORK SYSTEM. 425 



PI. XVII, fig. 2, represents a specimen of the kind last de- 

 scribed, where the surface presents slight ridges, as if the deposi- 

 tion had been made upon a surface previously hardened to a 

 considerable degree, and which was grooved and furrowed like 

 the surfaces of our present rocks. It is not usual to find grooves 

 in two directions, as in this instance. 



These appearances cannot be explained by supposing a mo- 

 tion in the mass, by which one part was made to slide over 

 another, forwe find them at about the same position in the rock, 

 at intervals,, over a distance of twenty or thkty miles, and it must 

 be remembered that the strata are almost horizontal, and quite un- 

 disturbed. The different degrees of hardness in the two masses 

 in contact would prevent any such conclusion, for the lower one 

 is too soft to impress the upper, which is the one presenting the 

 marks. The aspect of the surface is Likewise different from a 

 grooved or striated one, the lines being in relief, lying in corre- 

 sponding depressions in the shaly stratum below. 



Those appearances to whiclrl have strictly applied the term, 

 casts of mud furrows, are straight or sometimes slightly deflected 

 longitudinal ridges, which are found upon the lower surfaces of 

 strata. They appear to have been originally furrows made in the 

 mass beneath, while in a partially indm*ated condition, and filled 

 with the subsequent deposit, which has preserved their form and 

 inequalities. Some of these are deep and narrow, as if made by 

 a heavy body dragged over the surface ; others appear due to a 

 jagged or rough-pointed surface, impressing the stone or mud, and 

 the cast presents all these appearances. 



These ridges have all a uniform direction, varying but littie 

 from northwest and southeast. This uniformity occm's in localities 

 widely separated from each other, indicating some cause which 

 has operated over a large area, and in a uniform manner. Of- 

 tentimes several of these ridges are seen upon the surface of a 

 single slab, and, except in rare cases, are always parallel to each 

 other. In a few instances I have seen several of these parallel 

 ridges crossed by a single one at an angle of about thu-ty degrees, 

 its course as regular and well defined as the others. In no case 

 have I observed them upon the upper surface of the strata. 

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