UPON ROCKS OF THE NEW YORK SYSTEM. 423 



upon the surface of strata have been produced. Aside from these, 

 however, are forn^is somewhat resembling them, which bear no 

 marks of accretionary force ; and if due to that action, it must 

 have operated very differently from the same force in other well- 

 defined instances. Those to which I now allude, present all the 

 appearance of fluid mud ; of such a tenacity that the current was 

 broken, presenting several small streams, much resembling flow- 

 ing cinder from an iron furnace. The surface often presents a 

 series of interrupted semicylindrical ridges or corrugations, one 

 behind the other, which at first view strongly impress one with 

 the belief that they are due to some fluid body of a considerable 

 degree of consistence. 



This peculiar appearance sometimes covers only a small por- 

 tion of a slab of rock, where it thins towards one side, while in 

 others the whole surface is covered, giving the aspect of a body 

 cast in mud, which had been irregularly scooped out by the ac- 

 tion of shallow currents. Similar appearances are often seen in 

 flat beaches composed of mud and sand, over which a stream of 

 water spreads during the ebb tide. The whole surface, in such 

 cases, is scooped into little circular hollows, communicating with 

 one another by narrow depressions. If a deposition of sandy 

 mud could be made upon such a surface after it had become 

 sufficiently dry, its lower surface would present all the appear- 

 ances here described as seen in many of the higher strata of New 

 York. That such a condition may have existed, is demonstrated 

 by other phenomena, -^vhich are proved to have happened at in- 

 tervals, when the matter already deposited was near, or above 

 the surface. 



* The sketch, PL XVII, fig. 1, represents one of these surfaces near 

 the thinning edge of a sandy stratum,* and is one of the common 

 appearances in the Portage group. This form, under many modi- 

 fications, prevails extensively not only in the higher groups of 

 New York, but in specimens which I have recently seen from 

 the Connecticut River Valley, and which, Prof. Hitchcock in- 



• For an analoeoas recent production, see Geol. R.eport of Massachusetts, p. 348, 

 fig. 59. 



