426 CASTS OF MUD FURROWS, WAVE LINES, &C. 



In a few instances I have found these ridges or casts covered 

 with sliells, which apparently were floated over the even surface, 

 and lodged in the furrow previously made. These have adhered 

 to the next deposition, and are now found attached to the lower 

 surface of the same. 



PI. XVIT, fig. 3, represents the section of a thin slab of this 

 sandstone with one of these semicylindrical ridges upon its under 

 side. On the most convex part of the ridge, at «, the surface is 

 covered with small fossil shells, while the plane surfaces on either 

 side b b, are entirely free from them. Tins would se^m to indi- 

 cate that the shells were floated into the furrow or depression 

 previously existing, and there remained, the current not having 

 power to remove them, while it swept all from the even surface 

 to similar situations. 



Such are, briefly, a few of the facts attendant upon these ridges 

 or casts of mud furrows. I have not been able to trace any thing 

 like organization in any of them, and their variable character, as 

 respects form, size, and general symmetry, are strong arguments 

 against their having had such an origin. While, on the other 

 hand, their uniform direction, and parallelism, so essentially dif- 

 ferent from the fucoides, indicate a force that operated in one 

 general direction, varying sometimes to the amount of a few de- 

 grees. These ridges are sometimes much larger at one end than 

 at the other, as if the furrow which they filled was made by a 

 heavy body grounding, and then being gradually raised and 

 moved more lightly over the surface, and finally the impression 

 running out entirely. In some of these casts the peculiar frac- 

 tured character is apparent, as if the force making the groove had 

 a tremulous motion. The same is seen, in some degree, in the 

 more recent diluvial or glacial scratches. These appearances have 

 been traced from New York as far westward as Indiana, and all 

 possessing the general characters here described. At the same 

 time, fucoides frequently accompany the same strata, but these 

 can usually be distinguished from the casts. 



The numerous fragments of fossils and drifted shells which 

 accompany the same strata in many instances, sometimes form- 

 ing layers of themselves, together with other circumstances, indi- 

 cate a shallow ocean or a littoral position. 



