REPORT OF THE DIRECTOR I916 201 



fourth district, as well as elsewhere in this group; having from my own 

 observation detected them in other parts of New York, in Pennsylvania, 

 Ohio, and even to some extent in Indiana. 



These casts are elevated lines or ridges upon the surface of the stone, 

 varying from the size of the usual scratches upon the present surface of 

 the strata to the diameter of half an inch, and even one, two and three 

 inches, and in one case I have seen a specimen six inches in diameter. 



The only assignable cause for these ridges is the action of a current flow- 

 ing over the surface of the strata, sometimes transporting sand and at other 

 times coarser materials, which furrowed the surface upon which the sub- 

 sequent deposits were made. They are, in all cases, preserved upon the 

 under surfaces of sandstone or shaly sandstone layers, which rest upon soft 

 shale, so that the furrows or scratches must have been made in this mud. 

 They are not all confined to one position, but appear at different depths in 

 the group ; showing that the cause, be it what it may, operated through a 

 long period, and in a pretty uniform manner. The ridges are never curved 

 or bent on one side ; and though two sj'stems are sometimes observed cross- 

 ing each other, they are still as well defined and their course as unbroken 

 as in the glacial or alluvial scratches upon the surface of the present rocky 

 strata. 



From the frequent occurrence of these, and their continuation through a 

 great thickness of strata, we can hardly suppose the furrows to have been 

 made upon hard surfaces ; and if we suppose the mud in which they were 

 made to have been soft, it seems almost impossible to conceive how they 

 should be preserved. Still the numerous similar facts in other rocks prove 

 that even the most delicate markings are preserved, under even more un- 

 favorable circimstances. The tracks of birds and reptiles in the New Red 

 sandstone, with the impression of rain drops, is equall}^ difficult to compre- 

 hend, were it not demonstrated beyond all question. Again it has been 

 shown that in the Medina sandstone^ the delicate wave lines and the minute 

 ridges of sand, piled up before some little obstacle in the current, are pre- 

 served with the same integrity as they appear upon a sandy beach just left 

 to dry by the receding tide. In the same manner, the evidence of these 

 slight scratches and deeper furrows in the mud of the Portage group have 

 been preserved in the casts formed by the succeeding depositions. Nothing 

 can be more clear and convincing than the proofs, and nothing more beauti- 

 fully illustrative of the effects of oceanic currents upon the bottom. We 

 have little space here to describe these phenomena, and they should be seen 

 to be fully appreciated. This fact, however, may be added to the number, 

 if we have not already sufficient, to prove the condition of the sea in these 

 remote periods. 



The following woodcut represents a surface where three systems of 

 ridges appear, or where the grooves were made in three directions. 



This account is wise and safe ; indeed may well be regarded com- 

 petent so far as it goes. This phrase in the quotation may be empha- 

 sized, " they should be seen to be fully appreciated " — a characteri- 

 zation which in part justifies and leads to the present presentation. 



1 Pages 52 and 54 of the Geology of the Fourth District of New York. 



