PORTAGE GROUP, 
235 
ridges of sand, piled up before some little obstacle in the current, are preserved with the 
same integrity as they appear upon a sandy beach just left 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 beautifully illus¬ 
trative 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, how¬ 
ever, 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. 
