2G4 THE POLISHED KOCKS OF ROCHESTER. 



of Ihe specimens will show : so that if successive layers could be 

 cloaivd off, we should have so many ensjraved plates. Just so, 

 wh(>n we aetually eh'ave oil' the layers of rock, once mud, lying 

 above that layer on \\ hitli the track was oriii;inally made, we get 

 a track upon each layer; the successive tracks u))ward, however, 

 becoming less and less perfect, until in the vertical distance of one 

 or two inches, often less, they are entirely filled up. The vertical 

 distance through which an impression may be contiiuied in plastic 

 matt>rials, will depend upon the degree of plasticity, and the ra- 

 pidity with which they harden. Hence, as copper is entirely 

 dissolved until the moment of deposition, and is then instantly 

 made solid, we should expect it to exhibit this eflect in an eminent 

 degree, nuich more so than mud. And the specimens before us 

 fully confirm this expectation. They are, therefore, valuable in 

 illustrating a point wliich has been a stumbling block to some in 

 respect to fossil footmarks. 



On the Polished Rocks of Rochester, N. Y. By Rev. 

 Chester Dewey, of Rochester. 



The " Glacial Theory " proposed by Prof. Agassiz seems well 

 fitted to account for the natural polishing of rocks. It does not 

 appear essential to that theory, that there should be mountains or 

 hills, but only a belt of ice of sullicient depth and magnitude, and 

 a suitable climate, to produce such results, as are now found to 

 be taking place under the glaciers of the Alps. As the masses 

 of ice do not move or descend by gi'avitation, no deep or steep 

 valleys are necessary. We need only the mass of ice. Such 

 glaciers formed in a level, or nearly level country, would have no 

 moraines on the sides or middle, but only in front, or would 

 have only terminal moraines, from the carrying forward of the 

 sand, gravel, and loose blocks on the surface of the rocks. There 



