96 GEOLOGY OF OHIO. 



a depth of water sufficient to float large icebergs loaded with the rock 

 fragments, and when the muck bottom, long soaked with water, was a 

 soft, oozy, carbonaceous mud, into which a rock, when falling, would be 

 deeply buried. They are altogether too superficial to admit of the con- 

 clusion that they were thus deposited. They are also all of peculiar 

 shape, none of them approaching a round or spherical form, but are com- 

 paratively thin, with broad, 'flat surfaces. My conclusion is, that they 

 were lifted from the bottom of the old marsh by the growth of plants, and 

 that the carbonaceous soil accumulated beneath them. The modus 

 operandi was probably this : the roots and rootlets of trees and shrubs 

 once spread in a complete net-work on the surface of the bowlder-clay, 

 and insinuated themselves under all the flat rocks resting upon it. As 

 they increased in size, these roots and rootlets slowly lifted the blocks of 

 stone, making the intrusion of other roots more easy, all by their ultir 

 mate decay leaving a thin layer of humus between the rock and its old 

 bed. No roots would pass over it to bind it to its plac«, but their com- 

 bined growth and decay beneath it steadily lifted it upward, so that the 

 accumulation of vegetable debris under it kept pace with -that in other 

 parts of the swamp. Spherical or rounded rocks were probably too 

 deeply and firmly imbedded in the clay to permit the roots to insinuate 

 beneath them, and so lift them from their beds, and no such forms are 

 found upon the surface. 



The brick and stone sidewalks in our cities and villages, when they 

 surround trees, are lifted above the general level by the same means ; 

 and a tree planted upon a level lawn will in years become the center of 

 a gentle mound, not by the accumulation of material upon the top of the 

 soil, but by the roots lifting the turf upward through the steady pressure 

 occasioned by their growth. 



Since these observations were made, and this conclusion reached, my 

 attention has been called to a brief notice of two papers by Mr. Thomas 

 Mehan, read before the Philadelphia Academy of Sciences, on the " Influ- 

 ence of the Growth of the Roots," and the "Action of Frost in Elevating 

 the Trunks of Trees." There can be little doubt that the trunk of a tree 

 may be, and often is, lifted bodily upwards by the growth of the roots 

 when they rest upon a rock surface, or any material as unyielding as 

 bowlder clay, and that frost may aid somewhat in this elevation ; but 

 my observations do not tend to the conclusion that the top root serves 

 in any respect as an anchor to resist or counteract the action of the frost, 

 but that it aids very materially in its action. Seedling trees of the first 

 year are often lifted entirely out of their beds by the action of the frost, 

 and from this cause very many perish the first spring after planting. 



