118 GEOLOGY OF OHIO. 
all known coal fields, and some which have come under our observation 
will be referred to in another place. 
In the shales which overlie the coal seams we very frequently meet 
with the casts of the bases of the trunks of trees which were once 
rooted in the coal. These are generally short, showing that the upper 
portions of these trees rotted away before clay and sand were deposited 
around them; but in one instance I have seen a trunk of Sigillaria which 
projected fifteen feet above the carbonaceous mass in which it grew. 
The sections of these trunks are frequently seen in the roofs of our coal 
mines, each traced by a circle of carbonaceous matter. These circles 
are sometimes called ‘“pot-bottoms” by the miners, and they are re- 
garded by them with some dread, as from the conical form of the stump 
it sometimes drops out, and falls with sufficient force to crush any one 
beneath it. The first layer of the shale above the coal is generally filled 
with the impressions of the trunks, leaves, and fruit of the forest that 
was growing over the coal marsh at the time of its submergence, while 
the superincumbent layers of shale and sandstone may be entirely bar- 
rev of plant impressions. 
The alternation of sheets of vegetable matter with rooted trunks and 
other indications of the growth of a sheet of vegetation on a land sur- 
face, with layers of limestone full of marine shells, may be accepted as 
conclusive proof of great and repeated changes of physical condition in 
the area of our coal basin, and we may generally find evidence that these 
changes were produced by elevations and depressions of the bottom of 
the basin. The number of such alternations, however, is so great that 
some persons have found it difficult to believe that so many oscillations 
of level could have taken place in our terra firma during one chapter of 
geological history. It should be remembered, however, that the lapse of 
time recorded in our Coal Measures would, if expressed in years, be 
almost infinite as compared with the epochs of human history. We 
know, too, that warpings of the surface are now constantly taking place 
in all parts of the globe, and though accomplished so slowly that they 
are scarcely perceptible to our observation, evidences of recent changes 
of level have been gathered from many localities on the margin of our - 
own and the Kuropean continent. The shores of the Mediterranean 
afford many examples of local elevation and depression. The coast of 
Sweden is known to be now slowly rising, but most toward the north, 
and Lyell makes the average rate of movement four feet in a century. 
On the North American coast similar local changes are going on. In 
Greenland a slow subsidence is taking place; at St. John’s, in New 
Brunswick, the land is rising; sinking at the island of Grand Manan ; 
