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- 

 ren 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 firina 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 

 ia 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 European 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; 



