Geography of Ohio 177 



swamps existed. The coast was not regular, but arms of the sea 

 alternated with peninsular-like projections of the land. Between 

 these areas of land a swamp condition existed. These flat marshes 

 sometimes gave place to deeper water, as the coast, or as the land 

 drainage, shifted. When this happened, vegetation was killed 

 by the sediments brought into the water. As the shallow condi- 

 tion was again produced, the proper habitat for plants appeared, 

 and the marsh condition once more prevailed. If, for any reason, 

 the depth of water became sufficient, sea life appeared, and their 

 calcareous remains developed limestone. Thus, we have in the 

 rocks of this period two forms of sediment, organic and clastic; 

 the two types of the organic, plant and animal. It should be 

 borne in mind that on all sides of these bays or arms of the sea, 

 this condition prevailed. Obviously, then, the area of the bay 

 progressively decreased. The time finally came when the bay was 

 converted into one great marsh, representing some eight thousand 

 square miles, the area of the Pittsburgh coal seam. 



The origin of coal. All are agreed on the plant origin of coal. 

 It is further generally understood that where the coal is found 

 now, there the plants grew. In a few cases, the coal may represent 

 the assembling of plant remains by streams; wherever this hap- 

 pened, the fact is seen in the macerated condition of the plant 

 remains. 



Coal seams almost invariably overlie clay beds which contain 

 rootlets of the plants that made the coal. This is usually termed 

 fire clsiy. It probabh^ represents the soil in which the plant growth 

 started. Tree stumps, in an upright position, are also found 

 associated with coal seams. Coal seams contain fossils of plants, 

 embodying practically all parts of the organisms, leaves, fruits, 

 bark, pith, wood, and, as seen under the microscope, spores. 

 It is very seldom that any one plant exists so completely in the 

 fossil form as to show all of these parts. 



Many people understand that there was only one coal-making 

 period in the earth's history. This is a mistake. During several 

 of the geologic periods coal beds were formed. We have every 

 reason to believe that coal beds are being formed even to-daj-; 

 and this possibility aids us in unraveling the conditions that must 

 have obtained during the Pennsylvanian period, the greatest of 

 the coal making periods. In many parts of North America swamps 

 with abundant vegetation may be studied now. Along the Atlan- 



