260 GEOLOGY OF OHIO 



feet in thickness. We bring to our interpretation of a coalfield, thus con- 

 stituted, the points already made, viz., that every seam was accumulated 

 by vegetable growth in swamp, or marsh, or bog near the sea level. 



It goes without saying that in the explanation of a coal field, subsi- 

 dence of the coal forming area must be invoked. The swamps are suc- 

 cessively buried under sediments brought in from the adjacent sea, or 

 more infrequently, by sheets of limestone that grew in the invading 

 water. 



The northern extension of the Appalachian coal field, as it is found 

 in Ohio, is by far the most orderly field that has ever been described. 

 There is a regularity and simplicity of structure in it that makes it the 

 type and standard for this whole class of formations. The questions con- 

 nected with coal fields are found here in their simplest form. There 

 are no folds in its strata, and very few arches, and these few are low and 

 gentle. There is not, within the limits of the field, a fault that deserves 

 the name. Its reliefs are almost wholly due to erosion. The dip is re- 

 markably stead} r and uniform, and inasmuch as it very rarely rises as high 

 as one degree, it can be determined only by triangulation. The series 

 that is found on one side of a hill can be depended on with like intervals 

 and relations on the other. The general order of its most important coal 

 beds has long been known, the great economic interests involved leading 

 to their early and continuous exploitation on a large scale. 



What, then, do we find in this, the simplest and most symmetrical, 

 the least disturbed and complicated of all known coal fields ? 



We find a maximum of two thousand feet of strata covering ten 

 thousand square miles of surface. 



Of what does this series consist? Irregularly distributed through 

 it, but most largely in the lower portion, are found six or eight strata of 

 sandstone that can be followed with a good degree of steadiness through- 

 out the field. They have their names and places in the scale. Part of 

 them are conglomerates, characterized by the presence of quartz pebbles, 

 which sometimes are of large size and form the bulk of the stratum. 

 These sandstones and conglomerates constitute the largest single element 

 of the series. 



Next to them in aggregate thickness are beds of shale, frequently 

 replaced with sandstone layers, the shales being gray, blue, black or red 

 in color, and gathered in the largest quantity, in the upper half of the 

 Coal Measures. 



There are a dozen sheets of limestone distributed irregularly through 

 the entire series. All of them are thin. They are often to be measured 

 by inches instead of by feet, but they hold their places in the scale with 

 wonderful tenacity. Half of them are of marine origin, as is clearly 

 shown by the abundant fossils which they contain, and, on the same 

 grounds, the rest are found to be of fresh or brackish water origin. 

 Part of them are underlaid by seams of coal and others of the series 



