l80 THE JOURNAL OF GEOLOGY. 



derstood in the same way, a "Coal Horizon" represents an even 

 more limited expansion, where coal forming materials have 

 accumulated. Practicall}^ it is one of the greater planes of sedi- 

 mentation, marking a distinct episode in the deposition of a 

 series of strata. Theoretically it represents not a phenomenon, 

 but rather a set of conditions, a period during which the physical 

 circumstances were similar over a considerable marginal portion 

 of a geological province. From an economic standpoint it 

 stands not for a continuous bed of mineral fuel, but a strati- 

 graphical level where workable beds are more likely to occur 

 than elsewhere, and where the coal is to be especially sought for 

 in a wide belt fringing a great coal basin. It is not to be in- 

 ferred, then, that the mineral is equally developed on a given 



Fig. I. Coal Horizon at time of formation; parallel to shore line. 



horizon in all portions of this marginal border. In some places 

 the accumulations of plant remains are much greater than in 

 others ; limited basins and troughs of unusual thicknesses are 

 there found. Elsewhere the old vegetable materials are mea- 

 gerly represented ; only thin seams of coaly matter are there 

 preserved. Wide intervals of sandstone and shale often separate 

 adjoining basins, or ancient land elevations may cut off one area 

 from another. (Figure i). Yet, through all of the many irreg- 

 ularities of deposition and subsequent deformation there are 

 nevertheless discernible, certain levels quite well defined at which 

 coal beds are very much better developed than at others ; clearly 

 marked coal horizons they are, broad in extent and capable, in 

 the case of the greater ones under favorable circumstances, of 

 being traced over a large part of a given Coal Measure province. 

 The coal may not be present in a continuous seam over the whole 

 border district and probably never is ; but along much of the 



