1874.] ZJO [Stevenson. 



rivers. We may compare each group to an enormous bowl, somewhat 

 elongate and with tiattened base. 



At the beginning of the upper coal era this trough was a great arm 

 of the sea, closely land locked and communicating with the ocean at the 

 southwest by a comparatively narrow outlet. On the east and southeast 

 sides, rivers brought in their loads of detritus from the highlands to be 

 spread over the bottom, which gradually declined toward the west and 

 northwest. On the opposite shores few streams flowed out, and sucb as 

 came were sluggish, bearing no coarse material. The place of quiet, pux-e 

 water is marked by deposit of limestone in the north, while a similar 

 mass, traceable through Ohio south westwardly, marks the direction of the 

 outlet. The low shore of the southeast is marked by the shallow water 

 detrital deposits and the utter absence of limestones in West Virginia, 

 south from the N. W. Branch of the Ba,ltimore and Ohio Riilroad. 



The wedge-shape of the rocks intervening between the coals of this 

 group has been shown both in Pennsylvania and Ohio, east and west, as 

 well as in West Virginia, where the tapering is southeastward toward 

 that edge of tlie trough. The structure of the trough may be illustrated 

 as follows : 



Let a basin with gently sloping sides be lined with some black sub- 

 stance ; then filled with some material which will become hard, in wliicli 

 a similar black substance is arranged in layers, some of them covering the 

 whole surface, and others extending only part of the way from the border 

 toward the middle. Now break away the bowl, remove the black exterior 

 to near the base, at the same time cutting off portions of the hardened 

 mass around the border above, so as to give the whole an irregular sur- 

 face. Here we have a rude representation of the upper coal group, per- 

 haps as good as any that can be made on a small scale. If this mass be 

 divided vertically in two, the face of each piece will rudely resemble a 

 vertical section across the group from Harrison Co., Ohio, to the eastern 

 portion near the Pennsylvania and West Virginia line. 



In Ohio, Villa, VIII5, VIIIc and IX are traced directly to where they 

 have disappeared, while X and XI have been found successively approach- 

 ing VIII. In Pennsylvania similar conditions exist, but the extensive 

 erosion along the Alleghany slopes prevents us giving so full a presenta- 

 tion as that from Ohio. In each case we find the underlying Pittsburg 

 reaching farther east and west than the immediately overlying beds, and 

 continually approaching the higher ones, until, on both sides of the trough, 

 farther study is cut off by the completeness of erosion. 



I am, therefoi-e, compelled to believe that all the coals of the upper coal 

 group are off-shoots from one continuous marsh, loMch existed }rom the 

 beginning of the era to its close, and which in its full extent is now krioion as 

 the Piitsbu,rg Coal Seam.- During the whole time of formation of the 

 upper coal group the general condition was that of regular subsidence 

 interrupted by longer or shorter intervals of repose. During the time 

 of subsidence the marsh advanced up the sides of the trough, as new 



