1874.] ZJ'V [Frazei-. 



seem to have been branches of one great basin during the Upper Coal 

 epoch. They were separated by a tongue of land tapering southwardly 

 and terminating in West Virginia, not far from the Maryland line. The 

 eastern basin rapidly lost its widrh, and near the union was quite narrow. 

 The relation between the two basins, as I understand it, is rudely repre- 

 sented in the accompanying figure, in which A, is the western, and B, 

 the eastern, which latter now contains the fragmentary areas of semi- 

 bituminous and anthracite coal. 



"Whether or not this division of the coal-field existed from the begin- 

 ning of the period, I am unable to conjecture, as my material respecting 

 the Lower Coil Group is not sufficient. But that it had occurred before 

 the formation of the Barren Group admits of no doubt, as that group has 

 a well-defined saucer-shape in the Great Trough, and thickens eastwardly 

 from the dividing area. In like manner the Upper Coal Group thickens 

 east and west from the same region, the Pittsburg-, Redstone and 

 Sewickly Coals being as well marked in the eastern basin as in the west- 

 ern. 



The eastern basin, as might have been expected, shows little limestone 

 amid its strata. Surrounded on all sides by highlands, it was fed by 

 numerous streams, which brought down sufficient detritus to render its 

 waters turbid throughout. Its mouth was obliterated topographically by 

 the final convulsions of the Appalachium Revolution, so that its precise 

 position is to be ascertained only by close exploration. 



The common basin, below the junction of these branches, was broad 

 and never completely filled with detritus so as to permit the marshes to 

 cross it ; certainly at no time after the formation of the Pittsburg in 

 that region. This bed cannot be traced across the basin, owing to the 

 fact that it is deeply concealed in the centre, but the Waynesburg and 

 Brownsville thin out rapidly toward the west, and in W^est Virginia, 

 have almost disappeai-ed before reaching the disturbed region known 

 as the "Oil- break." Limestones are almost unknown, and for four 

 hundred feet on top, the rocks are entirely sandstone and shale, all the 

 limestones and coals belonging to that horizon being absent. 



ON EXFOLIATION OF kOiDKS^EAR GETTYSBURG. 



By p. Frazer, Jr. 

 {Bead before the American PMlosopMcal Society, Dec. AtJi, 1874.) 



During an examination which I made of the Syenite bouldei'S whi ;h 

 compose that part of the battle-field of Gettysburg, called the "Devil's 

 Den," (a collection of great blocks of this rock piled one on another in 

 the wildest confusion and lying about j mile west of " Granite Spur " 

 or Little Round Top, the ravine where Vincent's Brigade held their 

 ground so manfully on the afternoon of Thursday, July 2, 18G3, and 



