72 [A; 



SSEMBLY 



argillaceous red sandstones, seen in the northern counties, and 

 generally around the edges of the different coal-fields (especially 

 in the Sharp mountain south of Pottsville, where it has been so 

 - uplifted that its layers stand perpendicular in the walls of the 

 valley) ; but when traced southward through Virginia, is there 

 found to embrace near its middle several hundred feet of grey 

 and brownish limestone, which also extends far to the west, and 

 is known as the Carboniferous limestone. The St. Louis and 

 Keokuk limestones of Missouri and Ix)wa are part of the western 

 extension of these rocks. These limestones abound in fossils ; 

 but the formation in Pennsylvania (where it is all red shale and 

 sandstone), contains scarcely a shell or other fragment of any 

 living form. It, however, bears traces of life of a different 

 nature ; for therfe have been found on the surfaces of some of its 

 layers, which doubtless once formed a beach left bare by the 

 tide, tracks or footprints made by some fourfooted animal which 

 had walked over them while yet soft. The animal which left 

 these prints was in all probability a large lizard, or some similar 

 reptile ; and there are, between the right and left lines of the 

 tracks, obscure furrows, as if made by its tail dragging on the 

 soft mud or sand. These tracks are two or three inches in diame- 

 ter, and show five toes upon each foot. The layers on which 

 they are found often show little dots or pits scattered thickly 

 over them, which appear to have been the marks of heavy rain- 

 drops falling on the slimy sand when fresh and soft; and in 

 some instances they are intersected in all directions by small 

 cracks, just such as are at this day formed on muddy shores by 

 the drying action of sun and wind. These footprints are of the 

 greatest interest; for they are the oldest vestiges to show the 

 existence of any form of life higher in degree than fishes — and 

 thus the oldest relics of air-breathing animals. 



This ** Umbral Red shale " is covered by another thick series 

 of conglomerate strata, called by Prof. Rogers the " Seral con- 

 glomerate/' It is a gray and whitish conglomerate, in massive 

 beds alternating with gray sandstones. It is 1100 feet thick in 

 the Sharp mountain south of Pottsville, and often contains one 

 or more thin seams of coal ; being the lowest position in which 

 any considerable quantities of that mineral have yet been found. 



This is the base of the " Productive Coal-measures," as the 

 strata, containing workable layers of coal are called. These are 

 made up of thick beds of sandstones and black slate, with which 



