514 STEVENSON— THE FORMATION OF COAL BEDS. [Xov. i. 



there is evidence of marine life, may be the measure of the extreme 

 possible depth at which the Ames limestone was deposited. Indeed 

 one is tempted, in view of conditions observed on islands in mid 

 ocean, to suggest that the abrupt appearance of muds above the 

 Ames might be regarded as evidence that the limestone was deposited 

 in very shallow but very clean water ; and the temptation is the 

 stronger, because stream-sorting appears in the arrangement of 

 materials composing the overlying shale. 



Were these limestones deposited in deep water? The testimony 

 of the fossil remains in answer to this question will be examined 

 on a succeeding page ; but there are several matters to be considered 

 here. In some cases, the limestones can be followed to their disap- 

 pearance. The Vanport limestone in Pennsylvania ends in long 

 prongs within Pennsylvania, gradually passing into sandstone, debris 

 from the sides of the valleys in which the limestone terminates. 

 And fossils continue after the change has begun. The western edge 

 of the Ames limestone was reached by Condit in Meigs county of 

 Ohio, where that rock, still fossiliferous, is rippled and is con- 

 glomerate with quartz pebbles. This condition is not unknown in 

 other limestones of the Carboniferous; if verbal statements of geol- 

 ogists, who cannot recall localities, may be accepted, the condition 

 is familiar. The writer is indebted to Butts^"* for specific instances 

 in earlier formations. Shrinkage cracks and wave marks are abun- 

 dant in marine fossiliferous rocks of Stones River and Black River 

 age in southern Tennessee, where the deposits, he is convinced, are 

 of shallow water origin. In the Cahaba valley of Alabama, on the 

 border between the Bessemer and Montevallo quadrangles, he dis- 

 covered at the base of the Stones River (Chazyan) limestone, a 

 pebbly bed, which was examined in an area of several square miles. 

 At one locality, the pebbles are comparatively few and occur in a 

 layer never more than 2 feet thick ; but elsewhere the vertical dis- 

 tribution is much greater, becoming 20 feet, in which fragments 

 are abundant, while small pebbles occur throughout the higher por- 

 tion of the bed. The pebbles are of c|uartz, quartzite and chert, 

 varying in size from three fourths of an inch down. ]\Iany are 



"" C. Butts, letter of June 21, 1912. 



