GEOLOGICAL SCALE AND STRUCTURE. 23 



and in all sections the lower boundary of the formation is likely to be 

 uncertain, unless the bottom of the first bed of black shale found below 

 the Berea grit is in every case taken for the bottom of the Cleveland 

 division. If this is done, the Cleveland shale will be found to stand for 

 very unequal periods of geological time, as the uppermost black bed has 

 a considerable range in thickness. It often falls to thirty feet and some- 

 times extends to two hundred feet. It is probably the larger half of the 

 great black shale of southern Ohio. It is this element that proves most 

 persistent in the southerly extension of the black shale. The shale that 

 covers the Lower Silurian limestone in central Kentucky is the upper or 

 Cleveland division, as its most characteristic fossils, presently to be named, 

 prove. 



The mineral basis of all these shales, whether black, brown, blue or 

 red, is essentially one and the same thing, viz., afme-grained clay derived 

 from the waste of lands. As supplied to the sea basin, it was originally 

 blue or gray, but a small percentage of peroxide of iron goes a great way 

 in coloring such deposits red ; and in like manner, organic matter in 

 comparatively small amount gives them a dark or black color. The 

 organic matter that colors these shales was probably derived in large part, 

 as Newberry has suggested, from the products of growth and decay of 

 sea-weeds, by which these seas were covered like the Sargasso seas of 

 our own day. 



These organic matters seem to have accumulated along the shores 

 and m shallow water ingreater quantity than in the deeper seas. Hence, 

 ii the section of these shale deposits is taken near the old shore-lines, or 

 where shallow water occurred, a larger proportion is black than if the 

 more central areas are examined. The only land of Ohio at this time 

 was to be found in and along the Cincinnati axis, a low fold that had en- 

 tered the state from the southward at the close of Lower Silurian time, 

 and that had been slowly extending itself northwards through the suc- 

 ceeding ages. Southwestern Ohio was already above water, a low island "" 

 in the ancient gulf. But the shales on their western outcrop, where they 

 are largely black, are exactly equivalent in age to the alternating and 

 much thicker beds of black and blue shale, the latter being in large 

 excess, that were forming at this time in the central part of the basin, 

 viz., in eastern Ohio. The color of the shales is, from this point of view, 

 an accident, and cannot be safely used as a ground of division. The entire 

 shale formation that we are considering seems to have been laid down 

 without physical break or interruption. It must have required an im- 

 mensely long period for its accumulation. This is shown not only by the 

 fineness and uniformity of the materials which compose it, and which 

 could not have been rapidly supplied, and by the great thickness of the 

 formation in eastern Ohio, but also by the geological equivalent of the 

 shale in the general column of the country, which furnish even more con- 

 vincing proof as to its long-continued growth. The Ohio shale, as 



