CLAYS, THEIR ORIGIN, COMPOSITION AND VARIETIES. 57 



It is not a simple or homogeneous formation, but consists of alter- 

 nating bands differing slightly from each«other in composition. The two 

 leading types included in the formation are a black shale and a greenish- 

 blue shale, but several other varieties are occasionally met. All of them 

 are highly silicious in character, but the black shale contains eight to ten 

 per cent, of organic matter that gives to it its color, while the greenish- 

 blue variety derives its characteristic tint from silicate of iron. The 

 black shale is found at the bottom and generally again at the top of the 

 .formation, while the blue shale characterizes the middle of the series. 

 But no three-fold division based upon the facts above given can be any- 

 thing but misleading, for the reason that there are more likely to be thirty 

 or three hundred alternations than three. It is, however, true that beds 

 of black shale are almost certain to be found at the bottom of the series, 

 while at the middle of the column more or less blue shale is present. 

 The alternations are very frequent in this last named division, as a rule, 

 as is shown in a multitude of well records. 



There is but one point in the state where large use has been made 

 of this series, viz., Columbus. It has been here employed in the manu- 

 facture of sewer pipe of thoroughly approved quality and also of cqm- 

 mon and of building brick in the. large way. In both cases it is the 

 greenish shale, or some variety of it, that has been brought into requisi- 

 tion. The ppssibilities of the formation have scarcely been touched as 

 yet, however. There will, without doubt, be found large uses to which 

 parts of it will be found well adapted. 



The Bedford shale (No. 11a of the scale) is the next argillaceous de- 

 posit in our series that deserves mention. It cannot in all cases be dis- 

 tinguished from the underlying series that has just been described 

 Where the latter, for example, has the blue color that occasionally marks 

 it, and where the Bedford also has the same color, no line of demarka- 

 tion can readily be drawn between them. The only means of distinguish- 

 ing the two series in such cases would be by noting the fossils that they 

 severally contain; but the characteristic fossils of each are few in number 

 and uncertain in occurrence, at least in the great majority of its outcrops. 

 A ready means for the determination of the Bedford shale is, however, 

 at hand in most instances. It is generally a red shale and is thus sharply 

 distinguished from the hundreds of feet of shale below it that are really 

 continuous with it, and also from the great series that overlies it with 

 but one or two interruptions in the way of sandstones which occupy the 



interval. 



The Bedford shale can therefore be described as a stratum twenty to 

 eighty feet thick, red or light blue in color. In the great majority of in- 

 stances more or less of it is red, while bands of grey or blue shale are 

 sometimes intercalated. This color is its characteristic mark and by 

 means of it the stratum has been proved a very persistent one, having 

 been followed under hundreds and even under thousands of feet of cover 



