628 GEOLOGY OP OHIO. 



It must, however, be observed, that this ledge, in the central and north- 

 ern portion? of the county, is not in immediate contact with the Wa- 

 verly black slate, but is separated from it by a varying number of feet of 

 soft shales and clays. The distance between the upper surface of the 

 slates and the quarry beds is sometimes as great as twenty-five feet, but 

 is generally less than twenty feet. This heavy ledge fails altogether in 

 the exposed sections of Pebble and Benton townships, its place in the 

 scale being there occupied by shales, seams of fire-clay, layers of concre- 

 tions incrusted with a thin scale of iron, together with many thin and 

 worthless beds of impure sandstone. The above enumeration will also 

 apply with sufficient exactness to the one hundred feet overlying the 

 Buena Vista ledges in the region east of the Scioto. The general com- 

 position of these clays and blossoms of ore is shown in the appended an- 

 alyses. Number 1 is a clay from Gregg's farm, between the black slate 

 and the brown stone; number 2 is a fair representative of the clays 

 that compose a notable part of the series for one hundred feet at least 

 above the black slate in Pebble township ; and number 3 gives the con- 

 stitution of the scale of ore that incrusts the concretions to which refer- 

 ence has already been made : 



No. 1. No. 2. 



Silicic acid 61.00 61.-10 



Alumina 22.25 22.79 



Sesquioxide of iron 5.55 4.81 



Lime 0.70 0.70 



Magnesia 1.40 1.40 



Potash and soda 3.90 2.75 



Water, combined 4.80 5.80 



99.60 99.35 



No. 3. 



Silicious matter 32.12 



Sesquioxide of iron 53.97 



Alumina 1.80 



Manganese 1.00 



Phosphate of lime 2.11 



Magnesia and sulphur trace 



"Water, combined 8.30 



99.30 



Metallic iron 37.767 



Phosphoric acid .*. 1.279 



The large amount of potash and soda in analyses 1 and 2, and the equally 

 unusual percentage of phosphate of lime in No. 3, are points of sufficient in- 

 terest to be noticed here. These are the substances upon which, more than 



