656 GEOLOGY OF OHIO. 
SECTION II. 
THe Cuay Deposits or OHIO. 
The clay deposits of Ohio may be separated into two well-defined 
divisions, viz., the clays of the drift, and the far older group that is 
found interstratified at various horizons among the bedded rocks of the 
State. In geographical distribution, these two divisions are quite dis- 
tinct, the former having its chief developments throughout the northern 
and western portions of the State, and the latter throughout southeastern 
Ohio, but they overlap to some extent upon the boundary. 
The drift region of Ohio, as shown in a subsequent chapter, is 
bounded on the south by a sinuous line entering the State on the south- 
east in Columbiana county, and striking the Ohio river in Brown 
county. There are then in the south and east of the State twenty-one 
counties in which no upland drift is found. 
DRIFT CLAYS. 
The geological character of the drift is well known. It consists of 
deposits of sand, gravel, boulders and clay in no regular order. The 
item of interest to us is the occurrence of the large body of clays, which 
though of poor quality, as a rule, still play an important part in our 
economic geology in many ways. ‘The clays occur in shallow surface 
deposits, accompanied by alternating sands and gravels. 
They are distinguished as boulder clays and stratified clays, as their 
structure varies; only those of the latter class and of homogeneous 
structure, free from boulders and debris, are suitable for use. Valuable 
‘deposits seldom reach a thickness of ten feet and often have a thickness 
of only two or three feet, but in this thickness extend over acres of 
ground. Such as are fit for use are plastic, yellow or blue clays, which 
work very kindly with but little preparation.’ They are all impure, 
containing from 3 to 6 per cent. of iron or even more, from 3 to 12 per 
cent. of lime and magnesia and notable amounts of the alkalies. They 
are easily fusible, usually to a black glass; when burned but not hard 
