658 GEOLOGY OF OHIO. 
in the cut of a railroad under twenty feet of superincumbent dirt. It 
makes a beautiful cream-colored brick of fine, even, homogeneous frac- 
ture, and fit to be used in connection with the bricks from which our 
ornamental city fronts are built. 
The search for, and mining of, drift clays is of the most primitive 
kind, as they lie on the surface and are cut in every well. Mining is 
nothing but digging in low pits, taking everything as the work pro- 
gresses. The clays are oftenest used in close proximity to the diggings. 
BEDDED CLAYS. 
The clays which constitute bedded formations present a very 
different appearance from those of the drift, and have little in common 
in character. They may be divided into two groups, Coal Measure 
clays and the lower clays. This last division is made to include the one 
or two exceptions to the first group which have been noticed. In south- 
western Ohio, there is an occasional development of a clay between the 
Lower and Upper Silurian rocks. In one instance in Miami county, 
this has been made use of for making drain-tile and the like. The other 
notable ease is found at Columbus, where the decomposed top of the 
Huron shale deposit is utilized in making sewer-pipe. 
The geographical extent of the Coal Measures is bounded by a line 
somewhat similar to that which bounds the drift. Included within 
and cut by this boundary are thirty-three counties, but not more than 
twenty-two hold any large measure of mineral wealth. Asis well known, 
every coal vein carries its bottom clay. This at least is the rule, and 
the exceptions are rare. Ina few cases, coals have a slaty or shaly 
floor, but this is so nearly clay in nature, that the exception is only 
apparent. In England frequently, and in Ohio in a few cases, a hard, 
fine-grained sandrock called gannister, containing 98 yer cent. silica 
and 1 per cent. alumina, underlies a coalseam. Clays without the coals 
are much more common than coals without clays, and the presence of 
clay alone usually serves to mark the horizon of an absent coal. Lime- 
stone and ores are often also associated with constant deposits of clay. 
Some one has said, that the aggregate effect of all the coal deposits 
of the State would hardly be enough to make one steady, regular seam 
of moderate thickness over all the coal measure area. Our clay de- 
posits would be perhaps equal to a rather heavy vein of clay over the 
same territory ; heavy in proportion to the coal at least, for it has much 
less range in thickness and does not share largely in the fluctuations in 
