154 GEOLOGY OF OHIO. 
three grades from many. ‘These are respectively designated lump or 
round coal, nut coal, and slack. ‘The first commands the highest price, 
the last is often valueless, but within the last few years, the slack of — 
certain seams has come into demand for various purposes, and a large 
quantity is now finding its way to market. From the slack is also 
derived the grade of coal called pea coal, which a few mines are 
furnishing. 
The process of screening consists in passing the coal as it comes 
from the bank cars over one or more inclined screens on its way to the 
cars, boats, or wagons by which it is to be transported to market. 
There is no general system in force in the process of scieening. The 
length and width of the screens, the angle or pitch at which they are 
placed, the space between the bars, the width and shape of the bars, all 
these vary between wide limits in the different portions of the field, 
and even in different mines in the same district. The miner is paid in 
most of the districts on that portion of the coal only that passes over 
the screen, or in other words on lump or round coal, which is also called 
clean coal. The varying characters of the seams, the differences 
between different mines in the same seam, and different portions of the 
same mine, all these elements combine with the facts already noted as 
to the varying dimensions of the screens to make the question of wages 
for mining a complicated and troublesome one. ‘The screens are a 
fruitful source of discord between the mine operator and the miner. 
There is a rapidly growing appreciation of the second grade of the 
coal product, i. ¢., the nut coal. This grade results quite largely from 
the operation of “bearing in” upon the coal seam, or undermining it. 
In a large number of instances, the best portion of the seam occurs in 
the “bearing in” bench, and thus the nut coal often contains the 
choicest fuel that is produced from the seam. The lump coal must 
generally be reduced to smaller size before it can be used, but still the 
popular demand has hitherto been for large coal, and great quantities of 
the smaller sizes have been utterly wasted. The arrest of this waste, 
and the bringing into use of all the products of the mine are steps of 
great interest to the State at large. 
The nut coal is run with the slack in some fields, enriching this 
latter element so as to make it a fully marketable product, which still 
goes under the name of slack. 
Within a few years, two new departures have been made in the dis- 
