74 ANNUAL REPORT 
because as a rule these clays are finer grained. Such a clay is the Roseville 
stoneware clay in which the percentage silica alumina ratio is 3.63:1. The 
magnesia in most of these clays is low and the only possible detrimental 
constituent to be considered would be the concretionary iron sulphide, 
which is injurious both on account of its irregular distribution and the 
sulphur it introduces. 
Physically, these clays are rocky and hard when mined, but after 
being exposed to the weather for a short time they slake and crumble down. 
Their plasticity is developed only by grinding and working. 
The principal geological horizons furnishing these clays are the Lower 
Mercer, Upper Mercer, Brookville and Lower and Middle Kittanning. 
Especially prominent is also the horizon of the Putnam Hill limestone. 
Shales.x—Geologically, we speak of shales as being, as a rule, fine 
grained clays, deposited in still water and later subjected to pressure, thus 
giving them the characteristic cleavage and structure. If this is carried 
further the shale becomes slate, and 1f another agent, heat, is introduced the 
metamorphism may be carried still further and the clay reverts to an ig- 
neous rock. The term shale thus stands as a guarantee, so to speak, of 
fineness of grain and also uniformity of composition, but does not specify 
the chemical composition. We may have shales high or low in refractory 
power, high or low in fluxes, though each is tolerably uniform within a 
reasonable area. We must hence keep in mind that the term shale stands 
for no specific chemical composition, but has only a geological meaning. 
Alununous Shales.—Vhere is no reason why fire clays should not as- 
sume a shale structure. The most refractory clay known is a shale, the - 
Rackonitz shale clay. Ohio has buff burning clays with shale structure 
which in composition correspond about to the number two fire clays, some 
of them being reported from near Logan. But what has been said con- 
cerning the fire clays applies also here and we may dismiss this class of 
clays as being too high in alumina. 
Ferrugimous Shales.—Shales high in ferric oxide are plentiful in 
Ohio and may give rise to a valuable source of cement material. The 
chief advantage of shales is, as we know, the great fineness of grain and 
uniformity of composition, but it has as its great practical disadvantage 
the fact that in the unaltered condition within the hills, it is hard 
and rather difficult to reduce to a powder, though shales may differ con- 
siderably in this respect, some being easier to reduce than others. But 
nature in many localities has solved this problem by causing the shale 
structure to become softened, giving rise to the so-called “rotten” shale 
which has all the good qualities of a shale, but not its hardness. Examples 
of this may be noticed along the outcrop of most shales, and on the 
surface of surface beds. The question whether such softened shale 
becomes available for manufacture must be determined by the amount of 
such material at hand. If a clay property covers a large area or if the 
