SPATE GHOEOGISM | 69 
feldspar and grains of quartz. For the purposes of the cement manu- 
facturer neither the pure clay substance nor the quartz are of any direct 
value, -as the one brings in too much alumina to be suitable for cement 
making, while the other is too hard to be ground profitably on a large 
scale. Hence if the primary rocks are to be considered at all they must 
either be high in feldspar or high in very fine quartz. The latter is not 
all probable, as the primary rocks have not gone through a natural grind- 
ing process, and hence only the first case deserves consideration. 
-Feldspathic white clays may be used in the manufacture of white 
Portland cements, furnishing as they do the silica in the combined state 
and since they also introduce a powerful flux in the shape of the alkalies. 
Rocks of this type may even be considered commercially if they should 
happen to contain a small amount of coarse quartz. If the feldspathic 
minerals are fine grained while the quartz is coarse, the latter can easily 
be removed by washing, as is done in the preparation of the kaolins 
for the market. If the difference in size between the particles of feldspar 
and those of quartz is not sufficiently great to separate them, since their 
specific gravities are very nearly alike, it would probably pay to grind 
these materials to the required fineness. ‘This is due to the fact that white 
or very light colored cements sell at a much higher price, being especially 
suitable for decorative construction purposes. The white clay found at 
Mt. Holly, Pa., is an example of the kind of material under discussion. 
2. Ferruginous Rock Agegregate.—If on the decomposition of the 
granitic rock the iron carrying minerals, like augite and hornblende, are 
in part or completely broken down and these hydrous aluminum silicates 
are mingled with those from the feldspar and perhaps magnetite is 
ptesent also, the aggregate of these decomposition products may or may 
not be suitable for cement making purposes. Its suitability depends 
first on the softness of the decomposed minerals, the character and 
amount ef quartz present, the content of iron, the amount of clay sub- 
‘stance, the amount of undecomposed minerals and the uniformity of the 
clay bed. It is not often that a material of this character will be found 
satisfactory ; in the majority of cases it would be too heterogeneous to be 
of any practical value for the purposes of the cement industry. 
This statement applies to all primary deposits. Since no primary 
clays are found in Ohio a further discussion of them will be un- 
necessary. 
3. Fire Clays.—This is a somewhat indefinite term, standing in gen- 
eral for clays which do not melt at high temperatures. A great deal 
of confusion prevails in regard to the conception of high temperatures 
among practical men. Though no definite limit has ever been put down, 
it may be said that no clay can be called a fire clay or refractory clay 
unless it stands a temperature approximately 3000° F. or 1650° C., ex- 
pressed in the scale of fusible cones adopted by Seger as No. 26. 
