STATE GEOLOGIST. 63 
causing it to form balls or pebbles of various sizes. Such concretions 
are met frequently and of course cause the clays to be irregular in compo- 
sition. If they occur in too great a quantity they may render the clay 
worthless. Often, however, they may be removed by hand picking. 
Glaciation.—By glacial action we mean in general. the effect 
wrought upon rock material by the movement of ice upon it and the ad- 
mixture with the rock debris brought along by the ice. This action may, 
of course, be more or less pronounced. It may remove the original clay, 
entirely, replacing it by the glacial drift material, as we know it to have 
taken place in large areas of Ohio where for instance the black shale has 
disappeared entirely, or it may produce a heterogenous mass of clay con- 
taining bowlders of igneous rock and fragments from both igneous and 
sedimentary rocks. In any case extreme irregularity of composition will 
be produced and what might originally have been a good clay is now 
utterly unfit for the purposes of the cement industry. Unless such clays 
are worn away and redeposited there is little encouragement for their 
practical utilization. 
Composition of Clays. —We have thus seen that in adding clay to a 
Portland cement mixture, we are adding a complex mixture of minerals, 
each of which has its own characteristic behavior in the vitrification process 
to which the cement is subjected. While no two clays are ever exactly 
alike, and while the fluctuations in composition and properties of clays 
may be very wide, it is still possible to make some general statements con- 
cerning them. They are known to be composed chiefly of four kinds of 
mineral matter. These elementary minerals, of which three are silicates 
and the fourth is wholly silica, are 
Kaolinite or clay substance. 
Feldspar and the feldspathic minerals. 
Mica and the micaceous minerals. 
Quartz or crystallized silicas. 
These, blended in every conceivable proportion, form the predom- 
inating part of clays; in addition, there are frequently considerable quan- 
tities of ferruginous, calcareous, or carbonaceous matter, which may be 
in quantity sufficient to dominate and give character to the rock as a whole. 
Small quantities of other minerals are always present, but these are not 
factors of importance in the cement industry. 
The behavior of a clay in a Portland cement mixture will, therefore, 
be the resultant of the behavior of its component minerals, which will now 
be studied. 
Behavior of Kaolinite. —The importance of the clay substance in ce- 
ment-making becomes manifest when we study its behavior with calcium 
carbonate or limestone on heating, and compare it with ground quartz. 
While fine quartz, ground in a hall-mill, on being heated with pure, fine cal- 
