Si EGEOwOGISM 67 
of all sizes and the particles may be worn by the natural grinding process 
more or less or not at all. Since we know that it should be as fine as pos- 
sible so as to render it fit for the cement reaction without the expense of 
erinding, we should-select such clays as have been ground fine by natural 
processes and have been washed out and redeposited in still water. Such 
clays are the shales and the glacial clays of the lake regions of New York, 
Indiana, Wisconsin and Ohio, as, for instance, the Albany slip clay oi 
New York and the lacustral clay of Millbury and Bryan in Ohio. 
Geology thus aids us in finding and investigating suitable clay materials 
and determines whether a proper material can be furnished by a certain 
locality or not. 
The Rational Analysis of Clays.—In speaking of clays in general 
we mean, according to what has been already said, natural mixtures of 
three mineral groups: 
Clay substance. 
Feldspathic minerals. 
Quartz. 
It must be, therefore, of interest to know just what proportion of a 
clay is clay substance and what free silica or feldspathic minerals. ‘This 
is best determined by a chemical method, depending on the fact that clay 
substance is soluble in hot concentrated sulphuric acid and hot sodium 
carbonate solution, while both feldspar and quartz are not. By such a 
treatment all of the clay substance is removed, leaving only the quartz and 
feldspathic minerals. On analyzing this residue the amount of alumina 
and ferric oxide shows the percentage of feldspar present, since quartz can 
be assumed to consist of pure silica. Knowing the amount of the alumina 
and ferric oxide, the silica belonging to the feldspar is readily calculated, 
since the ratio between the R,O, compounds in orthoclase feldspar and . 
the silica is as 1:3.52, that is, for every per cent. of alumina plus ferric 
oxide, we have 3.52 times this amount of silica. The total amount of 
silica in the insoluble residue minus the calculated amount of silica belong- 
ing to the feldspar will give the amount of quartz and the total amount of 
feldspar is found by adding the silica (calculated) to the alumina and 
ferric oxide, the calcium and magnesium oxides and the alkalies all found 
in the insoluble residue. The details of this method will be given later 
under its proper heading. ; 
Though this method of mineral analysis can lay no claim to scientific 
accuracy, it is of great practical value in giving a picture of the mineral 
structure of a clay which is of much greater direct benefit than the mere 
gross analysis. 
Other Silicates——Other natural silicates than those composing the 
clays have so far found no general application in the manufacture of Port- 
land cement, but for certain purposes the use of certain silicious rocks 
low in iron is not an impossibility; for instance, for the manufacture of 
