644. GEOLOGY OF OHIO. 
Clay, instead of being a mineral formed by the ordinary processes 
of chemical synthesis, is the result of decomposition of granite rocks, or 
by a closer definition, is the result of the decomposition of feldspars or 
those rocks which yield them, notably granitic and gneissic rock. Ortho- 
clase, the feldspar from which the body of our clays is derived, is a 
double silicate of potash and alumina ; the other feldspars found accom- 
panying it are albite, or the soda feldspar, and oligoclase, the soda-lime 
feldspar ; these latter forms are in small proportion compared to ortho- 
clase. Ordirary aqueous and atmospneric agencies are sufficient to 
decompose feldspar, giving rise to hydrated silicate of alumina or kaolin 
and a soluble salt of potash, which is carried away by the water which - 
accompanies decomposition. Feldspar beds are fruitful sources of the 
finest kaolins and china clays, and are rarely found without some kaolin 
accompanying. 
The mineral elements in granite and gneiss, as is familiar to all, are 
quartz, feldspar and mica. The first mineral is not affected by air or 
water, but the feldspars and to some small extent the micas, are attacked 
by the atmosphere. When the feldspar decomposes, the bond which 
holds the other elements together is gone, and the quartz and mica are 
carried off by water or mixed with the clay in varying proportions as 
the conditions of formation vary. The more the water carries off, the 
purer the clay left behind. 
The irregularity of composition which is so characteristic of clay, is 
thus seen to be the result of the differing mechanical conditions which 
surround the clay as it forms; it is distinctively a mineral in which other 
forces than chemical affinity have left their mark ; and another element 
of uncertainty is added in the fact that if the mechanical conditions 
were constant, clays would vary with the parent rock, which has no 
fixed structure. j 
Thus is exposed, in the origin of clay, the reason of that irregu- 
larity which has so long baffled or retarded progress in its study, but 
which, once understood, proves the key tq all that follows. 
Chemical constitution claims the first attention in a study of clays, 
for in it lie the secrets which control its properties and qualities, to 
know which is the aim of all investigation. A knowledge of this sub- 
ject renders possible a comparisonof clays as to their value for any uses, 
selection of clays for special adaptations, insures sale of a worthy 
article to well informed purchasers, and relieves us from the tyranny 
