CHARACTER, ORIGIN, OCCURRENCE, AND USES. 
23 
The clay in this place is derived directly from the shale. But the 
shale itself is an argillaceous rock which was originally made from 
clays and sediments that washed down from land where some of the 
old feldspathic rocks were decaying or had decayed. This residuary 
clay is thus derived directly from shale, but indirectly from feld- 
spathic rocks. 
Fig. 4 shows the derivation of clay from limestone by decoihpo- 
sition, the lime of the rock being dissolved out and carried away", the 5 
clay 7 iron, sand, and whatever insoluble impurities the rock con- 
tained being left behind. This clay is of a deep red color. It con- 
tains some fragments of the limestone from which it is derived — 
small pieces not yet completely decomposed. This limestone con- 
tains nodular pieces of chert or flint, and as these do not decay, or at 
least do not decay so rapidly as the limestone, they are left as loose 
pieces in the clay. The clay in this case comes from the impurities 
Fig. 4.— Section on Dry Creek, Carroll County, showing formation of clay by decomposition of lime- 
stone. Is, Limestone; cl, clay. 
in the limestone, and these impurities, in so far as they form clay, 
were deposited as an argillaceous silt with the calcareous material 
that forms the limestone. This will serve to explain at once the 
origin of the clays of some limestone regions and the great blanket 
of clay found over most limestone regions, as well as that found 
in caves and in crevices and fissures in limestones. 
Clays thus formed and lying where they originated are the residue 
from the decay of some hard rock, and are therefore called residuary 
clays. Other illustrations might be given to show how clays arc 4 
derived from other kinds of rocks than those mentioned, but these 
are enough to show what is meant by residuary clays. The decom- 
position of rocks and the formation of residuary clays may be seen 
in almost every part of the elevated portion of the State. 
It is evident that the clays derived from any of the rocks men- 
tioned must vary more or less among themselves, according to the 
amount and character of the insoluble contents of the rocks from 
