22 
THE CLAYS OF ARKANSAS. 
organic acids which are derived from decaying vegetable or other 
organic matter, on penetrating these cracks causes the rock to decay, 
the decomposition processes attacking the masses at first along these 
openings and then extending inward until only a rounded bowlder is 
left ; later still the whole mass is converted into clay, the soluble parts 
of the rock having been washed out by # the percolating water. Vast 
quantities of clays are formed in this way by the decay of the crys- 
talline rocks, and no doubt such decay has been going on since the 
crust of the earth began to harden. 
Fig. 2, sketched in the railway cut 3 miles south of Little Rock, 
illustrates this method of forming clay, the clays at that place having 
been derived directly from the syenites by decomposition. 
Bruno Kerl a holds that feldspar may even be decomposed by 
water alone at high temperatures and under strong pressure, yielding 
Fig. 3.— Shales decaying at the surface in railway cut at Little Rock, sh, Shale; cl, clay formed by 
decomposition of shale in place. 
clay, silicate of potash, and silicic acid. Carbonic acid may produce 
similar decomposition at ordinary temperatures and without pressure. 
Fig. 3 is a sketch made in the railway cut about a mile west of the 
union depot at Little Rock. The clays at the top of the ground here 
are simply the decayed upturned edges of the shales exposed in the 
railway cut. Deeper down the shales are compact and undecomposed. 
The thin vein of quartz in the shale evidently once passed upward 
into the extension of the shales, but as the shales decayed and formed 
a soft clay the quartz, not being so readily attacked by weathering 
agents, simply broke into irregular fragments and remained in the 
clay or shale after it was decomposed. Any other insoluble mate- 
rials in the shale, such as sand, would thus be left as an impurity or 
grit in the clay. 
a Bandbuch der gesammten Thonwuarenindustrie, Braunschweig, 1S79, p. 29. 
