216 BOCK DISINTEGEATION AND DECOMPOSITION 



known as a polishing powder. This second stage is not so con- 

 spicuous a feature as the first, since the finer materials thus 

 formed are carried off by surface waters. The white residual 

 powder often contains masses of the porous, semi-decomposed 

 rock, the latter in turn encircling kernels of hard, unaltered 

 chert. Throughout this region, the cherts (of Carboniferous 

 age) are generally decomposed into the condition of a more 

 or less porous mass to all depths up to ten or more feet. 

 In all cases the disintegration may be traced to the removal, 

 by leaching, of a small amount of interstitial carbonate of lime. 



Weathering of Calcareous Rocks. — When we come to a con- 

 sideration of the Calcareous rocks, we find, almost invariably, 

 the chemical agencies of degeneration preponderating over those 

 that are purely physical. In arid regions, and with granular 

 crystalline types, physical agencies may for a time prevail, but 

 as a rule the process is largely chemical, and notable for its 

 simplicity. The decomposition is due mainly to the action of 

 meteoric waters trickling over the surface, or filtering through 

 cracks and crevices, under ordinary conditions of atmospheric 

 pressure and atmospheric temperature. Hence the process is one 

 of superficial solution, and the incidental chemical processes set 

 in motion, as in the feldspar-bearing rocks, are almost entirely 

 lacking. It follows that only the lime carbonate is removed 

 in appreciable quantities, while the less soluble impurities are 

 left to accumulate in the form of ferruginous clays, admixed 

 with quartzose particles, chert nodules, etc. Since in many 

 limestones the amount of these constituents is reduced to a 

 minimum, even perhaps to the fraction of one per cent, so it 

 happens that hundreds, or even thousands of feet of strata may 

 disappear without leaving more than a very thin coating of soil 

 in their place. 



An interesting illustration of the changes taking place in the 

 decomposition of an impure Carboniferous limestone is described 

 by Penrose in his treatise on the genesis of manganese deposits.^ 

 The stone in its least changed condition is of a granular crys- 

 talline structure and dark chocolate-brown color. The residual 

 clay from its decomposition is a trifle darker, highly plastic, and 

 quite impervious. On the next page are given the analyses of (I) 

 the fresh rock and (II) the clay, both being taken from the same 

 pit, the latter being of about fifteen feet in thickness and over- 



^Ann. Bep. Geol. Survey of Arkansas, 1890, p. 179. 



