PRIXCIPLES OF ROCK WEATHERING 87 1 



may be, of many a previous disintegration. Hence, the proc- 

 esses involved in the disintegration of the sandstones, shales 

 and argillites are mainly mechanical, with the exception of those 

 which carry a feldspathic or calcareous cement. 



It is, however, quite different with the calcareous members 

 of the group, where, with the exception of the granular-crystal- 

 line varieties, the process is almost wholly chemical, and notable 

 for its simplicity. In these latter forms, as the saccharoidal 

 marbles, expansion and contraction, from ordinary temperature 

 variations, bring about a more or less rapid disintegration. The 

 decomposition is, however, due mainly to the action of meteoric 

 waters trickling o\'er the surface, or filtering through cracks and 

 crevices under ordinary conditions of atmospheric pressure and 

 atmospheric temperature. Hence, the process is one of super- 

 ficial solution, and the incidental chemical proc::sses 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 i per cent., so it happens that hun- 

 dreds or even thousands of feet of strata may be removed with- 

 out leaving more than a very thin coating of soil in its place. 



George P. Merrill. 



