WEATHEEING OF AEGILLITE 213 



tion. It is well to note, however, that the materials composing 

 rocks of this type are themselves a product of these very dis- 

 integrating and decomposing agencies, but which have become 

 reconsolidated into rock masses and now, once more in the 

 infinite cycle of change, are undergoing a breaking up. It 

 follows from the very nature of the case that such rocks, with the 

 exception of the purely calcareous varieties, will undergo less 

 chemical change than do those we have been discussing. Their 

 feldspathic and easily decomposable silicate constituents long 

 ago yielded to the decomposing processes, and were largely re- 

 moved before consolidation took place. Thus, most sandstones 

 are composed largely of quartzose sand, the least soluble and 

 least changeable product, it may be, of many a previous disinte- 

 gration. Hence, the processes involved in the degeneration of the 

 sandstones, shales, and argillites, with the exception of those 

 which carry a feldspathic of calcareous cement, are largely me- 

 chanical. In these last-named, the cementing material gradually 

 gives way, and the rock becomes susceptible to the action of frost, 

 or falls away to loose sand simply through loss of cohesion. 

 Heusser and Claraz^ described the itacolumites of Brazil as sub- 

 ject to this mechanical degeneration, the process being charac- 

 terized by fissuration, succeeded by complete disintegration. 

 Among siliceous sandstone it is the binding constituent that 

 yields first, as is naturally to be expected, and as has been 

 shown by the investigations conducted by E. Schutze.^ 



Weathering of Argillite. — The rocks grouped under the name 

 of argillites, though composed of detrital materials from pre- 

 existing rocks, and of particles reduced to an extreme degree of 

 fineness, are, nevertheless, quite variable in composition. As a 

 rule, they are among the most indestructible of rocks, and on 

 breaking down yield only clays which differ from the original 

 argillites mainly in degree of hydration and condition of oxida- 

 tion of the iron and other metallic constituents. Those argillites 

 which carry appreciable quantities of still undecomposed sili- 

 cates, particularly alkali-bearing varieties, are, of course, more 

 susceptible, other things being equal. 



The deep blue-black argillites of Harford County, Mary- 

 land, as shown in the analyses given below, contain very con- 



^Ann. des Mines, 5tli, Vol. XVII, 1860. 



^Ueber VerwitteruDgsvorgange bei Krystallinisclien u. Sedimentargestei- 

 nen, Inaug. Dissertation der Friedrieh-Alexanders Universitat, Berlin, 1886. 



