EESULTS INCIDENTAL TO DECOMPOSITION 253 



is tlie tendency toward simplification in composition as mani- 

 fested all over the superficial portions of the earth. During 

 the process of decomposition there is a constant breaking down 

 of complex molecules of mixed silicates of alumina, iron, lime, 

 magnesia, and the alkalies, and a recombination of their various 

 elements as simpler silicates, carbonates, sulphates, and oxides. 



The production of carbonates, particularly those of lime, 

 is one of the most conspicuous results of rock weathering, and 

 according to Van Hise^ is a matter of paramount importance. 

 Through the decomposition of lime-bearing silicates, as certain 

 of the feldspars, pyroxene and amphiboles, the lime separates 

 out as ealcite or aragonite, as may be readily shown by micro- 

 scopic examinations or chemical tests. 



(12) Other Results incidental to Decomposition and Erosion. 

 — That all the minerals of a rock mass are not equally acted upon 

 by atmospheric agencies has been sufficiently noted in previous 

 pages. The more refractory, freed by the breaking down of 

 their host, remain to gradually accumulate in vastly greater 

 proportions than they existed in the original rock. If, in 

 addition to their refractory qualities, such possess, as is usually 

 the case, greater density, decomposition and erosion may act but 

 as agents of concentration, and in such residues minerals like 

 xenotime and monazite have been found in abundance, although 

 occurring so sparingly in the fresh rock that their existence was 

 scarcely suspected. 



It is in this manner that has originated the gem sand of 

 Ceylon. Precious stones have been found disseminated in limited 

 numbers in the granite converted into the cabook described on 

 p. 228. In weathering, the difBcultly decomposable precious 

 stones have not been attacked, or attacked only to a limited ex- 

 tent. They have therefore retained their origind form and hard- 

 ness. When in the course of thousands of years streams of water 

 have flowed over the layers of cabook, their soft, already half- 

 weathered constituents have been for the most part changed into 

 a fine mud, and as such washed away, while the hard gems have 

 only been inconsiderably rounded and little diminished in size. 

 The current of water therefore has not been able to wash them 

 far away from the place w^here they were originally embedded 

 in the rock, and we now find them collected in the gravel bed, 

 resting for the most part on the fundamental rock w^hich the 



^Treatise of Metamorphisms, p. 479. 



