/'AV.yC/PLES OF ROCK WEATHERIXG 869 



The researches of Hittcrman ' show, however, that carbonic 

 acid solutions may exert a scarcely appreciable effect upon mag- 

 nesian carbonates, which therefore accumulate in the residual 

 soils. In residues derived from limestones this authority also 

 found percentages of alkalies greatly in excess of those in the 

 unchanged rock, indicating beyond a doubt the occurrence of 

 these constituents in the form of insoluble silicates. 



It is safe to say that while the general process of rock- 

 weathering may be quite simple, as outlined, there are many 

 minor reactions which it is not possible to describe in detail. 

 It has been shown that even in firm rocks a mutual chemical 

 reaction is not uncommon among minerals lying in close juxta- 

 position, giving rise to what are known as reaction rims or zones 

 composed of secondary minerals. This is a particularly con- 

 spicuous feature in many gabbros where olivine and feldspar are 

 closely adjacent. In these cases a mutual interchange of ele- 

 ments ma)- take place, giving rise to garnets, free quartz or other 

 minerals, as the case may be. This is, to be sure, a deep-seated 

 change, to be classed as alteration rather than decomposition, and 

 taking place presumably under conditions of temperature and 

 solution quite at variance with those existing on the immediate 

 surface. It is, nevertheless, self-evident that when elements 

 are set free through an}^ process, they must almost immediately 

 recombine, taking those forms which existing circumstances 

 may dictate. In a mass of decomposing rock circumstances 

 are almost continually changing, and the inference is fair that 

 new combinations are continually being made and unmade, 

 the intricacies of which we are unable to follow. 



Among the siliceous crystalline rocks superficial disintegra- 

 tion is undoubtedly greatly aided by temperature variations 

 which, by rendering the rocks porous, facilitate chemical decom- 

 position. Such action must, however, be merely superficial, and 

 at considerable depths below the surface the change must be 

 purely chemical. The chief conditions favoring chemical action 



' Die verwitterunge Producte von Gesteinen der Triasformation Franker. Inaug. 

 Dissertation. Munich, 18S9. 



