226 EOCK DISINTEGEATION AND DECOMPOSITION 



reaction is not uncommon among minerals lying in close jnxta^ 

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

 composed of secondary minerals. This is a particularly cpn- 

 spicuous feature in many gabbros, where olivine and feldspar 

 are closely adjacent. In these cases, a mutual interchange of 

 elements may 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 decomposi- 

 tion, and taking place presumably under conditions of tempera- 

 ture and solution quite at variance with those existing on the 

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

 elements are set free through any process, they must almost im- 

 mediately reeombine, taking those forms which existing circum- 

 stances may dictate and that close contact of particles would be 

 favorable to the more rapid formation of new compounds. In 

 a mass of decomposing rock, circumstances are almost continu- 

 ally changing, and the inference is fair that new combinations 

 are continually being made and unmade, the intricacies of which 

 we are tmaWe to follow. 



