GEjSTEEAL BEDUCTIOlSrS 223 



and partial decomposition. Where thrown out upon the dumps, 

 this whitened mineral shortly falls away to fine sand, resembling, 

 at first glance, kaolin, but is distinctly gritty. 



Max Geldmacher noted^ that in the weathering of quartz 

 porphyry the oligoclase always gave way before the orthoclase. 



Indeed, as shown in our analyses, in certain phases of rock 

 degeneration, the potash feldspars may lose very little by 

 decomposition, but be converted into the condition of fine 

 silt merely through a mechanical splitting up. This fact will 

 in part explain the relative scarcity of free potassium salts 

 (carbonates, sulphates, and nitrates) as compared with those of 

 soda. 



The chemical processes involved in this f eldspathic decompo- 

 sition are of sufficient importance to warrant further discussion, 

 even though it may involve a certain amount of repetition of 

 what has gone before. 



Berthier, Forsehammer, Brongniart,^ Fournet,^ and others ex- 

 plained more than fifty years ago the process of feldspathic dis- 

 integration through the breaking up of its complex molecule 

 into alkaline silicates soluble in water, and aluminous silicates 

 which are insoluble. The loss in silica, as noted above, was 

 supposed to be due to the removal, by solution, of these alka- 

 line silicates. Bbelman,* however, subsequently showed that sili- 

 cate minerals poor or quite lacking in alkalies lost a portion of 

 their silica, as is also shown in the analyses of altered pyroxenites 

 on pp. 211 and 212. He accounted for this on the supposition 

 that the silica set free — in a nascent state — was soluble either 

 in pure water, or water containing carbonic acid. This observa- 

 tion is corroborated by the work of Kahlenberg and Lincoln,* 

 who showed in 1898 that in very dilute solutions such as natural 

 mineral solutions must necessarily be, the silica is present in a 

 colloidal form and not as silicic acid. Bischof states in his 

 earlier work (1856) that when meteoric waters containing car- 

 bonic acid filter through rocks containing alkaline silicates, the 

 first action consists in the partial decomposition of these sub- 

 stances by the carbonic acid and the formation of alkaline car- 



'Beitrage zur Verwitterung der Porpliyre, Inaug. Dissertation, Konigl. 

 Freidrieli Alexander Universitat, Leipzig, 1889. 



^Arch. du Museum, Vol. I, 1839 (cited by Ebelmen). 

 3 Ann. de CMmie et de Physique, Vol, LV, 1833. 

 *Ann. des Mines, Vol. VTI, 1845. 

 5 As quoted by Cameron and Bell, op. cit., p. 19. 



