PR/NCIPLES OF ROCK WEATHERING 857 



which by actual cxj)criment has been found capable of etching 

 marble. The exact nature of this acid is not accurately known, 

 but it is considered probable that in the rootlets of each species 

 of plant there exist a considerable variety of organic acids. ^ 



But the effects of plant growth are not necessarily always 

 destructive ; they may exert a conservative or even protective 

 action. In glaciated regions it is often the case that the striated 

 and polished surfaces of the rocks have been preserved only 

 where protected from the disintegrating action of the sun 

 and atmosphere by a thin layer of turf or moss. As a general 

 rule, however, the manifest action of plant growth is to accel- 

 erate chemical decomposition, through keeping the surfaces con- 

 tinually moist, and to retard erosion. 



Action of bacteria. — The researches of A. Miintz,- Widograd- 

 sky, Schl(>sing and others have shown that bacteria may exercise 

 a very important influence in promoting rock disintegration and 

 decomposition. Their influence in promoting nitrification has 

 been already alluded to. It would, appear that while these 

 organisms may secrete and utilize for their sustenance the carbon 

 from the carbonic acid of the atmosphere, as do plants of a 

 higher order, they may also assimilate the carbonate of ammo- 

 nium, forming from it organic matter and setting free nitric acid. 

 Being of microscopic proportions these organisms penetrate into 

 every little cleft or crevice produced by atmospheric agencies, 

 and throughout long periods of time produce results of no 

 inconsiderable geological significance. The depth below the 

 surface at which such may thrive is presumably but slight, and 

 their period of activity limited to the summer months. They 

 have been found on rocks of widely different character — granites, 

 gneisses, schists, limestones, sandstones and volcanic rocks — 

 and on high mountain peaks as well as on lower levels. The 

 Pic Pourri, or Rotten Peak, in the Lower Pyrenees of southwest- 



' See Application of Organic Acids to the Examination of Minerals. H. Car- 

 RINGTON Bolton, Proc. Am. Assoc, for the Advancement of Science XXXI, 1883. 

 Also Available Mineral Plant Food in Soils. B. Dyer, Jour. Chem. Society, March 

 1894. 



^CoMi'TES Rrndus de L., Academic des Sciences, CX, 1890, p. 1370. 



