EVOLUTION OF LIMESTONE AND DOLOMITE 



393 



cium to magnesium lost are based on the assumption that alumina 

 has remained constant. The percentage loss of calcium averages 

 higher than that of magnesium, a tendency generally character- 

 istic of metamorphic processes. 



TABLE V 

 Weathering of Acid Igneous Rocks 



Rock 



Source 



1 Granite. . . 



2 Granite. . . 



3 Granite. . . 



4 Granite. . . 



5 Granite. . . 



6 Granite. . . 



7 Granite. . . 



8 Granite . . 



9 Granite. . . 



10 Granite. . . 



11 Granite.. . 



12 Granite.. . 



13 Granite. . . 



14 Phonolite. 



15 Andesite. . 



16 Syenite . . , 



1 7 Gneiss. . . . 

 Average. . . 



Watson, Granites of Geor- 

 gia, 318 



Ibid., 315 



Ibid., 321 



Ibid., 320 



Ibid., 309 



Ibid., 312 



Ibid., 312 



Ibid., 327 



Ibid., 327 



Ibid., 325 



Average Georgia 



Merrill, Rocks and Rock 

 Weathering (Dist. of 

 Columbia), 207 



Ibid. 



Ibid. (Bohemia), 198 

 Ibid. (Grenada), 208 

 Clarke, U.S.G.S. Bull, jjo, 



412 

 Ibid. 



Materials lost by the weathering of basic igneous rocks. — In the 

 weathering of basic igneous rocks, the percentage losses of cal- 

 cium and magnesium average about even, in Table VI. The 

 ratio of calcium to magnesium lost varies between wide margins, 

 from one to infinity. The materials selected for analyses may 

 have unequaled value in the problem. 



Materials lost by the weathering of an average igneous rock. — The 

 ratio of calcium to magnesium in an average igneous rock (Clarke) 

 is about 1.37. The ratio of calcium to magnesium lost by its 

 weathering, calculated on the basis of average calcium and mag- 

 nesium decrements, is about 1 .85. 



