396 



EOCK DISINTEGEATION AND DECOMPOSITION 



tnre are — 8° and +100° Fahr., with an average annual tem- 

 perature of 62.1° and a rainfall of 50.38 inches.^ 



Weathering of Nepheline Syenite. — Passing from the most 

 acid group of granular crystalline roeks, we will next consider 

 the elseolite syenites of the Fourche Mountain region of Arkansas. 

 The Weather Bureau records for the region show a maximum 

 range of temperatures amounting to 118°, with an annual aver- 

 age of 61.5° and an annual precipitation of 53.63 inches. The 

 rocks are somewhat coarsely crystalline granitic-appearing, with 

 an orthoclase feldspar in broadly tabular forms as the prevailing 

 constituent, though always accompanied by nepheline, biotite, 

 pyroxene, titanite, and apatite, while fluorite! analdte, ani 

 thomsonite, together with caleite, occur as secondary products. 

 The rock weathers away to a coarse gray gravel which ultimately 

 becomes a clay, from which, by washing, may be obtained kaolin 

 in a fair degree of purity. 



Analyses of Fresh and Decomposed Syenite, Arkansas 



Constituents 



Fresh 

 Syenitk 



Bbcomposbd Syenite 



Kaolin-like 

 Kbsibue 





I 



II 



III 



IV 



Silica (Si02) .... 

 Alumina (AlsOg) . . 

 Ferric oxide (Fe203) . 

 Lime (CaO) .... 

 Magnesia (MgO) , . . 

 Potash (K2O) .... 

 Soda (ISra20) .... 

 Ignition (HgO) . . . 



59.70% 

 18.85 



4.85 



1.34 



0.68 



5.97 



6.29 



1.88 



58.50% 

 25.71 



3.74 



0.44 

 Trace 



1.96 



1.37 



5.85 



50.65 % 

 26.71 



4.87 



0.62 



0.21 



1.91 



0.62 



8.68 



46.27 % 

 38.57 



1.36 



0.34 



0.25 



0.23 



0.37 

 13.61 





99.56 % 



97.57 % 



94.27 % 



101.00% 



iThe following table shows the total losses calculated from a number of 

 analyses of more or less decomposed granites from Georgia, as given by 

 the authority quoted above: 



Biotite Granite (partially decomposed), near Elberton 7.92 



Oglesby 7.71 



(C 



ct 



ti 



tt 



it 

 it 



it 

 It 

 it 

 tt 



tt 



tt 

 tt 



tt 



tt 



tt 



Lexington 14.56 



Appling 15.84 



Lithonia 26.69 



Camak 34.04 



Coweta Station . . . 35.07 



Newman 38.45 



Oglesby 44.72 



Greenville 71.84 



