WEATHERING OF GEANITE 193 



— 12° and +97° Fahr. The annual precipitation is 48.88 

 inches. The rock is a coarse gray feldspar-rich variety with 

 abundant folia of black mica. Under the microscope it shows 

 the presence of both potash and soda-lime feldspars, a sprinkling 

 of apatite and iron ores, sporadic occurrences of an undetermined 

 zeolite, and an extraordinary number of minute zircons which 

 are mostly enclosed in the feldspars. There are also present 

 occasional small garnets and aggregates of decomposition prod- 

 ucts the exact nature of which was not made out. The residual 

 soil resulting from the decomposition of this rock is highly plas- 

 tic, of a deep red-brown color, and has a distinct gritty feeling 

 in the hand, owing to the presence of quartz and undecomposed 

 silicate minerals. In columns I and III on the next page are given 

 the results of analyses of fresh rock and residual soil, and in II, 

 IV, and V the analyses of the soluble and insoluble portions. In 

 columns VI, VII, and VIII are given the calculated percentage 

 amounts of the various constituents saved and lost, as before. 



The particular features to which attention need be called, 

 are (1) that 30.51% of the fresh rock and 69.18% of the 

 decomposed are soluble in hydrochloric acid and sodium car- 

 bonate solutions, and that more than half the potash and 

 nearly the same proportion of the soda in the fresh rock is 

 found in the acid extract. (2) That the insoluble portion of 

 the residuary material is mainly in the form of free quartz. 

 (3) That 44.67% of the original matter has been leached away, 

 and that (4) of the original silica 52.45% is lost, while 85.61% 

 of the iron and all the alumina remain. All the lime has dis- 

 appeared, 83.52% of the potash, 95.03% of the soda, and 74.70% 

 of the magnesia. The total amount of water, as indicated by 

 the ignition, has increased very greatly, as was to be expected. 

 The small original amount of phosphoric acid prohibits our 

 placing too much reliance upon the indicated gain in this con- 

 stituent, since it may be due to errors in manipulation. 



To guard against the danger of making deductions from in- 

 sufficient data, another fairly typical example may be selected — 

 this time a dark blue-gray rock of medium, massive texture from 

 near Greenville, Georgia, as analyzed and described by Dr. 

 Thomas Watson.^ The rock in its fresh condition consists of 

 quartz, the potash feldspars orthoclase and microcline, a soda- 

 lime feldspar near oligoclase, biotite, a little muscovite, and the 



^ Granites and Gneisses of Georgia, p. 313. 

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