186 



EOCK DISINTEGEATION AND DECOMPOSITION 



quite perfectly outlined crystals, is a strikingly abundant con- 

 stituent Small apatites, a few flakes of white mica (serieite), 

 and widely scattering black tourmalines and iron ores complete 

 the list of recognizable minerals. 



The outcrops from which the samples for the analyses to 

 which attention is first called were selected are shown in plate 

 16. At the very bottom, the rock is hard, fresh, and com- 

 pact, without trace of decomposition products other than as 

 indicated by minute infiltrations of calcite from above* Just 

 above the level of the small creek which flows at the foot of 

 the bluff at the point indicated by the first series of right-and- 

 left joints near the centre of the view, the character of the rock 

 changes quite suddenly, becoming brown and friable, though 

 still retaining its form and easily recognizable granitic appear- 

 ance. A few feet above a third zone begins, in which the rock 

 is converted into sand and gi*avel, which becomes more and 

 more soil-like to the top of the bank, where it becomes admixed 

 with organic matter from the growing plants. The amount 

 of organic matter is quite small, however, and in making the 

 analyses care was taken to remove such as was recognizable in 

 the form of rootlets, leaves, and twigs. 



Analyses or Fresh and Decomposed Granite, District of Columbia 



Constituents 



Ignition 



Silica (Si02) .... 

 Titanium (Ti02) . . . 

 Alumina (AI2O3) , . . 

 Iron protoxide (FeO) . 

 Iron sesquioxide (Ee20s) 

 Lime (CaO) .... 

 Magnesia (MgO) . . . 

 Soda (NaaO) .... 

 Potasli (K2O) .... 

 Phosphoric acid (PaOg) 



1.22 % 

 69.33 

 not det. 

 14.33 



3.601 



• » • « 



3.21 

 2.44 

 2.70 

 2.67 

 0.10 



00.60 % 



II 



3.27 % 

 66.82 



not det. 

 15.62 

 1.69 

 1.88 

 3.13 

 2.76 

 2.58 

 2.04 



not det. 



99.79% 



HI 



4.70% 

 65.69 



0.31 

 15.23 



4.39 

 2.63 

 2.64 

 2.12 

 2.00 

 0.06 



99.77% 



Bulk analyses of these three types, (I) fresh gray granite, 

 (II) brown but still moderately firm and intact rock, and (III) 

 *4.00% when calculated as FejO,. 



