328 G. p. MERRILL — DISINTEGRATION OF GRANITIC ROCKS. 



It is to be noted that the magnesia, together with the iron, exists 

 almost wholly in a soluble form. 



ANALYSES OF MATERIAL FROM OTHER LOCALITIES. 



Not wishing to attach too much importance to analyses of samples 

 from a single localit}', others were obtained from along the same belt. 

 In I of the columns below, is shown the composition of a soil from the 

 road-cut west of Pierce's mill, and in II and III material from the 

 deeper cut where this road crosses Connecticut avenue extended, num- 

 ber II being from some 3 feet beneath the surface where it was overlaid 

 by a thin layer of the Potomac gravel, and III from the bottom of the 

 cut some 20 feet below the present surface. The last sample, though 

 sufficiently soft to be readily removed with the fingers, showed scarcely 

 any of the oxidation which discolors the superficial portions, thus indi- 

 cating that oxidation itself is not an essential part of the disintegrating 

 process, but merely incidental to it. In column IV is given an average 

 of the three analyses, and in V the same calculated on a water-free basis. 

 For purposes of comparison the results given in column III, page 323, 

 are here repeated in column VI. 



I. II. III. IV. V. VI. 



Ignition..... 5.51 3.87 3.97 4.45 



Si02 64.25 64.87 63.42 64.15 67.13 69.10 



p^^Q^I 19.97 21.32 23.08 21.26 23.29 20.99 



MgO 3.12 3.01 2.69 2.94 2.07 2.77 



CaO.... 3.18 2.90 3.01 3.03 3.17 2.76 



K2O 2.17 2.39 2.15 2.24 2.34 2.10 



NaaO 1.55 1.86 1.77 1.72 1.80 2.23 



99.75 100.22 100.09 99.79 99.80 99.95 



It should be stated that in all these cases special care was exercised in 

 securing samples from areas which had never been under cultivation in 

 order that there might be no possible contamination or acceleration of 

 decay through the action of fertilizers or of plowing. Equal care was 

 taken to obtain material in place and where it had undergone only the 

 leaching of surface waters percolating downward from above. The re- 

 sults, though showing a somewhat more advanced condition of decay, 

 agree even more closely than could be expected from samples collected 

 from widely separated localities. 



Time-limit of Disintegration. 



A possible time-limit to the beginning of this disintegration is furnished 

 by the Potomac (Cretaceous) and more recent deposits of the region. 



I 



