AMOUNT OF MATEEIAL EEMOVED IN SOLUTION 245 



however, that this conclusion is not wholly satisfactory, since, i£ 

 correct, the intensity and depth of the colored zone should, to a 

 certain extent, be governed by the size of the pebble. Neither 

 are the conclusions in harmony with those of Walther,^ who 

 found in the Egyptian deserts a superficial discoloration and 

 induration common to all classes of rocks, quite independent of 

 iron and manganese as original constituents, and due, as he 

 believed, to prevailing climatic conditions and the prevalence of 

 a certain amount of silica. The more highly siliceous the rocks, 

 the darker the colors of the crust. Limestones he found to turn 

 light to dark yellow; sandstone and siliceous dolomite dark 

 brown; many granites, jaspers and flints, black; all, as a rule, 

 being most highly colored on the surfaces exposed to the at- 

 mosphere. 



No claim is made by "Walther of a complete solution of the 

 problem, though he regards the Egyptian occurrences as cer- 

 tainly not due to the solvent action of water, and the fact that 

 like crusts are found in pebbles on the banks of tropical rivers 

 is considered as having no bearing on the cases described. It 

 would, to the writer at least, seem probable that phenomena of 

 a somewhat similar character, so far as appearances go, but due 

 perhaps to quite different causes, have been confused by the 

 various writers on the subject. Attention, before leaving the 

 subject, may be called to the fact that the darkest colors described 

 by Walther, were on rocks which from their nature would 

 weather away the most slowly. A dark color on the surface of 

 a granite or quartzite pebble may be due in part to the fact that 

 the immediate surface of a rock of this nature has been longer 

 exposed than that of a limestone which is continually losing by 

 solution or abrasion. 



(8) Relative Amount of Material removed in Solution. — 

 Among siliceous rocks, chemical action proceeds but slowly, 

 and the amount of material actually removed in solution is 

 rarely over 60%, and may be so small that, as the writer has 

 shown,^ the residue in extreme cases occupies some 80% more 

 space than the rock from whence it was derived. Carbonate 

 of lime, the essential constituent of ordinary limestone, is, 



in the superficial deposits of the District of Columbia show at times a like 

 discoloration, due to a very thin coating of iron and manganese oxide. 



^ Denudation in der Wiiste, p. 117. 



2 Bull. Geol. Soc. of America, Tol. VI, 1895, pp. 321-332. 



