178 C. W. TOMLINSON 



question to be answered in this connection is : Was the ferruginous 

 matter deposited as a mechanical or as a chemical sediment ? 



The general absence of coloring matter from the non-clastic 

 members, 1 which are in very large part inorganic chemical precipi- 

 tates, indicates quite clearly that the conditions favoring free 

 chemical deposition of calcium and magnesium carbonate or of 

 calcium sulphate were not those under which the coloring matter 

 was usually deposited. The close limitation of ferruginous material 

 to the clastic sediments proves that the conditions under which 

 clastic sedimentation took place favored the deposition of iron 

 oxide also, and strongly suggests that that material itself was carried 

 and deposited as a mechanical sediment, for the most part at least. 

 In this connection it is of interest to note that Dawson 2 drew a 

 similar conclusion as early as 1848 with reference to the contrast in 

 color between the clastic and non-clastic strata of the Red Beds 

 of Nova Scotia. 



The condition of the ferric oxide in the Red Beds sediments, as 

 revealed by the microscope, is one of very fine division. Since line 

 division is to be expected from the mode of origin of the ferric oxide 

 in soils, 3 this cannot be taken as evidence that it is a chemical pre- 

 cipitate, in place, in the rocks. If it were the latter, definite orien- 

 tation of crystals of hematite with respect to peripheries of grains 

 might be looked for. I never have seen this phenomenon in thin 

 sections of Red Beds sediments. The microscopic evidence is 

 therefore rather noncommittal as regards the present question. 



The processes of weathering leave much the greater part of the 

 iron content of all types of rock as a residue in the soil, subject to 

 mechanical transportation. All of the other common chemical 

 constituents of rock, with the probable exception of alumina, are 

 leached from the soil more rapidly than ferric iron. The scarcity 

 of iron in any form in the surface waters of the continents, abun- 

 dantly shown by the analyses of lake and river water published by 

 Clarke, 4 and its even greater scarcity in the ocean, 5 testify to the 



1 See p. 157. 2 Op. cit. 3 See p. 164. 



♦F. W. Clarke, "The Data of Geochemistry," 2d ed., Bull. U.S. Geol. Survey 

 No. 401, 191 1, chap. iii. 

 5 Ibid., chap. iv. 



