1 64 C. W. TOM LIN SON 



Deposition of coloring matter contemporaneous with sedimentation. 

 — If the ferruginous material which furnishes the color was con- 

 centrated in the sediments chiefly at the time of their deposition, 

 this association of high color with fine sediment is explained readily, 

 as follows : 



In the weathering of igneous rocks, ferruginous material is 

 separated out chiefly by the chemical decomposition of iron-bearing 

 silicate minerals, and is therefore at the time of separation in a very 

 fine state of division. At the same time ferrous salts of iron usually 

 are altered to ferric oxide or hydrate. During surface transporta- 

 tion and sorting it is segregated in whole or in part, by reason of this 

 fine division (if it persists) and in spite of its high specific gravity, 

 along with other finely divided materials constituting muds and 

 "clays." 



This does not apply to iron occurring in the parent rock in the 

 form of magnetite or other very stable minerals', which in most 

 cases are concentrated with the coarser products of mechanical 

 disintegration, such as sands and sandy shales. Ferruginous 

 materials firmly cemented to sand grains during weathering may 

 also be transported and deposited with the sand. Iron taken into 

 solution will have yet a different history. 



In the weathering of sedimentary rocks, the behavior of their 

 ferruginous content is dependent upon the behavior of that material 

 in a former sedimentary cycle, and thus ultimately upon the con- 

 ditions already outlined for the weathering of the igneous rocks. 

 Ferric oxide, the form in which iron occurs most abundantly in 

 sediments, is, because of those conditions, usually finely divided, 

 and in a second cycle of transportation and deposition will be con- 

 centrated again chiefly with the muds. In so far as assortment is 

 imperfect, the ferruginous material may be deposited with any type 

 of sediment. 



Because of the usual absence of any commercial value in 

 the series, there is an unfortunate dearth of analyses of Red 

 Beds shales and sandstones. The fact of the concentration of 

 ferruginous matter in the fine-grained sediments is well illus- 

 trated, however, by composite analyses of shales and sandstones 



