1 62 C. W. TOM LIN SON 



is chiefly in the ferric state in light- and dark-red, buff, and yellowish 

 strata. A gray or green color, on the contrary, signifies a low pro- 

 portion of ferric oxide, and usually a preponderance of ferrous 

 over ferric compounds. The mineral composition of the coloring 

 matter is difficult to determine accurately because of its fine division. 

 A green color is "common with silicates in which ferrous iron is 

 prominent/' 1 and silicates may be important where the ratio of 

 ferrous to ferric iron is high. 



IS THE FERRUGINOUS MATERIAL AN ORIGINAL CONSTITUENT OF THE 

 SEDIMENTS, OR A LATER INTRODUCTION? 



Hypothesis of introduction of iron from igneous magmas. — If 

 the ferruginous matter was an integral part of the original sedi- 

 ments, we have no more difficulty in explaining its presence than in 

 accounting for any other common mineral constituent of sedi- 

 mentary rocks. Its source was most probably in the same rocks 

 which gave rise to other materials of the series, such as quartz, feld- 

 spar, and calcite, and the agencies of transportation were the same 

 as those which were responsible for the entire series. If, however, 

 we postulate that the iron has been introduced since the comple- 

 tion of sedimentation, after the series of Red Beds was otherwise 

 complete, we shall find it very difficult to reconstruct in imagina- 

 tion any natural agency which might have brought about such a 

 result, and we shall be puzzled to find an adequate source for this 

 vast amount of iron. 



There is one hypothesis of this kind which received credence 

 and vigorous support in America for several decades of the nine- 

 teenth century, in explanation of the red stain in the Newark series 

 of the Connecticut Valley. 2 The close association, in that series, of 

 clastic red sediments with contemporaneous extrusive and intrusive 

 basic igneous rocks of great thickness and extent made it a natural 

 suggestion that the exceptional color of the sedimentary members 

 was connected directly with igneous action. The absence of any 



1 E. S. Dana, A Text-Booh of Mineralogy (New York: John Wiley & Sons, 1909)) 

 p. 472. 



2 See J. D. Dana, Manual of Geology, ed. 1880, p. 764; and also his review of 

 Russell's bulletin on the "Subaerial Decay of Rocks" (U.S. Geol. Survey Bull. No. 52, 

 1889), in Am. Jour. Set., 3d Ser., XXXIX (1890), 319. 



