1 66 C. W. TOM LIN SON 



through the interstitial spaces. This material may have filtered in 

 during the process of cementation; or it may have been present at 

 the time of sedimentation and have been thrust out of the way by 

 the growing crystals during enlargement. Occasional specks of 

 red-stained material are found within the area of the enlargements 

 and distinct from the inner coating of the original grains ; and where 

 the enlargement is missing, the interstitial hematitic matter may 

 be difficult to distinguish from that of the primary coating. The 

 stained areas probably do not represent pure ferric oxide, but ferric 

 oxide in such ratio to silica or clayey material as to render the 

 mixture nearly opaque and quite uniformly red or reddish brown; 

 for chemical analyses of the same rocks show the percentage of 

 ferric oxide in the rock to be much lower than the percentage of red- 

 stained area in the sections. 



A similar microscopic investigation has been made by Richard- 

 son 1 in his study of the Spearfish formation of the Black Hills. 

 He says: "Amorphous red pigment is prominent in the slides. It 

 irregularly coats and spots the minerals, and .... constitutes 

 the chief interstitial substance." 2 He also presents an analysis to 

 show that the ferric oxide of the pigment is essentially anhydrous. 



Thin sections of Red Beds from all parts of the West are not 

 available at present, and it has not been practicable, therefore, to 

 make a thoroughgoing microscopic study of the group. Richard- 

 son's description suggests a relation of pigment to cementation 

 similar to that found in the Newark sandstones: a twofold relation, 

 indicating that part of the pigment was transported and deposited 

 as a coating on sand-grains, and that the remainder constitutes an 

 important part of the cementing material of the rock and is coeval 

 with the rest of that material. 



But what was the time of cementation ? Was this process com- 

 pleted before the exposure of the Red Beds series to erosion, or is it 

 still going on, or did it cease at some intermediate time ? In so far 

 as this question affects the iron content of the Red Beds we really 

 have answered it already; for if any considerable part of the iron 

 had been introduced as a cement later than the time of sedimenta- 

 tion it would not be found most abundantly in the strata which 



1 Op. cit. 2 Ibid., p. 379. 



