168 ANNALS NEW YORK ACADEMY OF SCIENCES 



■Where dryness prevails for most of the 3'ear, and where vegetation is as a 

 result scanty, snch oxidation may be especially favored. Thus semiarid 

 or even desert regions would furnish the best conditions for such oxida- 

 tion. On river flood plains there is always sufficient moisture to result 

 in the formation of hydroxides of iron, and hence the colors of such 

 deposits will range from yellows to ocher and brown. It is only under 

 conditions of intense heat that dehydration will result with a consequent 

 change in color toward the reds. Such change of color may, however, 

 take place as the result of aging of the deposits, as pointed out by Crosby" 

 (Grabau, 1914, 6). Crosby's statement is as follows: ". . . the 

 color of the deposit, so far as it is due to ferric oxide, is, other things 

 being equal, a function of its geological age. ... In o.ther words, 

 the color naturally tends with the lapse of time to change from yellow 

 to red; and, although this tendency exists independently of the tempera- 

 ture, it is undoubtedly greatly favored by a warm climate" (1891, 3). 

 Barrell has also discussed the causes of color combinations in continental 

 sediments (1912, 10). 



Without discussing the literature on this subject any further, tlie fol- 

 lowing conclusions may be made regarding the origin of beds of various 

 colors in the Morrison formation. As noted above (p. 159), the coarser 

 beds are usually gray or white, these beds often being cross-bedded, while 

 the finer beds are green, gray, white, blue or red. The red and reddish- 

 brown beds are not extremely fine, however, like the greenish clays. 

 This is probably due to the prevalence in them of quartz grains, while 

 the green clays are often composed largely of kaolinic material. There 

 is a considerable amount of gradation of color in the finer beds. The 

 coarser sands were probably deposited in the streams and as deltas in the 

 lakes. The finer red, brown and gray grits were deposited in both lakes 

 and streams, and also along river flood-plains. In most cases it would 

 be difficult to assign one of these brownish-red grits to a precise origin. 

 As noted above in discussing the petrographic characters of the forma- 

 tion, the green clays often grade iiito red and l)rown, and there is distinct 

 evidence of the origin of some of the red color, at least, by the alteration 

 of iron carbonate. It is possible, of course, that some of this oxidation 

 may have taken place before the burial of the material. It is much more 

 probable, however, that the process has been going on during a long 

 period of time subsequent to the burial of the deposits, and in some cases 

 is still going on. Many beds have been completely oxidized, there being 

 little but quartz and red-stained clayey matter in the rock. This material 

 is usually more abundant in the upper members of the formation than in 

 the lower. Other beds show the operation to have progressed to a con- 



