Tl40, 41] EFFECTS OF DENUDATION — CHEMICAL CHANGES. 23 



forms, in some measure, to the surface of the denuded mass. This sand is slightly 

 calcareous, and contains cretaceous fossils, which are best preserved in the 

 mass of the concretionary nodules appearing in the upper portions of the deposit 

 — indurated by a cement partly ferruginous, partly calcareous. The ancient 

 valley, the outline of which is marked by detrital fragments of slaty ferruginous 

 sandstone, adds interest to this section. Immediately back of tins cut rises 

 a high Orange Sand ridge, on the summit of which cretaceous fossils are found 

 in the ferruginous sand rock (nil). In the case before us, as in numerous other 

 instances seen in Tishomingo county, it becomes optional with the observer 

 to which of the two adjacent formations he chooses to consider the material in 

 question as belonging. 



40. It is perhaps in an analogous manner, that the anomalous basin-shaped 

 mass of black clay in the Orange Sand cut near Pocahontas (l[27) may meet its 

 explanation. If originally existing in sands belonging to the lignitic tertiary 

 (on whose territory we see similar phenomena), it might have resisted denuda- 

 tion by virtue of its extreme tenacity, while the adjacent sands might have 

 been partially removed, and their place filled with Orange Sand deposits. 



41. Chemical effects on other for mat ions. -We have thus far 

 noticed the changes which the materials of the more ancient 

 formations undergo when removed by the denuding action which 

 characterizes the Orange Sand period. But the chemical changes 

 effected on the upper layers of these formations, even when remain- 

 ing in place, are not less conspicuous at times. The effects produced 

 are referrible, in general, to one or more of three agents, viz : 

 Lixiviation and Oxidation, by the joint action of carbonic acid, 

 oxygen and water ; the action of siliceous solutions; and that of 

 ferruginous solutions. 



Instances of all of these effects within the Orange Sand formation have 

 • already been mentioned. The effects of oxidation may sometimes be noticed at 

 the planes of contact of the Orange Sand with the black lignitic strata, which 

 have been bleached on their surface ; but more usually this influence has been 

 accompanied or succeeded by one of the others, and most frequently, by the 

 ferruginizincj action which has already been cited as one of the prominent 

 features, especially of the upper stages of the formation. Thus in N. Tippah 

 county, and also occasionally in the more southerly portion of the northern 

 Lignitic region, we find the uppermost layers of the latter formation (there 

 composed chiefly of gray or black clays) transformed into red shales of consider- 

 able hardness — an excellent material for the preservation of organic remains, and 

 in some instances very rich in fossil leaves. One of these localities (Hurley's 

 Schoolhouse, at the heads of Ocklimita Creek (HT70), has furnished the best 

 collection of fossil leaves which I have as yet obtained from the Northern 

 Lignitic formation. Other instances of the occurrence of similar shales, will be 

 found mentioned under the head of the latter formation. 



