NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 43 



cemented, but I am inclined to think that it may be explained otherwise. 

 The "peculiar conglomerate" consists of small pebbles of about the size of 

 a large pea, mostly a light brown clay ironstone, frequently concretionary 

 in character, or weathering in concentric layers. The cement is limonitic. 

 For the purposes of this paper it may be best to refer to it hereafter as the 

 limonitic conglomerate. This conglomerate appears in sections i, 2, 4, and 

 7, but does not appear again as a bed of any extent farther west, i.e., higher 

 in the series. It must not be assumed that its appearance in the sections 

 mentioned indicates a continuous bed between them. It is altogether prob- 

 able that it does not, and in several places the recurrence of the same 

 material is demonstrable. 



The iron of secondary deposits is taken from the original minerals in the 

 zone of weathering (according to Van Hise and others), either by direct 

 conversion of the iron into ferric oxide, insoluble except in the presence of 

 CO2 or humic acids, or by conversion into iron carbonate and then into 

 limonite or other compounds. Limonite is formed from the carbonate by 

 the action of oxygen in limited cjuantities, and is deposited below or near 

 the water-level, where high oxidation is not likely to occur. 



We may be certain that the red clays of Texas, with their ferric oxide, 

 were deposited in the sea, or other bodies of water, in the condition in which 

 they now occur, and are not due to subsequent dehydration or decarbonation, 

 because (i) the color is uniform throughout; (2) because there is a solidity 

 and density in the clays, and a lack of filled seams and veins, which would 

 be impossible after such changes, which involve a decided decrease in volume; 

 and (3) because the red color transgresses into the limestones and sandstones 

 with marine fossils. The ferric condition of the iron may be due to either 

 the original formation of the ferric oxide from the iron-bearing minerals or 

 to the oxidation of carbonates in the course of prolonged transportation over 

 semiarid or, at least, well-exposed fiats previous to deposition. 



The presence of limonite seems to indicate a higher level of the ground- 

 water of the flats, for under such conditions the hydrated oxide derived 

 from the carbonate would not be so likely to make long journeys exposed to 

 the oxygen of the air. Thus we have an indication of a local increase in 

 humidity during an interval of time represented by the limonitic conglomer- 

 ates and associated light-colored clays and sandstones. The following state- 

 ment from BarrcU =" bears directly upon this point : 



"Sediment carried by rivers is subject to oxidation both while in transit and 

 after deposition on the surface of the flood plain, until its burial by overlying layers 

 carries a stratum below the level of ground-water. Where the ground-water level 

 is coincident with or higher than the surface, organic matter accumulates and de- 

 oxidizing processes take place. A certain fraction of delta deposits, depending on 

 the proportion of back swamps and coastal swamps, therefore show colors ranging 

 from green to blue, according to the state of the iron oxide, and from white through 



» Barrcll, Bull. Geol. Soc. Amer., vol. 2^, p. 4i6,fi9i2. 



