R. A. F. PENROSE, JR. TERTIARY IRON ORES. 47 



mammillary below. It directly overlies a bed of glauconite, which varies from 

 thirty to forty feet in thickness and which is underlain in turn by a sei'ies of sands 

 and clays. It sometimes crops out on the immediate summits of flat-topped hills, 

 but is more often covered by from one to twenty or more feet of sandy clay, which 

 represents the remains of the overlying strata, as already described. The glauconite 

 bed contains considerable quantities of iron pyrites and numerous Claiborne 

 fossils.* 



Sometimes thin seams of iron ore occur in the glauconite below the main ore bed, 

 but they are usually small. Between the main ore bed and the overlying sandy 

 clay there is a layer of dark -brown hard sandstone varying from one to six inches 



"-i1 i "i ^» »' ; 



Figure 17 — Ideal Section showing the Mode of Occurrence of the laminated Ores. 

 1 = Sands and sandy clays ; 2 = Ore bed ; 3 = Glauconite (greensand). 



in thickness and averaging probably one and a half inches. The ore crops out on 

 the brinks of the hills, forming a protruding rim or crown and often covering the 

 slopes with large masses that have broken off from the main bed. The accompany- 

 ing ideal section (figure 17) represents a characteristic mode of occurrence of the 

 laminated ores. 



Origin of the Ores. 



In inquiring into the origin of the iron ores of the Gulf Tertiary it is necessary 

 to consider the conditions surrounding the deposition of the great series of alter- 

 nating sands and clays which comprise the mass of the strata. That they are a 

 littoral formation is proved by the character of the organic remains enclosed in 

 them ; by the not infrequent occurrence of pebble beds (especially in Arkansas) ; 

 by the. lateral blending of marine and brackish water or lagoon deposits ; and by 

 the rolled and rounded character of many of the shell fragments, shaped as if by 

 continued beating on or near a sea beach. Again, the frequent occurrence of ex- 

 tensive beds of lignite at various horizons would indicate conditions of deposition 

 which permitted numerous ready transitions from marine to land or coastal-lagoon 

 environments. Such conditions doubtless gave rise to large areas of swamps and 

 shoals along the coast of the Tertiary embayment, probably not unlike those now 

 seen in places on the coasts of Florida and Louisiana, and around the lower part of 

 Sabine river in Texas. Into these basins the waters from the land drained and 

 probably often remained in a semi-stagnant state for considerable periods, under- 

 going a considerable evaporation. 



The rocks forming the coast of the Tertiary Gulf of Mexico all contained greater 

 or less quantities of iron-bearing materials : The glauconite of the upper Cretaceous 

 of Texas and the Paleozoic and pre-Cambrian rocks to the west of the Cretaceous 

 area were a ready source of iron to the circulating waters ; while the Carboniferous 

 and Silurian shales and sandstones of central and southwestern Arkansas supplied 

 an important quantity of iron to the waters tributary to the Gulf. These waters, 

 draining into the coastal lagoons and swamps, were subjected to active oxidizing 



*Kee Aiifielo Heilprin: The Koeene Mollusea of tin- State of Texas, ProC. Acad. Nat. Sci. I'liila., 

 part III, Oct-Dec, L890, pp. 393 W6. 



