II. A. F. PENROSE, JR. — TERTIARY IRON ORES. 40 



the top of the glauconite bed and immediately below the overlying clayey sand. 

 Here it occupies t lie same position as the laminated ore elsewhere and is frequently 

 associated with sands and clays which often contain lignite. The thin layer of 

 sandstone found overlying the laminated ore frequently contains masses of lignite 

 completely converted to iron ore, and these prohahly represent the alteration 

 product of the lignite originally associated with the pyrite. 



The following section at the McBee school-house, near Alto, Cherokee county, 

 Texas, shows a case of the original condition of the iron pyrites : 



1. AVhite sandy clay varying from 10-30 feet. 



2. Ferruginous sandy clay becoming indurated at base 1 foot. 



3. White sandstone with a cement of profusely disseminated iron 



pyrites 1-3 inches. 



4. White sand with lenticular masses of lignite (1 to 4 inches in thick- 



ness) and many disseminated particles of iron pyrites, passing 



below into a plastic greenish-brown clay •'! feet. 



5. Dark-green glauconite at hottom of section 



This section appears to represent the original condition of the strata before the 

 formation of the laminated ore. That ore usually occurs immediately above the 

 glauconite represented in number 5 of the section, but here the same position, that 

 is, above the glauconite and below the sandy clay, is represented by some four feet 

 of sandy and clayey strata highly charged with iron pyrites. This mineral, by its 

 oxidation, forms sulphuric acid and sulphate of iron, the latter sooner or later be- 

 coming still farther oxidized and going into a hydrous sesquioxide of iron. It 

 seems probable that the combined action of the sulphuric acid and sulphate of iron 

 percolating down from the pyritiferous sands into the clay causes an interchange of 

 constituents, and that the clay is to a greater or less extent converted into iron ore. 

 This would account for the considerable percentage of alumina usually found in the 

 ore, and also for its laminated structure, a structure often seen in the unaltered 

 clay. The thin layer of sandstone, which has already been mentioned as capping 

 tin' laminated ore, is probably due to the induration of the sandy stratum imme- 

 diately overlying the clay by the peroxide of iron derived from the oxidation of 



the pyrite. • 



The shape of the ore bed is strong evidence of the formation of the ore by the 



process just described: the upper surface of t he bed is usually flat, but the base of 

 it is very uneven and shows a series of bulging and receding mammillary forms. 

 These masses are often distinct from each other, but are closely assembled together 

 in a continuous or almost continuous stratum. The upper surfaces of the ore 

 masses are often concave, while the lower surfaces are convex, apparently pointing 

 to derivation bv t he downward percolation of the ferruginous solutions as already 



described. 



The glauconite itself may in some cases have assisted in the formation of the 

 laminated ore, hut its influence has probably been small. Glauconite is doubtless 



an important source of iron in surface waters, and the ferruginous -"hit ions derived 

 from it may often be precipitated elsewhere and accumulated in considerable 



beds of ore; but the case in question Ls one of the formation of brown hematite 

 in situ, and in such a process glauconite does nol seem, at least in the Tertiary area 

 of Arkansas and Texas, to have been so important a factor as the carbonate and 

 sulphide of iron. 



VI] Bi n. Gi oi 3oi Vm., Vol. 3, 1891. 



