82 GULF TERTIARY OF TEXAS. 



ferruginous pebbles and nodules in the beds of the streams. The pebbles 

 have originated both by the breaking up of iron ore beds, and by the collec- 

 tion of ferruginous concretions in the destruction of sand and clay beds. 

 The first class of pebbles do not necessarily all come from the large ore beds 

 already described, though many of them doubtless are derived from that 

 source, but a large quantity also come from the breaking up of thin seams of 

 hard-pan and iron ore often less than an inch thick. These are of universal 

 occurrence throughout the iron ore region, and readily succumb to the de- 

 nuding action of the atmosphere. They break into fragments, become rolled 

 and rounded in the creek beds, and are finally deposited in a solid stratum. 

 The second class of pebbles, i. e., those from the sand and clay beds, are nod- 

 ules resulting from the oxidization of iron pyrites and from the segregation 

 of ferruginous solutions. When the sand or clay beds containing these are 

 eroded, the pebbles are carried down into the streams and mixed with others 

 just described. These iron ore fragments and nodules collect in very large 

 quantities, especially in the creek beds, where the water has in many places 

 carried away much of the lighter sand and concentrated the pebbles. The 

 ferruginous matter in the sandy matrix of this rock owes its source to the 

 solution of iron salts in the streams and springs throughout the region. 

 These solutions run over and percolate into the originally loose bed of ore 

 pebbles, become rapidly oxidized, and deposit red peroxide of iron, which, 

 acting as a cement, binds the whole mass together. This action can be seen 

 going on in many places at the present time, and is only the natural result of 

 the physical and chemical influences now at work in the East Texas region.* 



Though this is the usual mode of formation of these beds, there are some 

 that have evidently originated differently. These have the appearance of a 

 mass of small nodules, one-eighth to one-quarter inch in diameter, in a porous 

 mass, with cavities between the separate nodules, and with little or no sand. 

 Such deposits are probably formed by the partial erosion in place of sandy 

 beds containing ferruginous nodules. The sand, being lighter than the ore, 

 is carried away by the surface waters, whi]e the nodules remain and become 

 solidified in a porous bed. Some of these nodules, of course, are carried 

 down into the streams, and go into the form of conglomerate just described, 

 but many of them doubtless remain. 



These ores are much richer in iron than the other variety of conglomerate 

 ores, and sometimes analyze over forty per cent of that metal. 



*These ores are of low grade and of little economic value. 



