R. A. F. PENROSE, JR. — TERTIARY IRON ORES. 45 



of Arkansas and the eastern part of Texas. On the northeast they commence a 

 few miles south of Little Rock, cross Saline river south of Benton and Ouachita 

 river between Arkadelphia and Camden, and reach Red river north of Lewisville. 

 Southwest of the Red river bottom in Texas they again appear in the border counties 

 of Bowie, Cass, Marion and Harrison, and around the upper waters of Sabine river. 

 Thence the belt bears southwestward across Angelina, Neches and Trinity rivers, 

 finally thinning out before the Brazos is reached. The length of this belt is over 

 300 miles ; the width varies from 1 to 50 miles. The ore is not found continuously 

 throughout this area but occurs intermittently, the ore-bearing areas being often 

 separated by much greater barren areas. The distribution is shown approximately 

 in plate 1. 



Associated with the ores there ai'e often found beds of sandstone, representing 

 local areas of sand indurated by the percolation of ferruginous solutions and often 

 locally mistaken for iron ore. Such deposits pass by abrupt gradations, both 

 vertically and laterally, into loose sands. 



With the exception of the iron ores and the sandstones, all the strata of the 

 region are of a loose, incoherent nature, and therefore these factors have been 

 largely instrumental in controlling the topography of the country. The strata are 

 all either horizontal or dip by almost insensible gradations toward the Gulf of 

 Mexico. They have suffered considerably from erosion, and the usual topography, 

 where the harder materials are absent, is almost flat or gently undulating in sandy 

 hills. Where the ore and sandstone are present, the region is much more broken 

 and is composed of abrupt hills and ridges, flat on top and sloping off rapidly 

 toward the creeks and river bottoms. Though these hills are rarely more than 

 from 100 to 300 feet above the surrounding drainage, they are in marked contrast 

 with the usual Tertiary topography, and are locally known as " mountains." Their 

 form has been regulated by the harder strata, namely, iron ore and sandstone, which, 

 resisting erosion better than the associated clays and sands, have protected the beds 

 immediately under them, while those above them have generally been largely and 

 sometimes altogether removed. As a result of this erosion, the iron ores as well as 

 the sandstones usually cap the hills, and the heaps of broken rock give a rugged 

 character strongly contrasted with the usual sandy or clayey Tertiary surface of the 

 Gulf states. Occasionally a covering of sand or sandy clay still overlies the ore 

 beds, and in such cases the ore is seen only where it crops out on the slopes, form- 

 ing a rocky rim around the hills or along the slopes of the ridges. Besides the ore 

 and sandstone on or near the tops of the hills, similar beds are sometimes found 

 below, cropping out on the lower slopes. 



As the ore is of only local extent, so the flat-topped hills are only local, 

 while elsewhere the less resistant strata have been eroded down to the surrounding 

 level. 



Geologic Relations of the Ores. 



The geologic position of the ores is in the Eocene scries of the Tertiary, and 

 probably in or below the ( Jlaiborne horizon of thai series. Two principal divisions 

 in the Eocene contain noticeable quantities of ore, though more or less iron is 

 characteristic of the whole series. The lower one is in the greal section of sands 

 and sandy clays which form the central partof the Eocene; the upper one is at 

 the top of the Claiborne glauconite thai overlies these }>v<\*. The lower deposits 

 are extensively developed in both Arkansas ami Texas, and comprise by far the 



