24 GULF TERTIARY OF TEXAS. 



able quantities of dark brown or gray *nica. The clay beds of this division vary 

 from a pure white highly plastic clay to a dark brown, or even black, ma- 

 terial containing large quantities of lignitic matter. They are generally lam- 

 inated, or finely stratified, and frequently occur interbedded with thin seams 

 of sand, the latter often in lenticular streaks, while the clay is generally con- 

 tinuous. A very characteristic deposit of this kind is seen underlying the 

 Claiborne greensands in the iron ore regions. The seams of clay vary from 

 one-twentieth to one-eighth inch in thickness, and the sandy seams are but 

 very little thicker. The whole formation shows a peculiar undulating section, 

 the undulations being due to the thinning and thickening of the sandy seams, 

 and not to lateral pressure.* The lignite beds of this series are composed 

 mostly of brown or black varieties, which have not as yet been put to any 

 important economic uses, and which will be treated more fully under "Eco- 

 nomic Geology.' Silicified wood is of very frequent occurrence in these 

 strata; sometimes occurring as small fragments, and at others as large trunks 

 of trees. On the Brazos River, in the northern part of Milam County, was 

 seen a trunk one and a half feet in diameter, protruding from a clay bed. 

 Ten feet of it were exposed, while the rest was imbedded in the clay. In 

 many places such fragments are collected in great quantities, but it is espe- 

 cially plentiful in the lower part of the Fayette Beds. It is generally dark 

 brown or black inside, and weathers gray or buff color on the outside. Some- 

 times it occurs partly lignitized and partly silicified. It frequently shows 

 shrinkage cracks which are filled with quartz or chalcedony, and often lined 

 with quartz crystals. Carbonate of iron, in the form of clay ironstone, is of 

 very frequent occurrence throughout the Timber Belt Beds. It rarely occurs 

 in a continuous seam, but is found in lenticular masses and nodules, often oc- 

 cupying the same plane of stratification for considerable distances. Some- 

 times these masses coalesce into a bed continuous for a few hundred yards. 

 They are rarely over three or four inches in thickness, and are generally 

 rusty from oxidation. They are probably the source of some of the brown 

 hematite iron ores in the counties north of the Sabine River. (See Iron Ores.) 



*The sand seams look like a series of connected lenses blending into each other at their edges. 

 This interlamination of sand and clay was caused by the different velocity of the waters that 

 flowed over the beds during their deposition — the swifter waters carrying and depositing the 

 sand and the more sluggish waters depositing the clay. It is natural that such waters as would 

 carry sand would have sufficient velocity to give a gently undulating surface to the beds 

 that they are depositing, and not the smooth level surface of a still- water sediment. Thin 

 beds of clay laid down afterwards on such a surface would naturally conform to the inequal- 

 ities of the surface, and hence the undulating section that we see does not require the suppo- 

 sition of a lateral pressure for its formation. It seems possible that this same phenomenon 

 may also account for the undulations in many of the old gneissic and schistose rocks, many 

 of which may have once been in the form of sands and clays. 



