154 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



being the newest. Coal beds from a few inches to lo or more feet 

 thick are numerous in the Timber Belt. The enclosing clays, in many 

 cases, are extremely dark and contain much silicified wood as well 

 as lenticular masses of iron carbonate. Silicified wood is abundant 

 in the Yegua. 



Penrose,-^' in a publication of somewhat earlier date, described 

 the lignite as occurring in a broad area, which in some portions ex- 

 tends eastward to within 150 miles from the Gulf coast. He sepa- 

 rated the rocks into two divisions of which the upper may be Mio- 

 cene. The coal beds are often double, as shown by a section in 

 Robertson county, where the benches, 12 and 2 feet thick, are sepa- 

 rated by 2 feet of clay. This is the important bed of the lower group 

 and its coal is lignite, black, friable and woody. The upper group, 

 along the Colorado River, has beds one to 10 feet thick, some of 

 which contain masses of wood, including tree trunks partly silicified, 

 partly lignitized. The coals of this upper group are all in lenticular 

 deposits. Texas brown coals hold not only trunks, branches and 

 leaves of trees but also reeds and other forms characteristic of 

 swamp vegetation. In some beds, the coal shows distinct vegetable 

 structure, but generally the mass of the material has been so 

 thoroughly converted that no trace of structure is visible to the 

 unaided eye. Frequently the coal is amorphous and soft, while at 

 others it is hard, black, brilliant, with either cubical or conchoidal 

 fracture — but all possible gradations exist between these extremes. 

 The rocks throughout are undisturbed and coal of both types appears 

 often in a single section. At the San Tomas mines, 25 miles above 

 Laredo on the Rio Grande, a coal bed was seen with this structure : 

 lignite, 2 inches; clay, 4 inches; coal, i foot 3 inches; black clay, 2 

 inches; coal, i foot 3 inches. The underlying clay contains just 

 below the coal streaks of lignite — a faux-mur. The coals are mas- 

 sive glossy black and with conchoidal fracture, without trace of 

 vegetable texture ; but the thin top bench is a true lignite with the 

 plant texture well-preserved. Kennedy-^^ in the same state found 



21^ R. A. F. Penrose, Jr., " Preliminary Report on the Gulf Tertiary of 

 Texas," First Ann. Rep. Geol. Survey of Texas, 1890, pp. 26, 43, 52, 53, 94, 95. 



218 W. H. Kennedy, " Harrison County " ; J. H. Herndon, " Smith 

 County"; Second Ann. Rep. Geol. Survey, pp. 155, 156, 267. 



