48 GULF TERTIARY OF TEXAS. 



largely of sand, with calcareous seams and nodules. The local differences 

 will be mentioned farther on. The clays vary from laminated chocolate-col- 

 ored beds to massive light watery-green and pale sky-blue strata of a most 

 characteristic and unmistakable appearance. This latter variety increases in 

 abundance as we approach the middle of the Fayette Beds, and the clays that 

 are associated with the upper sandy part are all of this character. They are 

 hard, massive, heavy, with conchoidal fracture, and cut like talc. They weather 

 a pure White. Such clays underlie the town of Rio Grande City, on the Rio 

 Grande, and the climatic conditions here are such that they have become in- 

 durated by the heat and dryness of the region into a soft rock. Large quan- 

 tities of lignite are frequently found in the clay beds, especially in the choco- 

 late clays near the base; and even where lignite is absent, the strata are very 

 often highly impregnated with vegetable matter, and contain the remains of 

 many plants. The study of these will doubtless throw much light on the 

 geological position of the Fayette Beds, but they have not yet been deter- 

 mined. Faulting on a small scale is of very frequent occurrence and consider- 

 able jointing is seen everywhere. Figure 2, page 52, shows a faulted lignite 

 bed on the Colorado River, in Fayette County. Sulphur and gypsum are of 

 very frequent occurrence, the latter often being found as twin crystals in the 

 shape of an arrow head. The beds also frequently contain carbonate of lime 

 in the shape of nodules, or impregnating the strata. One of the most marked 

 characteristics of the clays, and especially of the chocolate-colored beds, is a 

 white bleached appearance on the surface, while, a few inches in, they regain 

 their dark color. This, and the presence of sulphur and gypsum, are 

 intimately connected phenomena, and can be easily explained by the com- 

 bined decomposition of the iron pyrites, carbonate of lime, and the vegetable 

 coloring matter of the dark clays. The iron pyrites decomposes with tne form- 

 ation of sulphate of iron and sulphuric acid; the sulphuric acid attacks the 

 carbonate of lime, forming gypsum and carbonic acid; the former is de- 

 posited as crystals, and the latter goes off in the air and surface waters. The 

 sulphate of iron attacks the organic matter in the clays and is again reduced 

 to iron pyrites with the evolution of sulphuretted hydrogen and oxygen. The 

 oxygen forms carbonic acid with the vegetable matter, and rapidly goes off 

 into the air. This reaction repeats itself until the clay finally becomes devoid 

 of all vegetable material, and hence of coloring matter, and exposes a white 

 surface. The sulphur, which originally formed a part of the sulphuretted 

 hydrogen, but which has now lost its hydrogen, is deposited as a yellow or 

 white crust on the surface and in the cracks of the strata. 



The sandy strata of the Fayette Beds are very variable in thickness and 

 consistency, though their composition is very constant. They consist almost 

 entirely of pure coarse siliceous sand, generally sharp, and containing red 



