456 GEOLOGY OF NORTHWESTERN TEXAS. 



the masses of gypsum. This is the case where the original beds of limestone 

 were interbedded with seams of clay. The limestones on both sides of the 

 clay would be changed into gypsum by the action of the sulphuric acid, but 

 the clay would be unaffected by it and remain in its original form. 



The other source of gypsum is by the deposition of material from sea water 

 in enclosed seas. All sea water contains many materials dissolved out of the 

 soil and carried down to the sea in solution. This material would only be 

 precipitated by evaporation of the solvent. Common salt and gypsum are of 

 this class. When the sea becomes enclosed so that there is no inlet from 

 the ocean, and not enough fresh water flowing into the sea from the rivers 

 and creeks to keep up the supply of water, at a certain stage of evaporation 

 the various materials will be precipitated to the bottom, and if the evapora- 

 tion continues for any length of time, heavy beds will be formed. Sulphate 

 of lime, or gypsum, will be precipitated at one stage of the evaporation and 

 common salt at another. The clay beds, sandstones, and limestones will be 

 interbedded with these as the material is brought into the sea from the ad- 

 joining land. Thus in one place we find beds of gypsum pure and white, 

 and in another rock salt, and in others limestone, sandstone, and clay beds 

 without any admixture of either of the others. 



The gypsum found in Western Texas was formed by this latter process. 

 At one time the Permian sea was cut off from the main ocean, and as the 

 waters of that sea became exhausted from time to time by evaporation, the 

 beds of gypsum were deposited. At other times the sea would receive an 

 amount of water, either from the open sea or by an influx of fresh water from 

 the surrounding country, heavily laden with other material. Thus we have 

 clay beds, limestones, and sandstones interbedded with the gypsum and salt. 



The gypsum fields of Texas and the Northwest are the most extensive of 

 any such formation in the United States. In Texas it reaches from the north 

 line of the State to the line of the Texas and Pacific Railway on the south, 

 and is from twenty to fifty miles wide. The beds are of varying thickness, 

 from that of a knife blade to twenty feet. The east line of the deposit begins 

 on Red River near the mouth of the North Fork, and thence by a line to 

 Sweetwater, on the line of the Texas and Pacific Railway. 



The western boundary is at or near the foot of the Staked Plains. The 

 entire thickness of the bed containing gypsum about the middle of this dis- 

 trict is about nineteen hundred feet. I have placed the gypsum beds in the 

 Permian. There are various forms in which the gypsum occurs, but it all 

 has the same chemical ingredients, the forms differing only in the manner of 

 crystalization. 



Selenite. — This is a clear, transparent variety, and has the quality of being 

 split into thin sheets like mica or isinglass, and is very often taken for that 



