APPENDIX D. GEOLOGY. 149 



gypsum is embraced in the red clay formation already described, for 

 most of this mineral occurs above the red clay, though sometimes em- 

 braced within it. Yet the importance of the gypsum justifies me in 

 coloring that portion of these strata as the gypsum formation where it 

 is most abundant. It is several times mentioned as occurring in other 

 parts of the region, marked as red clay. But on the 3d of June, high 

 bluffs were met of red and blue clay, with interstratified layers of snow- 

 white gypsum. From this time till the 12th the same formation was 

 found, and' also from the 21st of June to the 9th of July. But your 

 own description of this formation in your letter of November, 1852, 

 contains a better account of its extent than I can give. 



"I have traced this gypsum belt," you observe, "from the Canadian 

 river, in a southwest direction, to near the Rio Grande, in New Mexico. 

 It is about fifty miles wide upon the Canadian, and is embraced within 

 the 99th and 100th degrees of west longitude. Upon the North, Mid- 

 dle, and South forks of Red river it is found, and upon the latter is 

 about one hundred miles wide, and embraced within the 101st and 103d 

 degrees of longitude. I also met with the same formation upon the 

 Brazos river, as also upon the Colorado and Pecos rivers, but did not 

 ascertain its width. The point where I struck it, upon the Pecos, was 

 in longitude 104£° W. 



"Wherever I have met with this gypsum I have observed all the 

 varieties from common plaster of Paris to pure selenite ; and among 

 specimens of the latter were pieces three feet by four, two inches in 

 thickness, and as perfectly transparent as any crown glass I have ever 

 seen. It is to be regretted that I could not have brought home some of 

 these beautiful specimens; but my means of transportation were too 

 limited. I regard this gypsum belt as a very prominent and striking 

 feature in the geology of that country. From its uniformity and extent 

 I do not think there is a more perfect and beautiful formation of the kind 

 known. I have myself traced it about three hundred and fifty miles, 

 and it probably extends much further." 



The position and thickness of the gypsum beds may be learnt from 

 Dr. Shumard's sections, especially No. V, where they are from ten to 

 fifteen feet thick. I do not wonder that you have been deeply impressed 

 with the vast extent of this deposite. Prof. D. D. Owen, in his late 

 valuable report of a geological survey of Wisconsin, Iowa, &c, (1852) 

 describes a gypseous deposite, twenty to thirty feet thick, in the car- 

 boniferous strata, and occupying an area from two to three square miles ; 

 and he says, that " for thickness and extent, this is by far the most 

 important bed of plaster-stone known west of the Appalachian chain, if 



