GEOLOGY OF THE ELEVATED PLAINS.—SAND HILLS. 19 
shaft of the mine, I observed five distinct beds of gypsum alternating with the blue clay, and 
varying in Ó— from five to twenty feet.” 1 
The formation which was partly traversed by the survey, although, perhaps, equalled in the 
thickness of the beds by other localities, is nevertheless the most remarkable for its extent, and 
for the number and variety of the gypseous beds of all’ degrees of transparency and of many 
colors. Captain Pope observes that it extends over a distance of one hundred and fifty miles 
along the route. It was found along Delaware creck, the Pecos, and again at the sources of 
the Colorado, at the eastern margin of the Llano. The same fo Finition is largely developed 
along Red river and the Canadian; and the explorations of Captain Marcy and of Lieutenant 
Whipple have added greatly to our knowledge of its character and extent in that direction. 
Captain Marcy, in a letter to Professor Hitchcock in 1852,? observes that he has ‘‘traced this 
gypsum belt 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, Middle, and South forks of Red river it is 
found, and upon the latter is about one hundred miles wid, and embraced within the 101st 
and 103d degrees of longitude.”’ 
With regard to the geological age of this interesting formation, we are yet without full and 
satisfactory evidence. Observations on this subject, and additional remarks upon the gypsum, 
its uses, value, &c., will be found in a subsequent portion of this report. 
Plains between the Rio Grande and the Guadalupe mountains.—It is difficult to form a satisfac- 
tory conclusion regarding the geological character ef the plains between the Guadalupe, the 
Hueco, and the Organ mountains. The collection is without specimens from the strata underly- 
ing this part of the route. It is only possible, therefore, to infer the geology from the topog- 
raphy and the general character of the regions as compared with the adjoining plains, the geo- 
logical character of which is already indicated by fossils. These determined points are the 
Llano Estacado on one side, and more particularly the well-known Cretaceous plains of the Rio 
Grande, at Doña Ana, on the other. This locality of the Cretaceous was well determined by 
the collections of the United States boundary survey, many Cretaceous fossils having been 
obtained. The elevation of the locality is nearly that of the plains under consideration; and 
I am led to infer that they are formed of Cretaceous strata, and have so colored them on the 
map. Further explorations of the region will, perhaps, lead to the discovery of Tertiary strata 
overlying the Cretaceous, and the exact limits of the formations will only then become known. 
Sand hills.—Hills of dry, blown sand, appear to be a prominent characteristic of the surface 
of the Llano, and must greatly add to its desert-like and barren appearance. From the Diary 
of Mr. Byrne, it appears that they were encountered before reaching the Llano, and on the west 
side of the Hueco mountains. Between camps 11 and 12, on the plateau between the Hueco 
and Guadulupe mountains, a long belt of sand hills was alo found, and the wagons travelled 
for seven miles at their base. Smaller hills were also seen for a long distance from camp, all 
composed of fine white sand. But these accumulations of sand do not compare in magnitude 
or extent with those encountered on the broad Llano, where, according to the same observer, the 
hills extend for fifty miles from north to south, with a breadth of fifteen miles from east to 
west. This is the great accumulation of sand on the Llano, and forms a conspicuous feature on 
the map; it is certainly the most extensive accumulation yet known in the interior; the belts 
of sand hills on the Colorado desert of California being much inferior in magnitude. In the 
remarks of Captain Pope upon the Llano Estacado, he states that the Llano is bordered on the 
south by a range of sand hills of white drift-sand, absolutely destitute of vegetation, and rising 
seventy feet above the general level of the plain. He further observes: ‘‘These hills, over a 
distance of thirteen miles, present steep ascents threugh short distances in many places; and 
the loose, moveable character of the sand, and its depth, render the passage through it with 
1 Quart. Jour. Geol. Soc. London, vol. i, p. 2 
2 See Report of Captain Marcy —Appendix D,—Geology by A Hitchcock, p. 172. 
