CHAPTER H. 
GEOLOGICAL NOTES 
OF A SURVEY OF THE COUNTRY COMPRISED BETWEEN PRESTON. RED RIVER, 
AND EL PASO, RIO GRANDE DEL NORTE. 
(Extract from Report of Exploration of a route for the Pacific Railroad, near the thirty-second parallel of latitude, from 
the Red River to the Rio Grande, by Brevet Captain John Pope , Corps of Topographical Engineers. CHAPTER 
XIII. — GEOLOGICAL REPORT, page 125 etc. Washington, 1855. House Doc. 129.) 
Boston, September 21** 1851. 
Dear Sir, 
I have the honor to submit tliese geological notes in relation to the route you have surveyed 
from El Paso del Norte to Preston, with a view to the construction of a railroad from the Missis- 
sippi valley to the Pacific coast. 
I have availed myself of the notes which you sent to me in your letter of the 12"' of September, 
and of the excellent collections of minerals, rocks, and fossils which you made during your sur- 
vey. The short time allowed me to prepare this Resume must be the excuse for its brevity and 
imperfection. To complete the study of your collection will take several months, and then I shall 
be able to send you a detailed Report, with a map, descriptions, and drawings of the new fossils, 
and an analysis of the minerals you have collected in your very interesting survey. Meanwhile, 
I hope these notes will suffice to give yon a slight sketch of the mineralogical and geological re- 
sources of the country. 
Preston and its environs are formed of the Cretaceous Rocks that extend along the Red River 
and the False Washita, and as far as the Canadian river. These rocks form also the beds of se- 
veral tributaries of Trinity river, especially of the Elm Fork of Trinity, where your survey has 
found this formation very well developed with numerous fossils. The Cretaceous Rocks consist, 
at the base, of yellowish-gray limestone, filled with broken oysters, of which the most common 
species is the Grypheea Pitcheri; then pale grayish-blue clays are superposed, containing numerous 
fossils, such as: Grypheea sinuata var. Americana', Exogyra Texana, a species of Ostrac®, having the 
greatest similarity to Ostrea jlabellala Goldf. ; Ostrea carinata-, Pecten quinquecostatus I'oxaster Texanus', 
etc. Upon these clays are sandy limestones , grayish-white , containing large Ammoniies , Hamites , 
pmA Baculites'. the most common are Hamites Fremonti, and several other new species, such as: Am- 
monites Shumardi, Ammonites Belknapii, Ammonites peruvianus, Ammonites Gibbonianus ■, Isocardia Washita; 
Ilolaster Comanchesi ; that you collected , and which I will describe in the final Report (See : the fol- 
lowing Chapter III, Paleontology.'). The lower part, formed of limestone with Grypheea Pitcheri, Ca- 
protina Texana, and blue clay with Toxasler Texanus, corresponds to what is called by geologists 
