No. 7.— The Jura of Texas. 
By Jules Marcou. 
The first time that Jurassic rocks are mentioned as occurring in 
road near the thirty-second parallel of latitude, by Capt. John Pope, 
Chap. 13: Geological report; Geological notes of a survey of the 
country comprised between Preston, Red River, and El Paso, Rio 
Grande del Norte, p. 127. House document, 129. Washington. 
1854. 8°. 
In the “geological report” by Jules Marcou, published in 1855 
(dated, September, 1854), the author says that the collection made 
by Capt. Pope contained “ fragments of the Gryphaea Tucumcarii , 
found in a limestone ” on the Llano Estacado; and adds that the 
grayish Jurassic limestone was found in ascending to the Guadalupe 
Pass in the Sierra of Guadalupe, western Texas. 
In Geology of North America, by Jules Marcou (Zurich. Feb¬ 
ruary, 1858. 4°), we read: “Fossils of the Jurassic rocks: Capt. 
Pope has found the Gryphaea dilatata var. Tucumcarii in the 
southern part of the Llano Estacado, and Dr. Kennerly gave me 
specimens of that species picked up by him at Leon Spring, between 
Fort Inge, Texas, and El Paso, Chihuahua”; and on the Carte 
geologique des Etats-LTiis, etc., accompanying the Geology of North 
America, the Jurassic series occupies the whole Llano Estacado, 
between the Rio Pecos, the Canadian River, and the headwaters of 
the Red River, the Rio Brazos and the Rio Colorado of Texas, as 
well as the whole plateau or mesa between the Rio Pecos and the 
Rio Grande del Norte round Leon Spring. It must be said that, out¬ 
side of the route followed by the present writer along the thirty-fifth 
parallel of latitude, the limits assigned to the Jurassic series were 
hypothetical, and that more recent formations, as the Cretaceous, 
Tertiary, and Quaternary, may have covered and consequently con¬ 
cealed the Jurassic series. 
In the same volume, at p. 33, under the heading, Fossils of the 
Cretaceous rocks, I have described and correctly figured Ammonites 
