GEOLOGY OF NORTH AMERICA. 
27 
the ^eocornian formation i:)-, while the upper portion, containing Ammonites Shumardi, A. permianus, 
and Hamiles Fremonti, corresponds to the formation designated as Green-Sand and Marly-Chalk. 
The Cretaceous rocks are very useful in the construction of a railroad. Lying in horizontal 
strata, they form excellent embankments, contain numerous springs of water, and the limestone 
especially yields e.xcellent lime for building. The road is upon these rocks from Preston to the 
Lower Cross Timbers, where, beneath the Cretaceous, is seen pinkish-gray and often violet sand- 
stone, with an inclination south-south-east, and in discordant stratification with the Cretaceous rocks. 
This sandstone formation belongs to the Carboniferous rocks, whose different beds occupy the whole 
M The Neocom.an rocks were not known to e.vist in North .America before my exploration in company 
with Caiit. A^iipple. More than ten years previous d’Orbigny had recognized the Neocomian in the col- 
lections made and the sections constructed by my friend, the late Colonel Acosta, in New Grenada 
South America; which rendered it probable that these rocks existed also in the United States and in 
Mexico. The Neocomian formation of the environs of the town of Preston, of Fort AVasliita and of the 
banks of the False Washita river , rests in discordant stratification either u|)on the Carboniferous rocks 
or upon the New Red Sandstone; further, all the fossils found in it are either identical with, or have 
forms analogous to those that form the Neocomian fauna of the environs of Neuchatel (Switzerland) and 
the Lower Green Sand of England ; and as the rocks in which they are found belong to the lowest beds 
of the Cretaceous formation of America, it is natural to conclude that this formation corresponds to the 
Neocomian of Europe. Thus, when in « .4 Geological Map of the United States, toith an Explanatory Text 
etc.* published in Boston in July 18o3. I said page 45, that « it was highly probable the Neocomian 
would be found m the Indian Territories of the Far West*; I was right, and only two or three months 
after the publication of this book I discovered the Neocomian formation at Fort Washita, on the False 
Washita ri\er, and in the Prairies (Longitude 99°, Greenwich, latitude 35°.). Since then, from speci- 
mens submitted to me and descriptions , I do not hesitate to regard as belonging to the’ Neocomian, 
a band of limestone extending from the river Vert-de-Gris making a curve towards the west to redescend 
the banks of the False Washita river in passing by Preston; this band forms afterwards the first pla- 
teaux of Texas, especially those west of Now Braunfelds and Fredericksburg, from whence issue the Rios 
San Saba, Piodernales and Guadalupe; finally it continues upon these plateaux as far as the southern 
end of the Llano Estacado, and there at the sources of the Rio Colorado of Texas, a great number of 
Exogyra Texana are found. Its thickness varies from 6 to 50 feet. It is thinnest in the places where it 
has been very much washed away , and where there are only the remains of beds, as on the banks of the False 
^ashita. As to its elevation above the sea-level, the Neocomian occupies a much lower position than 
the Jurassic rocks of the Llano Estacado, and aff the same time superior to that of the Upper Green 
Sand and the Marly Chalk which succeed it in relative age and superposition. Thus in my line of ex- 
ploration of the Prairies, following nearly the 35“' degree of latitude, the following are the heights. The 
Vppen- Green Sand found in the very bed of Little river and almost at its fall into the Canadian at a 
Shawnee mdian village, is 800 feet above the level of the sea; the ISxocomian, which is 200 miles to 
the west of this point, has an altitude of 2,000 feet; finally the first place on this line where the Ju- 
rassic formation is found, is on the Llano Estacado at 230 miles distance from the Neocomian rocks, 
and has 4,300 feet of elevation. I think that further south on the 32'* degree of latitude, these diffe- 
rences of altitude and the distance between these three formations are much diminished, and further I 
am very much inclined to helieve that the Neocomian formation will be found superposed upon the Ju- 
rassic, somewhere upon the plateau extending from the source of the Rio Colorado of Texas to the Rio 
Pecos and Leon spring. At any rate these differences sufficiently show that great changes have taken 
place in the relief of these regions between the Jurassic and Neocomian epochs , and it' is at this mo- 
ment I place the dislocations and elevations of the principal chains of the Rocky Mountains. 
