106 GEOLOGY. 
Marl series. 
2. Red, white, and green sandstones, and shales. 
3. Soft white calc. sandstone. 
4. Soft red calc. sandstone. 
5. Red and white sandstones and shales with gypsum. 
Saliferous group. 
6. Red and green sandstones, conglomerates and shales. 
Carboniferous. 
7. Limestones with beds of shale and sandstone. 
I had no means of measuring the interval between the surface of the Carboniferous limestone 
and the summit of the Marl series, but judged it to be considerably less than 2,000 feet. Mr. 
Marcou estimates the thickness of his Triassic rocks (variegated marls and red sandstones) on 
the Sandia mountain at from ‘‘4,000 to 5,000 feet,’’ a difference at localities but fifty miles apart, 
which, if it really exists, is quite surprising. 
The limestone at Pecos is highly fossiliferous, and has often been referred to in the preceding 
pages. Its fossils are nearly all common in the coal measures of Kansas and Missouri, of which 
this deposit is undoubtedly the geological equivalent. It is called by Mr. Marcou the Lower 
Carboniferous or mountain limestone, and it is barely possible some portions of it may repre- 
sent the period of deposition of the Lower Carboniferous strata of the Mississippi valley, but as 
yet we have no proof of it. On the contrary, all the fossils it has yielded, Productus semire- 
ticulatus, P. Rogersi, Spirigera subtilita, Rhynconella Rocky montana, etc., etc., are in North 
America characteristic of the coal measures. 
Along the Pecos, from Pecos village to San José, the saliferous sandstones and shales form the 
bottom of the valley over which the road passes, while the adjacent hills are composed of the 
Marl series and capped with Cretaceous sandstones. 
At Ojo Vernal an arch of the strata has brought the Carboniferous limestone within reach of 
the powerful erosive action which has scooped out the valley, and for a short interval it forms 
the surface rock. The overlying strata here form picturesque mesa walls and isolated buttes, 
of which the height, according to our estimate, is about 1,500 feet. 
From Ojo Vernal to Las Vegas we had constantly in view the strata included in the section 
at Pecos. They become mofe and more disturbed, however, and, within five miles of Las 
Vegas, are broken through by a mass of protruded granite. Along this line of upheaval the 
order of succession of the strata is very much obscured, and in passing it we entered an en- 
tirely new geological field. The Carboniferous limestone here sinks below the surface, and 
does not appear again on our route for more than six hundred miles ; with it also sink our old 
friends the variegated marls and saliferous sandstones, and of these our farewell was final ; for, 
although they were immediately beneath our feet through more than one day’s march in East- 
ern Kansas, they do not rise above the soil and grass, and were not recognized. Even the 
Lower Cretaceous sandstones, the summit rock of the highlands bordering our route west of 
Las Vegas, here for a time disappear, and the surface for many miles is exclusively occupied 
by Upper Cretaceous rocks. 
+ These were first noticed some ten miles west of Las Vegas ; thence to the vicinity of the 
Moro they appear wherever the surface of the prairies is broken. They consist of greenish 
brown or gray shales, and compact, blue, argillaceous limestones. The shales include thin 
arenaceous bands, containing fragments of shells of Inoceramus, which at first sight resemble 
fish scales. The limestone is very homogeneous and compact, but usually in thin layers ; 
a yu she quite dark internally, it weathers white, and exposed surfaces of it in the distance 
appear as white as chalk. We found these strata much better displayed further eastward, but 
