Memoir of a tour to northern Mexico/The Geological Sketch
THE GEOLOGICAL SKETCH,
Which I have drawn, does not make any pretensions to a geological map, which even a more able geologist than I am could not give in the short time and haste, in which I travelled through that country, but it may elucidate and concentrate at least what little information I have acquired in relation to that object. To make it more intelligible, I will add yet a short summary of the various geological observations # spread over the whole extent of the journal.
Independence, near the western frontier of the State of Missouri, is situated in the great Missouri coal basin, which occupies more than one-third of that State.
The first rock in situ which I saw in the prairie, after leaving Independence, was on Rock creek, (about 79 miles from Independence.) It was a yellow-brownish compact limestone, with encrinites, and similar fossils of the carboniferous limestone, as found in Missouri.
On Pleasant Valley creek (125 miles) the bluffs are formed by two different limestones: the one is white and compact; the other grayish, soft, and argillaceous. The first contained some indistinct fossils, but in too imperfect a state to determine what formation they indicate.
In Council grove (143 miles) a horizontal, grayish, argillaceous limestone prevails, without fossils.
Leaving Cottonwood creek (185 miles,) irregular heaps of bog-ore are seen" in the prairie, and a ferruginous sandstone of yellow, brown, and blue color, extends from here to Pawnee fork, (a distance of about 100 miles.).
The bluffs of the Little Arkansas consisted of a spotted, yellow, calcareous sandstone, and isolated pieces of ferruginous sandstone.
Between Camp Osage (the first camp near the Arkansas river) and Walnut creek, (263 miles,) I met with a very porous and scoriaceous rock in situ, apparently the product of action of subterranean fires upon t lie ferruginous sandstone. Most likely a large coal-field lying beneath here has become ignited, and produced this change of the rock. The so-called Pawnee rock (between Walnut creek and Ash creek) consists of the same ferruginous sandstone, changed by fire. On Pawnee fork (292 miles) I saw the last of it; the ferruginous sandstone there was more compact, and deep red.
On a branch of Big Coon creek (332 miles) I found the bluffs to consist of common standstone below, and a white, fine-grained marl above it. This marl resembles very much some from the cretaceous formation of the Upper Missouri; but finding no fossils, I could not ascertain it.
Two miles beyond that place (341 from Independence,) I had the first chance to examine the bluffs on the Arkansas; it was a grayish,conglomerated limestone, with a few small fossils, that were rather imperfect, but seemed to belong to the cretaceous formation. The neighborhood of the above mentioned marl raises this presumption nearly to certainty. I have, therefore, not hesitated to lay it down as cretaceous fornation. About 20 miles higher up on the Arkansas, I saw, upon a second examination, but a coarse conglomerate of sand and limestone. At the usual fording place (373 miles!) where I left the Arkansas for the Cimarron, no rocks were in situ. Having crossed the Arkansas, I met with the first rocks again, on the "middle springs of Cimarron," (468 miles;) it was a sandy limestone above common sandstone.
Six miles west of the Crossing of Cimarron (500 miles from Independence,) light bluffs rise in the prairie, of a yellow, reddish, and spotted sandstone, combined with lime and argile.
A few miles beyond them a large, isolated mountain of boulders stands in the plains, composed of heavy blocks of quartz and quartzose sandstone, and maiy erratic rocks were afterwards found on our road.
On Cedar creek, McNees' creek, and Cottonwood branch, a yellow sandstone prevailed.
On Rabbit ear creek I met for the first time with amygdaloidal basalt, a black, heavy, basaltic rock, with a great many irregular, vesicular cavities, that are generally hollow—sometimes, too, filled with lime; in rare instances, with olivin. This rock is most common throughout the high mountains of Mexico. It occurs in most irregular masses, in whole mountains, as well as in millions of pieces strewn over the surface of the country. Here it rose in high perpendicular walls, as bluffs of the creek, and a very compact quartzose sandstone was below in horizontal layer.
The Round mound, a mountain in the prairie about three miles further wert, which I ascended, is formed of a brown, decomposed basaltic rock.
On Rock creek, and Whetstone creek, the amygdaloidal basalt with underlaying sandstone was found.
In going from there to "Point of Rocks," (600 miles.) extensive strata of a yellow, compact quartzose sandstone are passed, dipping gently towards the east. Point of Rocks itself, a spur of the western mountains, is a mass of sienite.
Some 12 miles beyond it, rises a hill in the plains, composed of very compact, black basalt, with underlaying white sandstone.
The bed and bluffs of the Rio Colorado and Ocate creek (627 miles,) are formed by quartzose sandstone.
The Wagon mound, an isolated mountain in the high plain, consists of a compact, black, and spotted basalt, rising in columnar shape.
On Wolf creek (664 miles) the amygdaloidal basalt and quartzose sandstone reappeared, both in horizontal layers.
Reaching the Gallinas creek, near las Vegas, (690 miles.) I met, after a long interval, with limestone again. It was a dark blue, with casts of Inoceramus of the cretaceous series.
From here we penetrated into the very heart of the mountains. At first we met but with sandstone, common and quartzose, and of most different colors.
Near San Miguel (707 miles,) a coarse conglomerate was found of decomposed granite, sandstone, and lime, and large blocks of decomposed granite lined the Pecos river, opposite the old Pecos village, (737 miles.)
In the cañon leading from here to Santa Fe,at first sandstone is found, common, quartzose, and calcareous, of various colors and granulations, tilt about 15 miles from Santa Fe, granite in situ appears, and continues all the way to Santa Fe. Near where 1 met for the first time with granite in situ, the sandstone, if I may judge from a very limited examination, was suddenly uplifted and thrown back in an angle of nearly 100 degrees.
West of Santa Fe, granite seems also to prevail. In my excursion to the Placers, southwest of Santa Fe, I found sandstone below, and on the height of the mountains granite and trap rocks. In the mountains of that neighborhood common limestone and sulphate of lime are said to exist; but on the road over which I travelled 1 had no chance to see any.
Granitic and trap formations seem to predominate, too, in the valley of the Rio del Norte below Santa Fe; but as the road leads always along the river, and the mountains on either side ar$ generally about 10 miles distant, 1 could not examine them as I wished to do, and had often to depend alone upon the external form of the mountain chain, apparently indicating unstratified and igneous rocks. Whenever the maintains approached the river, I gained more information. So, for instance, 1 found between Joyita and Joya (about 115 miles from Santa Fe) quartzose sandstone and quartz in a spur of the eastern mountain chain; and in Joyita itself, bluffs near the river of amygdaloidal basalt.
Some miles west of Socorro, (140 miles,) on the right bank of the river, I examined the western mountains, and found porphyrhic and trachitic rocks.
Near the ruins of alverde (165 miles) I met with bluffs of a dark brown, nodular sandstone; and about eight miles beyond, with amygdaloidal basalt again.
In the Jornada del Muerto granitic and basaltic formation, to judge from their shape, exists in the distant mountain chains; part of them in the eastern chains is called, for their basaltic appearance, Organ mountains.
Below Doñana I perceived some primitive rocks again, near the river, resembling a decomposed porphyry.
The mountains above el Paso belong mainly to the trap formation.
During my short stay in el Paso 1 made an excursion to the southwestern mountains of the valley, and was rather astonished to find mountains of limestone. The foot of the mountains was formed by a horizontal quartzose Standstone, similar to that underlaying the amygdaloidal basalt. The very compact and gray limestone, intersected with many white veins of calcspar, rose upon it to the crest of the mountains; but on several places, granite and greenstone seemed to have burst through it and formed partial eruptions. After a Ion? search 1 discovered some fossils, and though much injured and imperfect, they are nevertheless sufficient to determine the age of this formation. The fossils are a coral: Calamopora, and a bivalve shell of the genus Pterinea. This limestone is therefore a Silurian rock. Several mines have formerly been worked in it.
On the road from el Paso to Chihuahua I met in the first day or two with the same limestone. The pieces lying on the road were generally surrounded with a white crust of carbonate of lime; pieces, too, of what appeared io be fresh-water limestone, occurred. It is rather probable that this is the same material as the white crust of the blue limestone, and that both are the result of calcareous springs.
About 50 miles south of el Paso the limestone seems to cease, and porphyritic rocks of the most varied colors and combinations continued from here as far as Chihuahua, interrupted sometimes only by granitic rocks. The base of the porphyry is generally felspar.
Around Chihuahua and some distance to the south and west of it, in the Sierra Madre, porphyritic rocks predominate, and valuable mines are found in them.
Near Chihuahua, I understood, about 12 miles northeast of it, mountains of limestone appear; and through the favor of Mr. Potts, in Chihuahua, I received a piece of limestone from there, containing some casts of the chambers of an Orthoceras, proving that this limestone belongs also to the Silurian system. Mines are also found in it.
Another fossil I received in Chihuahua, said to come from the limestone near Corralitas, a mining place about 250 miles northwest of Chihuahua. It is a Pecten quinquecostatus (Sowerby,) of the cretaceous series; but not having travelled through that part of the State, I am not able to give any comment upon it.
From Chihuahua to Matamoros, travelling with the army as a surgeon, my time was so occupied that 1 could not make any distant excursions from the road; but generally, too, the geology of the country seemed to be very uniform and uninteresting.
From Chihuahua some distance south, the porphyritic rocks continued. In Saucillo (70 miles from Chihuahua) I perceived the first limestone From there to Santa Rosalia I passed some hills of amygdaloidal basalt, but the main chain of the mountains was all limestone, and continued to be so throughout the whole eastern ramification of the Sierra Madre, over which we travelled from here down to Saltillo and Monterey, where the low country begins. This limestone forms steep, often rugged mountains, rising on an average 2,000 feet above the plain; it is metalliferous, and has all the appearance of the Silurian limestone, found at el Paso and Chihuahua, but I was never able to discover any fossils on this route. Silver and lead mines are of various occurrence in it; in the limestone surrounding Cadena, coal has been found, I was informed, but 1 had no time to verify it.
From Monterey to the seashore I made but one interesting discovery; near Mier. On the bank of the Alamo river, about four miles above its mouth into the Rio Grande, I found an extensive bed of large fossil shells of Ostrea, belonging to the cretaceous formation. As the same formation has lately been discovered by Dr. Roehmer, of Berlin, as extending in Texas from the San Antonio to the Brazos, this cretaceous bed near Mier is in all probability a continuation of it. In looking over the recent publication of "Notes of the upper Rio Grande, by Bryan Tilden," I found, in a description of the river bank of the Rio Grande below Laredo, that 41 entire hills are to be seen, composed almost wholly of what appears to be a collection of large sea oyster-shells." I presume, therefore, that the same cretaceous formation extends in this direction higher up on the Rio Grande.