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[ 26 ] 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.