Page:Memoir of a tour to northern Mexico.djvu/137

Rh 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 felsparfeldspar [sic].

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,