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limestone above probably belongs to the Trenton period, indicated by shells of Orthis, Strophomina, Leptena and crinoids, specimens of which I collected last Spring.

The lower Potsdam sandstones have the peculiar green sand (glauconite), common to the rocks of the period in Burnet and Llano counties. The entire strata of the mountain has a thickness of about one thousand feet and all horizontal, most of the adjacent mountains being granite.

Sandstones of the Potsdam period abound in Mason county, and extend westward in Menard county, nearly to Menardville.

There is a fine exposure of these rocks in the western part of Mason county, on the Llano river, at the foot of Loyal valley.

Rocks which may and probably do belong to the lower silurian are upraised at the base of the Organ mountain at Fort Bliss, near the town of El Paso, where there is a fine display of uptilted strata of lower silurian, carboniferous and cretaceous rocks to a nearly perpendicular height of 1600 feet above the valley.

The rocks of this age in Burnet and Llano have been alluded to in former reports, and are noticed elsewhere under the head of coal.

These rocks have been traced the past season from a few miles west of Menardville to Fort McKavit, thence northward as far as Kickapoo springs, south of Fort Concho, again, from the Twin mountains, about six miles west of Fort Concho, westward to near Barilla springs, beyond Fort Stockton ; thus far with no interruption by other rocks ; thence westward to the Rio Grande. We saw no other sedimentary rocks except cretaceous, and only these at rare intervals, in uptilted strata among or on the sides of the granitic and basaltic mountains of that region. North of the Toyah valley are many limestone mountains which are probably of cretaceous age.

About four miles north of El Paso, above the Pass of the Rio Grande through the mountains from which the