Between the 116th and 117th meridians is a region of granitic and volcanic rocks, including two principal north and south ranges, and many spurs and side ranges. The Toiyabe range, in which are the mines of Austin, or the Reese River mines, and which is a little east of the 117th meridian, is mostly granitic. Stratified but highly metamorphosed rocks are said by Mr. Blatchley to occur on its east flank, a few miles south of Austin; these rocks contain fossils, which, however, are nearly obliterated by metamorphism. A box of these, forwarded some time since, has never been received, so that no positive statement can be made as to the age of the formation. From Mr. Blatchley’s description of the forms observed by him, it would appear that we may have here rocks older than the Carboniferous. East of the 116th meridian, our collections indicate the existence of a broad area covered chiefly by rocks of Carboniferous age, which seem to occupy most of the space between the 115th and 116th meridians. The most western locality of Carboniferous fossils known to us is that on the west side of Diamond Valley, about 70 miles east of Austin, and in lon. 116°. Here, in a dark, crystalline limestone, a variety of species have been obtained by Mr. Clayton, all in a poor state of preservation, but of which the Carboniferous age may be without difficulty made out. Among the genera represented at this locality are _Productus_—two species, one of which is _P. semireticulatus_—_Spirifer_, and _Fusulina_, probably _F. gracilis_, besides some indistinctly marked corals. The indications are that these beds are of Lower Carboniferous age. On the east side of Diamond Valley some imperfectly preserved corals have been found, probably Devonian, or possibly belonging to a still older group. The collections from the region included between the 115th and 116th meridians indicate the association of rocks of both Devonian and Carboniferous age in the outcrops. Here our collections do not enable us to do anything more than to corroborate the previously published statements of Messrs. Meek and Engelmann in regard to the age of the formations.