| OCR Text |
Show 77 species oolumbi, from this bed, and found of the arroyo. Shells of Planorbis, Phy8a, & c, indicated the lacustrine character of the deposit, which may be known as the Zandia clay. The Cretaceous No. 3, here, as on the Galisteo Creek, is underlaid by the red sandstones of the Galisteo group, which here dip 20° or more away from the mountains, close to the latter, north of the canon, from which the Placita Creek issues. These beds constitute an important element in the landscape, as several lines of bare, rounded hills, whose red strata are curved and twisted so as to resemble at a distance low anticlinals. Immediately behind them, the Carboniferous limestone rises from beneath them. This formation, as has been already described by Professor Newberry, and previously by Prof. Mar-cou, constitutes the northeastern and eastern face of the Zandia Mountains, having evidently been tilted into its present position by the protrusion of a vast body of granite. The granite forms the precipitous southwestern and western escarpment of the mountain, displaying from this side its cap of Carboniferous limestone. This face is eroded into many gorges, leaving numerous irregular and picturesque peaks between them. The Carboniferous beds dip at an angle of 20° to the northeast. I obtained from them, on the sides of the canon of the Placita, numerous fossils, includingFenestella, Crinoids, Brach-iopods, Aoephala, & c, which indicate the horizon to be that of the Coal- Measure*. The total thickness of these beds is considerable; 1,000 feet being visible in the canon, which does not penetrate them. The age of the Galisteo sandstone is a point not satisfactorily decided. So far as their position on the flanks of the Zandia Mountains indicates, they may belong anywhere from Cretaceous No. 3 to the Coal- Measures. That they are not of Tertiary age, as has been supposed by some, is clear. Dr. Hayden's observations lead to the conclusion that they are not older than the Cretaceous, since he states that they overlie the coal of Placer Mountain, which itself is superior in position to undoubted Cretaceous beds. If these positions be correct, this group constitutes a special member of the Cretaceous formations. The age of the volcanic outflow which has covered such extensive areas in the valley of the Rio Grande is more modern than that whose remains are seen in Central Colorado. Portions of the trachyte are there inclosed in beds of Miocene age, although the outflow of trachyte has covered the same beds at a later period. In the region observed by me, I noticed indications of three successive periods of eruption. The Loup Fork marls of the San Antonio bones of the same species in place in the banks |
