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Show 67 me to correlate them with the Lonp Fork Tertiary of Colorado and Dakota.* The species discovered by our party number twenty- nine, of which twenty- four are Mammalia, three birds, and two reptiles. An enumeration of them is given in the chapter which follows the present one. Twenty- five miles west of the Rio Grande, at San lldefonso, the eastern masses of the Jemez Mountains rise. The greater part of this interval is occupied by a plateau which is traversed by more or less parallel ravines, which issue in the trough of the Rio Grande. The mesas which separate the ravines terminate abruptly like the wharves of a city- front. Their material consists of sandstone, conglomerate, and arenaceous marl, of whitish, gray, and drab colors, having a gentle dip to the northwest. Many of their upper beds contain numerous pieces of pumice, which readily disintegrate, and the resnlting siliceous dnst under the inflnence of wind excavates the surrounding sandstone into caverns and pigeon- holes of many sizes and shapes. Nearer the mountains the northwest dip of the beds is distinct, and they accordingly present escarpments to the southeast and gentle pine- covered slopes to the northwest. The ravines have a northeast and southwest direction, and extend to the base of the mountain. The escarpments are composed of orange- colored and reddish rook of uniform constitution, which breaks into prism- like masses as it falls, forming talnses below. It is entirely distinct in character from that of the bluffs nearer the river, which form part of the Santa Fe" Tertiary marls, as proven by the occurrence of the bones of Mastodon and Aoeratkerinm Jemezannm, Cope, near Santa Clara. The orange beds are doubtless older, and were afterward seen on the Chama River; but I was unable to determine their age, or their precise relation to the overlying sands and marls. They are covered near the mountains by a mass of basalt, which forms the floor of a higher mesa, from which rise the basaltic cones of the Jemez Mountains. Some of its peaks were doubtless sources of discharge of lava at a former period. I did not observe that the orange beds were tilted, or rested other than nearly horizontally against them. In the ascent of the Rio Chama, we pass over the Santa F6 marls exclusively until reaching the town of Abiquiu. Here are bluffs of TOO feet in elevation, of a soft sandstone, havingthe same character and dip ( 10° to 15° northwest) as those above described as at the eastern base of the Jemez Mountains. In a bay on the western side of one of these bluffs is a patch of picturesque bad- lands of the Santa Fe* marls. Five miles above Abiquiu, the brilliantly- colored yellow and red beds which form such an important feature in the geology of Western New Mexico, appear in high blnfls on the north side of the river. They are several hundred feet in thickness, but near the Rio Chama descend so as to permit of a view of their relations to the superincumbent beds. The brightly- colored beds are cut by a ravine to a depth of about 150 feet. The upper portion is yellow, and they dtp 25° southwest. They are overlaid by a shale of 15 feet in thickness, whose lamince are frequently contorted. The lower part of the bed is finely laminated, and the upper portion consolidated into a very hard rock. Above it is a bed of 20 feet of a very coarse conglomerate, whose cement is arenaceous. ( See Fig. 2.) These details are entered into for the purpose of exhibiting the unconformability between the late Tertiary beds of the Rio Grande Valley and the formations constituting its western shores. The beds lust described are believed to correspond with those called Jurassic in the section taken at Colorado Springs, and quoted in my introductory remarks. Red beds, supposed to correspond with the Trias of the same section, were observed by me to form the northern boundary of the basin a few miles north of the town of £ 1 Rito, east of the Rio Chama. These beds crop out in high bluffs, and doubtless formed the precipitous western shore of the fresh lake whioh during the * See Report on the Vertebrate Fossils of New Mexico, Annual Report Chief of Engineers, 1874, p. 603. jy FIG. 2.- Outcrop of " Jurassic'' strata near Abiquiu. |
