6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



and latite, in contrast to the andesitic volcanics in the lower part of 

 the formation. The remainder of the coarse elastics consists largely 

 of limestone, chert, and granitic and dioritic rocks in varying pro- 

 portion. The matrix is in part tuffaceous but contains also a reddish 

 residual soil formed in the source area and carried with the coarser 

 debris down into the basin of deposition. This reddish color is char- 

 acteristic of these early Cenozoic clastic deposits. It apparently is 

 the product of weathering in the upland areas of that time, where 

 probably moist temperate climates prevailed, and was preserved by 

 rapid accumulation and burial in drier lowlands formed by active 

 downfaulting. The formation is in strong angular discordance with 

 the Mesozoic rocks and has itself been broken by faults into many 

 wedge-shaped and irregular blocks, which dip generally between 20° 

 and 35° to the northeast, except adjacent to faults. Inasmuch as the 

 conglomerate was derived from a high area to the northeast, the 

 present strong dips are the reverse of the originally low dips to the 

 southwest. 



The conglomerate passes abruptly into thin-bedded tuffaceous 

 sandstone {Tmv in fig. 2), which was apparently deposited nearly 

 horizontally over a broad area. The sandstone shows crossbedding 

 and ripple marks and seems to have been deposited on a flood plain. 

 The beds aggregate from 20 to 40 meters in thickness, decreasing 

 from northeast to southwest, and pass abruptly upward into massive 

 beds of what may be devitrified, rhyolitic welded tuffs. These are 

 followed by a thick sequence of normal tuffs, breccias, and some lava 

 flows, intercalated with a few thin beds of conglomerate. An angular 

 and erosional unconformity separates these volcanic rocks from over- 

 lying poorly consolidated pale-yellowish to grayish conglomerate 

 (Tpc), which, by lithologic, structural, and stratigraphic similarity 

 to conglomerate in the Lajas River Valley to the northeast and to 

 other conglomerates in Guanajuato, Hidalgo, and Mexico State, is 

 believed to be of middle and late Pliocene age. Thin fine-grained 

 elastics (Qal) rest unconformably upon the conglomerate of Pliocene 

 age. 



HISTORY OF THE FOSSIL DISCOVERIES 



Vertebrate remains were found by J. D. Edwards and G. Ortiz in 

 August 1950 in the red conglomerate about 2,000 meters south of 

 the town of Marfil (see fig. 2) in thin beds of poorly consolidated 

 reddish-brown sandstone and bentonitic siltstone dipping about 35° 

 to the southwest. The direction of dip in these beds is the reverse of 

 the general dip of the conglomerate and is probably due to drag along 



