The valley of Tehuacan is mil wide and less flat than to the north. Looking back from the station one sees again strata appearing horizontal of a yellowish color. The rocks stand not in bold cliffs but the hill outline are as usual unequal and jagged. Had a talk with Ordway and learned the following, The valley of Tehuacan is large and in it on the eastern side rests nearly horizontal strata (Conglomerate with pebbles at least of the Cretaceous or even the volcanics) of Tertiary. The age is not younger than Miocene. Thickness of it 2000 meters. To the west are the Cretaceous. The valley was produced by faulting, the faults especially were cranked to the east, in some places the Tertiary rests on the Cretaceous. Upon the Archean promontory rests the Cretaceous, In other words at the close of the ? Cretaceous the present mountains were elevated at the strata much higher than now for they furnished 2000 meters of Tertiary. The age of those higher re- mains stills I do not know. Throughout the time of this erosion the valley of Tehuacan was formed and eroded down to the Archean. Beginning with the Miocene the valley of Tehuacan received its erosional material and has again