1890.] NATURAL SCIENCES OE I'HILADELEHIA. 463 



fnictures and dislocations follow rapidly on one another, and in 

 short intervals the beds assume all positions and reversions. No 

 more magnificent display of mountain architecture can be conceived 

 than that which is presented in the steep upper face of the plateau 

 scarp. In some places, as at Alta Luz, the strata become " flaggy," 

 and appear in thin shales verging on to slates. Possibly it is some 

 of this rock, having a clearly ancient look, which geologists have 

 from time to time considered as being Palteozoic. Dollfuss' men- 

 tions the occurrence of Palaeozoic strata near the Boca del Monte, 

 but I failed to find any such outcrop; and I am certain that the 

 series of PaUeozoic strata, from the Carboniferous to the Silurian, 

 which Packard mentions'" as outcropping between the eastern crest 

 of the great central plateau and the basal plain does not exist. 



For some distance after the summit of the plateau is reached, up 

 to and beyond the town of San Andres, the limestone appears in low 

 ridges trending southward, which ridges are but the backbones of 

 the main chain rising through the flat surface of the plateau. We 

 traced these seemingly low ridges southward to Tehuacan and 

 beyond, following in the line of the rapidly declining valley of the 

 Salado. With our descent from the plateau the hills, becoming more 

 and more exposed to their bases, rose majestically above us, and 

 were seen to constitute a true axial system of mountains, the inner 

 folds, manifestly, of the system which builds up the eastern face of 

 the plateau. Black cinder cones appear at intervals planted on 

 the white limestone. In the region immediately about Tehuacan, 

 the height of the ridges locally known as the Sierra de San Antonio 

 Tlascala and Sierra San Felipe Maderas cannot be less in places than 

 8000-9000 feet (absolute elevation). The dominant dip of the beds, 

 which outcrop in sharp lines on the eastern faces of the hillsides, is 

 westward, with varying angles. 



In the trough of the long sloping valley which leads southward 

 from the plateau vast deposits of detrital material have accumulated ; 

 masses of shingle and boulders, representing largely the debris from 

 the table-land, are exposed in all the stream-cuts, and build up 

 river-terraces of broad extent and distribution. Secondary lime 

 deposits, known as tepetate, have resulted from the re-deposition of 



^ Observations Geologiques, Arch. Comm. Scientifique du Mexique; Felix 

 und Lenk, Beilrage zur Geologic und Palaontologie der Re[mblik Mexico, p. 11, 

 1890. 



2 American Naturalist, XX, p. 122. 



