464 PROCEEDINGS OF THE ACADEMY OF [1890. 



of the higher volcanic ash-sands, or through simple precipita- 

 tion from waters highly charged with lime-carbonate. Much 

 of this deposit is strikingly firm and compact, and in parts so com- 

 pletely interwoven with the basal limestone of the region as to be 

 only with difficulty distinguished from it. Bones of one or more 

 forms of extinct elephant are found imbedded in the tepetate. 



From any of the eminences about Tehuacan the valley can be 

 traced southeastward very nearly to tiie limits of vision, with the 

 bounding ridges following in the same direction into the State of 

 Oaxaca (where they are met by a more or less transverse system of 

 elevations). The fact that these ridges pass for such long distances 

 beyond the true edge of the plateau, and retain throughout a general 

 parallelism of structure, is to me sufficient evidence that the plateau 

 is not the result of uplift along an east-and-west line of faulting, such 

 as has been assumed by Felix and Lenk, and which is made by 

 these authors to conform with the (assumed) east-and-west fissure 

 upon which the principal volcanic vents — Orizaba, Popocatepetl, 

 Nevado de Toluca, Jorullo — are supposed to stand. The plateau, 

 in this part at least, represents compressional uplift, in which an east- 

 and-west thrust has produced a series of folds running in a direction 

 more or less at right-angles to this line. The inequalities or saddles 

 of folding have been largely filled in through volcanic and fissure 

 discharges, which have thus mainly been instrumental in sha])ing 

 the existing physiognomy of the plateau. Parallel chains of hills or 

 mountains, similar to those of the region about Tehuacan, also pass 

 southward from the plateau in the State of Morelos, near Cuautla 

 and Yautepec, and between Yautepec and Cuernavaca, but these 

 will be considered later on. 



About ten miles to the southward of Tehuacan, following in the 

 direction of San Antonio and Zapotitlan, are the hills upon which 

 are situated many of the quarries of the famous Mexican onyx 

 (" Esperanza marble "). They lie at an elevation of between 6000 

 and 7000 feet, where the chain is deflected slightly to the northwest, 

 with a dip of the strata varying from west to southwest. This 

 so-called onyx occurs in light and heavy beds, sometimes several 

 feet in thickness, and is manifestly an infiltration product of a stal- 

 agmitic nature. It is, in fact, a floor or crust stalagmite, inter- 

 bedded with the distinctive Cretaceous limestones. I found numer- 

 ous fossils of the Hippuritic period in beds both underlying and 

 overlying the onyx, so that there can be no question as to the posi- 



