1890.] NATURAL SCIENCES OF PHILADELPHIA. 465 



tion of that rock ; but when it was formed — how soon or how long 

 after the close of the Cretaceous epoch — there seems to be no data 

 for determining. And for aught we know to the contrary, the onyx 

 may be forming to-day.^ 



Westward of the region that has just been described the limestone 

 ridges are continued through much of the State of Puebla, both on 

 and off the plateau ; while many of the more characteristic fossils 

 that occur elsewhere have not yet been recognized in these deposits, 

 their position and association leave no doubt that they are a belong- 

 ing, in part at least, of the same general Cretaceous formation which 

 occupies so much of the Republic.- Some of the most extensive 

 deposits of Mexican onyx, here known as " Puebla " marble, are 

 worked in the region about (southwest of) Tecali, a few leagues to 

 the south of the city of Puebla.^ 



In the State of Morelos, most marked in the region of Cuautla 

 and Yautepee, limestone ranges similar to those of Tehuacan trend 

 southward from the plateau scarp ; more properly, perhaps, they 

 might be described as abutting against the southeastern face of the 

 plateau, whose giant volcanic masses are reared up in imposing 

 grandeur in bastion-like towers and mural serrations, thousands of 

 feet in elevation. The eniergence of the limestone from the abrupt 

 wall of the plateau is here clearly marked. Between Cuautla 

 and Yautepee the parallel ridges of limestone are but feebly devel- 



1 Many workers of tlie onyx believe the rock to be a united breccia, or a 

 compound of large angular fragments which cross one another in all directions. 

 This misconception arises from the peculiar interblending of the laminal lines in 

 geodic sections, appearing as though distinct blocks of rock had been brought in 

 opposition. The broken appearance is in reality only the result of involution, by 

 which different sections of the same series of laminae are brought into a variety ot 

 antagonistic positions. 



2 Barcena describes Ammonites Ja»ics-Da7iae from Acaxochitlan, Ferreria 

 de la Trinidad, and Abra de Huilacapixtla (Anales del Mu^eo Nacional, I, p. 286 

 1877). The author justly calls attention, as had already been done by Mr. Meek, 

 to the Jurassic aspect of this Ammonite [Arietes ?), wiiich much recalls the type 

 of A. Bucklandi. Whether the presence of this form alone can be considered suf- 

 ficient evidence for recognizing in the region of its occurrence a Jurassic formation 

 I am not prepared to say. It may be a Jurassic type which has lingered well on 

 into the Cretaceous period; or, the discovery of additional fossils may clearly 

 establish the existence of true Jurassic deposits. There is nothing in the strati- 

 graphy of the rocks of the region which, as far as I can see, is opposed to the pos- 



■'• Barcena — Las Rocas de Tecali — La Naturaleza, III, 1876, pp. 7-9 ; Proc. 

 Acad. Nat. Sci. Phila., 1876, p. 166. 



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