466 l'KOCEEDIN<i.S OF THE ACADEMY OF [1890. 



oped, rising from 150 to 200 or 300 feet above the general level of 

 the country ; yet, if their elevation above the level of the sea is con- 

 sidered, they are mountains of noble proportions, rising to nearly 

 5,000 feet West of Yautejjec, however, they gradually increase in 

 height, and in what is locally known as the Sierra Blanca — the long 

 divide which separates the region of Yautepec from that of Cuer- 

 navaca — they attain an absolute elevation of probably not less than 

 6,500 or 7,000 feet, thus reaching to within a short distance of the 

 actual level of the plateau. Mr. Baker and I traversed the eastern 

 slope of the divide to an elevation a little exceeding 6,000 feet. 

 The general features of the region, both in their geological and bot- 

 anical aspects, are very similar to those of the mountains of Tehua- 

 can and Zapotitlan. Heavy beds of white or gray limestone crop 

 out on the hillsides and give to them that distinctive light color 

 which so eminently serves to distinguish the sedimentary deposits 

 from the dark, in some places almost black, volcanic masses ("Sierra 

 Negra") with which they are associated. We found the dip of the 

 beds to be uniformly east or southeast, wnth a declination ranging to 

 nearlv 40 degrees. A short distance to the westward of Yautepec, 

 a volcanic cone, whose lava-stream reaches the town, rises through 

 the limestone ridge, but I could not detect that the dip of the beds 

 was materially affected by its presence ; both east and west of it the 

 beds dipped eastward, and at almost identical angles. A slight 



sibility or piohabiliiy of Jurassic masses being here and there protruded through 

 the Cretaceous limestones, and indeed, such protrusions and intercalations may be 

 much more numerous than the general similarity of the rock-masses would lead 

 one to suppose. Nikitin has also quite recently described (Einiges iiber den Jura 

 in Mexico und Centralasien— Neues Jahrbuch fiir Mineralogie, 1890, p. 1273) 

 what are assumed to be Jurassic fossils [Aticella, closely related to A. Pallasi, 

 etc.) from the region about San Luis Potosi, and he identifies the formation con- 

 taining the fossils as the equivalent of the Russian " Aucella beds." It is, how- 

 ever, not at all unlikely that a number of Jurassic forms have actually survived in 

 the Mexican region into the Cretaceous period, and possibly the Aucella (which in 

 Europe survives into the Tithonian etage=passage beds between the Jurassic and 

 the Cretaceous and into the Lower Cretaceous itself) is one of these. Barcena 

 assumed that the Nerincea which he referred to N. hieroglyphica was a Jurassic 

 form, and he also cites the occurrence of Ostrea {^Gryphea) virgula in the lime- 

 stone of Apasco (Datos pare el Estudio de las Rocas Mesozoicas, p. 18) ; but this 

 limestone, as has already been seen, is a part of the true Hippuritic series, and a 

 member, consequently, ol the Cretaceous system. I have also found the same 

 species of oyster, whether true Ostrea virgula or not, in tiie Escamela rock of Ori- 

 zaba, where it is very clearly associated with the large NerincBa, N. Castilli, and 

 with various Hippuritida; its position there is thus firmly fixed. 



