144 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Sei. 



lamellibranchs. Similar marls with apparently the same Schi- 

 zaster are found at Los Naranjos, Tempoal, Zacamixtle and 

 elsewhere, proving in some measure the extent of the Alazan 

 beds in this region. 



Taken altogether, therefore, it would now appear that the 

 section south of Aquismon probably corresponds closely with 

 that described by Bose from Chiapas and Tabasco, but is 

 more extended. The Tamasopa limestone, with occasional 

 remnants of San Juan and Papagallos, is followed by Eocene 

 deposits characterized by nummulites, orbitoides, etc., succeeded 

 by Upper Eocene (Jackson) and this by Lower and Upper 

 Oligocene. 



OLIGOCENE 



After the deposition of the Eocene sediments they were ele- 

 vated and folded and, in this area, were base-levelled so that at 

 the present time they form a comparatively level floor, the gen- 

 eral surface of which is not far below the water-level of the 

 region. 



Upon this floor of Eocene sediments are found those of the 

 Oligocene, which includes the greater part of the materials 

 forming the various mountains, hills, and mesas of the region 

 as well as those portions of the intervening valleys in which 

 erosion has not reached the underlying Eocene. In many places 

 they are penetrated by dikes and necks of basalt, and, at others, 

 are covered by basalt flows. Some sedimentary deposits of 

 Quaternary age also occur overlying them. 



The Oligocene deposits consist of .sands and sandstones, 

 clays, marls, shales, with more or less calcareous matter, and 

 limestone. These, where unaltered, are brown, gray, or blue, 

 but are usually weathered yellow, which is their prevailing color 

 throughout the region. By far the greater part of the beds are 

 clays with more or less sand, the shales and limestones being 

 most abundant in the middle portion of the beds. 



These deposits were first studied by us on the lower Conchos 

 River near the town of San Fernando, and that name was used 

 to designate them^®. Finding that the name was already in use 

 the name San Rafael was adopted as a substituted^. 



" Tertiary Deposits of Northeastern Mexico, E. T. Dumble. Science, No. 841, pp. 

 232-4. 1911. 



Tertiary Deposits of Eastern Mexico, E. T. Dumble. Science, Jmo. 910, pp. 901-8. 

 1912. 



"A Medial Tertiary Fauna from Northeastern Mexico. Proc. Cal. Acad. Sci. 1917. 



