22 I E. T. DCJMBLE GEOLOGY OF THE RIO GRANDE. 



This .section is at road crossing of the creek, half a mile from the Rio 

 Grande. At the mouth of the creek the limestone passes under the 

 water, and just below is succeeded by the beds of the Eagle Pass divis- 

 ion. The contact is covered by river drift, but may be found further 

 up the creek. 



Throughout the entire range of this chalky limestone the conditions 

 of deposition seem to have been quite similar. The beds become 

 somewhat more massive but broken toward the top. They are sepa- 

 rated by limy shales at the base, then by calcareous clays, then by 

 purer clays, and finally by calcareous clays again. Inoceramus and 

 A in mi miles seem to be the only fossils ranging entirely through the 

 Yal Verde flags and Pinto limestone, and the occurrence of Exogyra 

 ponderosa and E. costata so far down in the Pinto limestone is worthy of 

 note. 



The Eagle Pass Division. — Immediately overlying the Pinto limestone 

 there is a great series of clays, sands and greensands, with more or less im- 

 pure limestone and beds of coal, to which I propose to give the name Eagle 

 Pass division. This name was suggested for a portion of these deposits 

 by Dr. C. A. White in 1887, and I now extend it to cover the entire 

 series of deposits lying above the Pinto limestone and below the Webb 

 bluff beds. It has a surface exposure along our line of section of nearly 

 sixty miles. It comprises a number of more or less distinct members 

 which may be described separately. 



Upson Clays. — The basal member consists of yellow clay containing 

 calcareous nodules of septarian character, the crevices or septa? of which 

 are filled with dogtooth spar. These nodules occur in large geodic forms 

 scattered through the clays, and contain Exogyra, ponderosa, Roemer. 

 Numbers of specimens of these fossils are found in geodes as well as on 

 the hillsides, where they have been left by the disintegration of their 

 matrix. The nodules or geodes seem to occupy pretty definite horizons, 

 and sometimes form benches on the hillsides. The uppermost member 

 of this series, as I observed it, is a clay shale. 



San Miguel Beds. — Resting on the clay shales, which form the upper 

 member of the Upson clays, there is a deposit of sandstone, thin to heavy 

 bedded, separated by bands of clay, and containing seams of glauconitic 

 material with many fossils, as well as occasional heavy beds of clay, 

 especially toward the top. I have called this deposit the San Miguel hols 

 from the locality at which it was first observed by Dr. Comstock and 

 myself. In the Rio Grande section it first occurs in the hills north of 

 Carter's ranch, where the hills show exposures of it from 75 to 100 feet 

 in height. The exposures are excellent for several miles south of this 

 point, and a very rich fauna, which is now being studied, was secured. 



