jgg CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



of the eastern face of the Tamaulipas Range, permitting the 

 later deposits to overlap the earlier, and its close was marked 

 by gradual dessication and the formation of the numerous 

 beds of gypsum found in the Frio clays. Here, as in Texas, 

 no beds have been recognized which are in any way refer- 

 able to the Upper Claiborne. 



*. 

 Upper Eocene 



Between the exposures of the Frio on the Rio Grande and 

 southward and the first deposits clearly referable to the Oligo- 

 cene there is quite a belt of country, largely covered by the 

 Reynosa. It is entirely possible that within its limits there 

 may exist representatives of the Upper Eocene or Jackson 

 such as occur in eastern Texas, but which are apparently en- 

 tirely wanting in the valley of the Rio Grande. The only 

 deposits actually observed that seem in any way related to 

 this period were found overlying the Frio on the road from 

 China to Laguna de los Indios. The Frio clays and soft 

 shaly sandstones formed the country rock from Chilarios 

 southeastward for some distance. No change in character of 

 deposits were seen for 24 miles, but at a so-called Mina An- 

 tigua some three miles southeast of Rancheria, the pld shaft, 

 20 feet deep with tunnel to east of 40 feet, showed princi- 

 pally sands. The tunnel was in a bed of coarse-grained bluish 

 sand carrying pyrite, gypsum, and sulphur and entirely dif- 

 ferent from any Frio materials. From this point to Laguna 

 de los Indios, some 18 miles, the surface is sandy. The well 

 at the ranch is 80 feet in depth, the materials through which 

 it was sunk being coarse-grained bluish sand with gypsum 

 similar to that found at the old mine. The sand was here 

 interbedded with yellow clay which carried a number of 

 poorly preserved fossils. Dr. W. H. Dall, who looked them 

 over, stated that they contained a Pecten recalling P. poul- 

 soni, Tellina and Cardium or Venericardia, which, while not 

 characteristic, seemed to indicate an Upper Eocene or Oligo- 

 cene horizon. This may prove to be the southern extension 

 of the Jackson of east Texas. Similar sands covered by the 

 Reynosa occur also in ridges northwest of the ranch. 



