80 A GEOLOGICAL SURVEY OF LANDS 



3. The exposed west ledge of the middle sandstone group 

 forms the second range of hills extending from north to south, 

 three miles east of Eagle Pass. In survey 1 in block 16 the es- 

 carpment formed by this sandstone makes a turn to the south- 

 east and reaches its highest position in the rock capping Muerto 

 hill, where again it turns to the east, or a little south of east, to 

 survey 8 in block 6. From this point it was traced with some in- 

 terruptions northeast and then north to Chilpotin tank and 

 northwest to the Vela Ramos survey on Salad o creek. It is prob. 

 able that this same horizon is represented by some ledges which 

 run from here north and west, to survey 25 in block 7, and from 

 there north and east across Chacon creek, and then east on the 

 south side of the Anacacho mountains. 



While apparently more persistent than the lower sandstones 

 this middle group is more variable in its nature than the former. 

 It is frequently slightly coarser and more open in texture. Quite 

 often it is calcareous and contains an appreciable ingredient of 

 organic fragments. In places it is wholly composed of entire and 

 broken pieces of the valves of an ancient oyster, Ostrea cortex. 

 Near the northwest corner of survey 225 a breccia of this shell 

 measures nearly forty feet in thickness. 



The thickness of the strata belonging to this sandy group, 

 including the sands, the interbedded clays, and the shell breccias, 

 is in the neighborhood of 130 feet. 



4. The middle clay underlies the basin of Lampasitas creek 

 and is also found on some of the high divides on either side of 

 Rosita creek. Occasionally it is of light gray color and contains 

 seams with small flakes of gypsum. Shells of Ostrea cortex are 

 frequent throughout. In its upper part I found large concretions 

 of yellow carbonate of lime and many of these contained the fossil 

 Spheuodiscus pleurisepta. It contains some ledges of shell breccia 

 locally known as shell rock, but these are absent in many places. 

 Some of these are seen near Stone Ranch on Willow creek. 



5. The upper sandstone was traced in a continuous outcrop 

 for fullj' ten miles on the east side of Palo Blanco and Willow 

 creeks. It forms a very sinuous line of bluffs from survey 116 in 

 block 6 northwestward to survey 98 in the same block, and thence 

 northward to the head of Palo Blanco creek. In this region it 

 measures from five to thirty feet in thickness. In texture it is 

 usually fine grained. 



