716 TRANS-PECOS TEXAS. 



ism of the rocks in the different localities. Here are found compact, clear, 

 sharp sands in massive beds and in false laminated strata, the same in general 

 lithologic character as the Trinity beds of Hill. 



The relations of this bed to that of the Fredericksburg in the different 

 localities do not lie altogether in the character of the rock, but in its relations 

 to the overlying Texana and Caprina horizons, below which it occurs in com- 

 plete conformity, or as nearly so at least as a littoral sand bed can conform to 

 that of a limestone. 



The principal occurrence of this sand is in Flat Mesa, which is composed 

 of a range of hills beginning one mile north of Sierra Blanca Junction and ex- 

 tending north northwest about six miles. The central hill has the true mesa 

 features, which is a broad flat top. with precipitate sides. The east and north- 

 east sides of Flat Mesa are especially precipitous, while the west side has a 

 gradual descent (but little more than the dip of the rock, which is from five 

 to ten degrees sonthwest) to the basin leading to Sierra Blanca and Quitman 

 Mountains. From the base of the scarp on the east side the almost level 

 basin extends eastward to the Diabolo Mountains. 



The cap rock of Flat Mesa is from thick bedded to massive limestone, be- 

 neath which extend the Texana bed and Trinity division, as in the following 

 section : 



1. Caprina limestone 40 feet. 



2. Flaggy calcareous sandstone and siliceous limestone, with numerous gastero- 



pods, bivalves and Exogyra texana 15 feet. 



3. Brown quartzitic sandstone 35 feet. 



4. Siliceous conglomerate and grit 10 feet. 



5. Brown sandstone 46 feet. 



6. Arenaceous limestone in which are numerous individuals of Actceonella dolium 



and a small exogyra 4 feet. 



7. Brown quartzitic sandstone, disappearing at the base of the scarp beneath the 



debris of the basin, exposed 100 feet. 



Total 250 feet. 



On the northeast side of Flat Mesa the Trinity sand is thrown down by a 

 succession of steep faults until the Caprina limestone, No. 1 of the section, is 

 brought to a level with the basin. 



A laccolitic uplift composing the clump of hills at the extreme northwest 

 end of Flat Mesa has brought the Trinity sand up, where by erosion it is 

 beautifully exposed, the dip of the rock being forty-five degrees to the south- 

 west toward Sierra Blanca Peak. 



On the east side of this uplift erosion and deposition of basin debris have 

 concealed all stratified rocks, and the surface of the basin passes up against 

 the porphyry core of the laccolite. 



At numerous places in the southern half of Flat Mesa dykes of porphyry 



