122 CRETACEOUS ROCKS OF TEXAS. 



the basal Caprotina horizon to the culminating horizon of E. texana inclusive 

 is a height of 406 feet.* The beds of magnesian limestone which mark the 

 central third of this division are especially worthy of future study and ob- 

 servation from a petrographic and chemical standpoint. They are sometimes 

 accompanied by pockets of calcite, aragonite, celestite, and epsomite, as near 

 the summit of Mount Bonnel, near Austin This basal subdivision of the 

 Fredericksburg division is not well known in the northwestern portion of the 

 State, but apparently diminishes in importance in that direction, being only 

 about 100 feet thick in Comanche County, but maintaining considerable 

 thickness at Comanche Peak, Hood County, 100 miles northeastward. 



Economic Features of the Basal, or Alternating, Beds. — While the 

 basal Fredericksburg division no doubt possesses many valuable quali- 

 ties as a source of building material, and many mineral substances, as an 

 agricultural region, it is practically sterile, as can be readily seen in the so 

 called mountain slopes of western Travis and eastern Burnet counties. In 

 fact it does not even possess a favorable grazing region, except for goats and 

 scrubby horses, the cattle being usually small and poor. 



In the valleys of its creeks and streams small patches of soil have collected, 

 which afford a meagre subsistence for a few farmers; but as these do not 

 compose one per cent of the aggregate area, the fact remains that the alter- 

 nating beds, from the present agricultural standpoint, remain valueless. What 

 the application of the soil-saving methods of Europe and China may ulti- 

 mately do for the same is not in our province to predict. 



The reasons of this sterility are apparent. (1) Chemical. There is an 

 excess of lime, sulphur, magnesia, and other salts in the rocks and waters, 

 which are deleterious to vegetation and animal life. (2) Physical The 

 slopes are steep and prevent the accumulation of soil. (3) Geological. There 

 is an absence of sufficient clays m the substructure to retain for the use 

 of vegetation the moisture which this formation so readily imbibes, while 

 the strata at the surface store a great amount of heat. These two geologic 

 causes render the surface outcrop of these basal alternating beds far more 

 drouthy than the adjacent regions, although possessing an equal amount of 

 rainfall. 



While void of any great agricultural possibilities, however, the basal, or 

 alternating, beds, abound in useful material to mankind, and especially in 



^Throughout the series fossils are occasionally found, especially E. texana, Nerinea, 

 Requienia {Caprotina), Gyphosoma, and casts or moulds of Tylostoma praigrandis, Roemer, 

 Area, Trigonia, and especially the peculiar globular foraminifera-like form which has been 

 called Goniolina f by D'Orbigny. There are also horizons more or less chalky throughout, 

 one of which is especially interesting in that it is a perfect mass of small coin-shaped fora- 

 miniferae called Orbitolina (Tinoporus) texana, Roem. 



