THE LOWER OR COMANCHE SERIES. 127 



The Washita Division along the Colorado is composed of the following 

 well marked subdivisions: 



Estimated 

 Thickness. 



The Shoal Creek Limestone + 80 



The Exogyra Arietina Clays — 80 



The Washita or Fort Worth Limestone +1 50 



The Caprotina Beds + 20 



The Flagstones ■ + 10 



Total thickness in feet + 340 



Of these horizons only the Washita limestone and the Exogyra arietina 

 clays are known to have any persistent extent, these being found as far north 

 as the Arkansas-Choctaw line and southwest to the Pecos. 



3a. THE FLAGSTONES. 



These can be seen at McDonald's brickyard, Johnson's quarry, Taylor's 

 lime kiln, and other points immediately west of Austin. They consist of thin 

 flagstones, of almost pure white chalky limestone, varying from one to three 

 inches in thickness, and are void of fossils. 



The surfaces of the slabs, which are quarried for paving and building 

 stone, are sometimes covered with the pentagonal markings usually attributed 

 to mud cracks, and these are filled with soft yellow lime material. These 

 beds are only eight or ten feet in thickness, and their occurrence elsewhere 

 than in the Colorado section has not yet been reported. 



3h. THE UPPER CAPROTINA LIMESTONE, OR AUSTIN MARBLE. 



Immediately above the flagstones, along the line of the Bonnell fault, and 

 in West Austin at nearly all the localities above mentioned, is a massive stra- 

 tum of limestone often metamorphosed into marble, which is composed al- 

 most exclusively of the calcified shells of Requienia (Caprotina), Nerinea, 

 etc., accompanied by occasional Hippurites. This horizon was confused by 

 Dr. B. F. Shumard with the Caprotina horizon some 1000 feet below, which 

 marks the beginning of the Fredericksburg division. Away from the met- 

 amorphism of the Bonnell fault and local igneous action, the bed has not the 

 crystalline consistency of marble. This bed is interesting, inasmuch as it 

 represents the final appearance of the more or less continuous Requienia 

 fauna which outcrops at various places from the bottom to the top of the 

 Fredericksburg division, and it is possible that this horizon may in reality 

 represent the close of that division. Between the Caprotina limestone and 

 the Flagstone horizon there are beds of yellow laminated calcareous marls 

 of a few feet in thickness, with the latest known horizon of E. texana and 



