THE UPPER OR BLACK PRAIRIE SERIES. 115 



is the most extensive and valuable, but least appreciated, geological formation 

 in the United States — a remarkable deposit of chalky clays, aggregating some 

 twelve hundred feet in thickness, according to reported well borings and esti- 

 mates of the normal dip. In fact these clays are so little known that no pop- 

 ular name has been found for them, and hence they are called after the im- 

 mense fossil oyster which is found in them. These clays occupy the whole 

 of the main Black Prairie region east of the Austin-Dallas chalk, and form 

 the basis of the rich black waxy soil. Notwithstanding their areal extent, 

 good outcrops of the unaltered structure are seldom seen, owing to their 

 rapid disintegration. Usually they are seen only in ravines, creeks, or fresh 

 diggings. However, at the Blue Bluffs of the Colorado, six miles east of 

 Austin, a good exposure is afforded, where these clays can be readily studied 

 and diagnosed. They are of a fine consistency, unconsolidated, and appar- 

 ently unlaminated until exposed to weathering, when their laminated char- 

 acter is developed. They are light blue before atmospheric exposure, but 

 rapidly change into a dull yellow, owing to the oxidation of the contained 

 pyrites of iron. Their chief accessory constituent is lime in a chalky con- 

 dition, and they are more calcareous at their base than at the top. Near 

 the top of these and other exposures there is to be seen a rapid transition 

 into the black calcareous clay soil, characteristic of chalk and chalky clays, 

 whenever their excess of lime comes in contact with vegetation. 



The minute details of these clays have not yet been ascertained, and from 

 the nature of the problem it is not evident that they can be discovered speed- 

 ily; but it is apparent that they are more calcareous and fossiliferous at their 

 base, where they probably gradate into the Austin chalk. Their middle por- 

 tion is apparently void of well preserved fossils, yet impressions are abundant 

 in places. Toward the top, as seen one mile north of Webberville, ten miles 

 east of Austin, they become slightly arenaceous and concretionary and very 

 fossiliferous, indicating a gradation into the Glauconitic division. The fauna 

 of these concretionary clays at Webberville, Corsicana, and elsewhere begins 

 to partake of the character of that of the Glauconitic division, and yield an 

 abundance of species. 



Although not the top of the series, the "Webberville beds are its upper- 

 most exposures seen along the Colorado River section, for at that place they 

 are overlaid by the Lignitic or Basal division of the Eocene Tertiary. To the 

 northward these beds thicken. 



The economic value of these chalky clay marls is in that they are the foun- 

 dation and source of the rich soil of the main Black Waxy Prairie of Texas, 

 the largest continuous area of residual agricultural soil in the United States, 

 apparently inexhaustible in fertility; for as the farmer plows deeper and 

 deeper he constantly turns to light the fertile marls which renew the vitality. 



