BASAL OR WILLS POINT CLAYS. 21 



bluff showing them, coming down the river, is in the corner of Milam County. 

 It is about a third of a mile long and forty feet high. The lower part of it is 

 composed of very dark, almost black, clays, containing fragments of shells, and 

 running into a lighter yellowish and greenish clay towards the top. This upper 

 part contains highly calcareous indurated strata, showing a nodular structure 

 and containing many fossils. The lower part of the bluff is also calcareous, 

 but not as much so as the upper part. Dip, three degrees southeast. At a point 

 two miles above Pond Creek is another bluff of inter bedded dark gray clays 

 and white and gray sands, containing many flat calcareous concretions, weath- 

 ering in concentric layers and one to ten feet in diameter. They are dark gray 

 inside and brown on the outside. They are in the sand seams, and are prob- 

 ably simply part of the inclosing stratum indurated by the large amount of 

 calcareous matter that they contain. In a thin bed of clay, four inches thick, 

 in this bluff are found many shells of an oyster. The dip of the bluff is very 

 gentle to the southeast. It becomes much more sandy towards the top than 

 at the base, and doubtless represents the transition bed from the Basal Clays 

 to the great overlying series of sandy strata (Timber Belt Beds.) The ex- 

 tension of the Basal Clays between the Colorado and Rio Grande has not as 

 yet been studied, but it is probable that it becomes much more sandy in that 

 region than it is to the north, as the first Tertiary beds found going down the 

 Rio G-rande are very sandy, being composed mostly of siliceous grains and of 

 glauconite, This fact, however, is to be expected, as the Basal Clays doubt- 

 less owe at least part of their existence to the clayey matter derived from the 

 Upper Cretaceous marl and " glauconitic " strata, and on the Rio Grande these 

 strata have not undergone so much erosion as those to the north. There is 

 little or no lithological change in the Rio Grande region from the Upper Cre- 

 taceous strata to the Lower Tertiary (Laramie?). They both consist of silice- 

 ous and glauconitic sands, and therefore, as no unconformity can be seen, the 

 evidence as to a break, if indeed any does exist, m the deposition of the strata 

 must depend upon paleontological evidence. The thickness of the Basal Clays 

 is difficult to determine, as it is rarely exposed in high bluffs, and the vague 

 records of well borings make that source of information unreliable. As 

 stated before, the dip of all the Tertiary strata is so uncertain that no reliance 

 whatever can be placed on estimations of thickness based on it. From an 

 examination, however, of all the details now available, it is probable that the 

 thickness of the strata from the top of the Cretaceous to where the Basal Clays 

 merge into the overlying sandy strata is about two hundred and fifty to three 

 hundred feet. 



SOILS OF THE BASAL CLAY REGION. 

 These soils vary from clay to clay loams, are of a dark gray or black color, 



