602 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



division, while slightly resembling others in the series, have a sufficient proportion of grit, and 

 sometimes of iron, to make them relatively impure. The beds are successively shallower in 

 origin in ascending series. 



In general, rocks of the Trinity division were laid down upon a subsiding bottom of a former 

 land surface; rocks of the Fredericksburg division (in this portion of Texas) upon a stationary 

 offshore bottom of Trinity sediments; and rocks of the Washita division upon a shallowing 

 bottom of Fredericksburg sediments. 



TRINITY DIVISION. 



This division includes the lower or initiatory beds of the Cretaceous formations of the Texas 

 region, embracing all the rocks lying below the Walnut beds of the Fredericksburg division. 

 * * * In general these strata consist of sands, clays, marls, and massive limestones (including 

 in the latter shell breccias, agglomerates, and chalks), all of which grade imperceptibly into one 

 another, both vertically and horizontally, according to their proximity to the shore line against 

 which they were deposited. * * * 



Studies of these three areas, the western border, the incised sections of the Grand Prairie, 

 and the embed beneath the Black Prairie, show great differences in thickness and composition 

 of the rocks of the Trinity division, indicating that they are thinner and more arenaceous along 

 the western border region, where they are best exposed, and that they thicken and become 

 calcareous to the east, being buried in the region of their greatest development beneath the 

 Black and Grand prairies, where a knowledge of their nature can be obtained only by careful 

 interpretation of the artesian drillings. * * * As a result of these variations of thickness 

 and composition the rocks of the Trinity division along the eastern portion of the general area 

 of exposure within the incised valleys of the Edwards Gut Plain present the aspect of several 

 well-defined and mappable lithologic units of various kinds of rock, which so coalesce along the 

 western border exposures into a general basement formation of sands that they are there usually 

 inseparable. 



As a whole these strata of the Trinity division may be subdivided into the following 

 conspicuous formations and beds : 



"Upper sands. 



Paluxy formation ■ Thin limestones. 



Lower sands. 

 Glen Bose formation. 



{Hensell sands. 

 Cow Creek bed. 

 Sycamore sands. 



The term Basement sands will be used as the equivalent of any of the above formations 

 where they rest upon the underlying Paleozoic floor. The name Antlers sands' wUl be apphed to 

 the equivalents of all these formations as they coalesce along the western border region north 

 of Parker County. 



FREDEEICKSBTJRG DIVISION. 



During the Fredericksburg subepoch the Cretaceous shore reached west and north approxi-. 

 mately to a line extending from El Paso north into southwestern Kansas. The area lying south 

 and east of this line, as far at least as the eastern border of the Black Prairie, as shown by the 

 extent of the Edwards limestone, was a great arm of the sea, the bottom of which was covered 

 with calcareous organisms as certain banks of the present West Indian seas are now covered. 

 Over the entire area thus covered by the oceanic waters the sea deposited a mantle of arena- 

 ceous, argillaceous, and chalky sediments which now make the rocks of the Fredericksburg 

 division. * * * 



As a whole, these sediments thicken and become more calcareous seaward, away from the 

 position of the old peripheral shore line at the close of the Fredericksburg epoch. * * * 

 This thickening took place by the accretion of calcareous layers offshore and horizontally away 



