CHARACTERS OF THE TRINITY SANDS. 505 



THE TRINITY OR BASAL DIVISION. 



Separation. — lu previous papers I have defined this as an arenaceous lit- 

 toral deposit at the base of the Coraauche series in Arkansas and Texas. 

 During the past year, however, I have discovered that the beds described 

 under this general term really include two stratigraphic subdivisions sepa- 

 rated by distinct lithologic and paleontologic characteristics, the Trinity or 

 basal sands, and the Glen Rose or alternating beds, respectively. 



The Trinity Sands. — While in many places these vary in composition 

 ^vith that of the underlying floor, they are usually composed of fine, white, 

 cross-bedded sand, mostly unconsolidated, very porous, calcareous, some- 

 times free from lime. In places there are deposits of small jasper and quartz 

 pebbles, seldom" exceeding a pigeon's egg in size, exceedingly rounded and 

 worn, and often cemented by a matrix of iron and lime, sometimes harder 

 than the pebbles. This pebble deposit is of various hues — white, black, and 

 jaspery red — and often remains as a residuum, over large areas of the red 

 beds and Carboniferous strata, from which the post-Trinity beds have been 

 denuded, as seen in Taylor, Tom Green, Nolan, Montague and many other 

 counties of the Abilene and gypsum country.* Silicified wood and occa- 

 sional fragments of hard lignite occur, the latter seldom, if ever, in continu- 

 ous beds or strata, but as if the remnant of some solitary log or tree floated 

 out from shore. 



These sauds can be seen in contact with the underlying Carboniferous 

 and the overlying Glen Rose beds all along the western margin of the 

 Comanche area except around the immediate perimeter of the Burnet-Llano 

 region, where in places the Paleozoic continent persisted above the Trinity 

 shore-line until the Comanche Peak epoch. Fifteen miles south of Burnet, 

 however, in another pre-Trinity topographic valley, now followed by Colo- 

 rado river below Smithwick Mills, where the lithologic nature of the beds 

 is entirely diflerent, consisting of coarser rounded pebbles of Silurian and 

 and Carboniferous limestone and Llano schists, as well as quartz from the 

 Burnet granite, and fine cross-bedded sands and shell debris (resembling, 

 as seen at Travis Peak post-oflice, in the bed of Cow creek, the Florida 

 coquiua). Here also there is an unstudied molluscan fauna including 

 ammonitid^e, ostrseidse, trigoniadae and other forms, not one of which occurs 

 in the hitherto supposed Cretaceous and overlying beds. Here the Trinity 

 beds are in contact unconformably with hard Carboniferous and Silurian 

 limestones, and contain much debris of the Burnet granite. They also vary 

 in composition and thickness with the irregularities of the floor. 



West of the 98th meridian the Trinity sands are deposited unconformably 

 upon the various beds of the " Triassic," or gypsiferous red beds, as seen 



* In places these pebbles are cemented into large masses of conglomerate, as at Saa Angelo, in 

 Tom Green county, where it attains a great thickness and areal development. 



