SIGNIFICANCE OF THE TIIINITY FORMATION. 507 



from the materials of the underlying contact beds, mostly the sandstones of 

 the Carboniferous series, except in the valley of the Colorado east of the 

 Burnet granite area, as previously noted. The origin of the fine, rounded 

 quartzite and jasper pebbles, however, is still problematical ; although I have 

 some evidence that it is the redeposit of a conglomerate that belongs to the 

 earlier beds, which was degraded and redistributed over and far west of the 

 present Carboniferous area, as now revealed by the removal of the overlying 

 Comanche series. When one considers the immense degradation of the sand- 

 stones of the Carboniferous system in Texas and Indian Territory, the source 

 of the Trinity materials is obvious. 



These sands record the beginning of one of the most important events of 

 Mesozoic time, to wit : The invasion of the area of the interior lake region 

 of the red bed epoch (Triassic) by the marine waters of the Atlantic, and 

 the degradation or base-leveling of the narrow continental divide which 

 separated them. The extreme paucity of land debris in the sediments is 

 indicative of the limited -area of this divide, for where the Trinity waters 

 bordered the Appalachian continent, as in Arkansas, plant remains are ex- 

 ceedingly abundant. 



The Glen Rose Beds. — Immediately overlying the basal sands of the 

 Trinity division just described, and no doubt succeeding them by continuous 

 subsidence, lies a group of strata which are of great importance in our geo- 

 logic history. These are composed of soft, yellow, magnesian fossiliferous 

 beds, silicious at base, alternating in dimension layers with an exceedingly 

 fine argillaceous sand, with occasional dimension layers of almost pure 

 crystalline limestones, chalk, and magnesian limestones, often oolitic in 

 structure. At Mount Bonnel, west of Austin, there is a distinct oolitic 

 structure in many of the layers of indurated stone and marls, while nodules 

 and geodes of beautiful anhydrite, calcite and strontianite crystals are also 

 quite abundant. 



The unequal weathering of the hard and soft layers produces in the 

 eroded topography a beautiful bench-and-terrace effect, so much resembling 

 ancient shore lines along the western escarpment of Grand prairie, where it 

 overlooks the Trinity valley and the lower Paleozoic beds from which it has 

 been eroded, that earlier geologists have often confused these features with 

 shore topography. On fresh fracture these rocks are usually either white or 

 of an intense orange or gamboge color, but weather into a dull gray. 



North of the Colorado-Brazos divide the beds contain an abundant and 

 unique moUuscan fauna, composed in the lower part of littoral species de- 

 scribed by me in a report on the Neozoic geology of southwestern Arkansas, 

 to wit: Pleurocera strombifonnis, Scloth ; Corbicalidce, sp. ; Ostrcea franklini, 

 Coquand; besides numerous undescribed forms, but not one species of the 

 great lower Cretaceous fauna, such as the characteristic Gryphcea, of the 



