Area in Louisiana and Eastern Texas. 89 



very little relief but at other times, as during the Ordovi- 

 cian and Silurian periods and the Mississippian and 

 Pennsylvanian epochs, it was mountainous. It was 

 depressed and entirely submerged during Lower Cre- 

 taceous time, and later depressions carried the sea 

 across it during Upper Cretaceous and Tertiary times, 

 so that its rocks are now covered and concealed by depos- 

 its of these ages. The discovery of pre-Cambrian schists 

 directly beneath Cretaceous strata at Waco, Georgetown, 

 Maxwell, San Antonio, and Leon Springs, Tex., suggests 

 that the rocks of this buried land area were similar to the 

 crystal-line rocks that are now exposed in the Piedmont 

 Plateau of the eastern United States. If so such rocks 

 underlie the Cretaceous strata over much of Louisiana, 

 eastern Texas, and perhaps adjoining areas to the south 

 and east. Prominent structural features of the Gulf 

 Coastal Plain, including the Preston anticline and Sabine 

 uplift, may mark the location of some of the folds that 

 were produced in the rocks on the old land area but that 

 have undergone further movement since they were buried 

 by Cretaceous and later sediments. The increase in the 

 intensity of the folding of the rocks in the Ouachita 

 Mountains and Arkansas Valley toward the south sug- 

 gests that the deformation of the basement rock of the 

 Gulf Plain south of these regions was still greater. 



The results of future deep drilling in the Gulf Coastal 

 Plain and further study of the Paleozoic and older rocks 

 that are exposed around the borders of the Gulf Plain 

 will add greatly to our imperfect knowledge of the old 

 land area considered in this paper. 



