ABSTRACTS OF PAPERS 41 



tion on the subject was obtained by the late Dr. A. H. Purdue and the writer 

 during several years' study of the rock formations in the Ouachita Mountains 

 and Arkansas Valley of Arkansas and Oklahoma, beginning in 1907. The fol- 

 lowing conclusions are based on this information and on the data published 

 by other geologists. 



A land area, which has been called Llano by Willis, Schuchert, and Ulrich 

 and Llanoria by Dumble, existed in Louisiana and eastern Texas during much, 

 if not mo^t, of the Paleozoic era and during the Triassic and Jurassic periods 

 of the Mesozoic era. It varied in outline from time to time. It may have 

 occupied a part of the area of the present Gulf of Mexico ; at times it was 

 doubtless connected with large land areas occupying at least much of central 

 and northern Texas, southern Oklahoma, and southern Arkansas, and for short 

 periods it may have extended eastward across the lower Mississippi Valley 

 and joined the southwest end of the Appalachian area. It furnished most of 

 the sediments that formed the clastic rocks of Pennsylvanian age in north- 

 central Texas, and for those of Ordovician, Silurian, Mississippian, and Penn- 

 sylvanian age in the Ouachita Mountains and Arkansas Valley of Arkansas 

 and Oklahoma. At times, as during the Devonian period, it had very little 

 relief, but at other times, as during the Ordovician and Silurian periods and 

 the Mississippian and Pennsylvanian epochs, it was mountainous. It was 

 depressed and entirely submerged during Lower Cretaceous time, and later 

 depressions carried the sea across it during LTpper Cretaceous, Tertiary, and 

 Quaternary time, so that its rocks are now covered and entirely concealed by 

 deposits of these ages. The discovery of pre-Cambrian schists directly beneath 

 Cretaceous strata at Waco, Georgetown, Maxwell, San Antonio, and Leon 

 Springs, Texas, suggests that the rocks of this old buried land area were 

 similar to the crystalline rocks 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 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 abstract. 



Eead from manuscript, illustrated with charts. 



Discussed by Messrs. E. C. Moore, E. H. Sellards, G. H. Chadwick, 

 and E. S. Bastin, with replies by the author. 



IGNEOUS GEOLOGY OF SOUTHEASTERN IDAHO 

 BY GEORGE ROGERS MANSFIELD 



Presented by title in the absence of the author. This paper is printed 

 in full in this volume. 



The section adjourned at 6 o'clock p. m. 



