14 University of Texas Bulletin 



of 2100 feet, apparently without intervening Ellenburger, and at Waco 

 th,e pre-Cambrian is present at about 2500 feet. However, at points still 

 farther south and southwest, as at Gatesville and Leander, the Ellenburger 

 is present but thin, and overlies pre-Cambrian shales or slate-graywacke 

 which near Leander are stated to be identical in appearance with the 

 Virginia shales and to be of upper Huronian age. 1 At Georgetown, Leon 

 Springs and Camp Bullis, 2 the Trinity directly overlies the pre-Cambrian 

 schists; 3 the depths of the contact are respectively 1100 feet, 1015 feet, and 

 1790 feet. 



The depression which is triangular shaped and narrower to the north- 

 west, is flanked to the north, at least in Cooke County, by a structurally 

 high area of Ellenburger at -900 feet, and a thin Pennsylvanian ; and to 

 the west by the structurally high Bend Arch in which the -3000 and -3500 

 Ellenburger contours have a trend east and south. 4 Whether this Ellenbur- 

 ger depression opens to the southeast or to the northwest is not known to 

 me. It was, however, invaded by the Pennsylvanian seas which deposited 

 in its central part at least 3600 feet of sediments, and left a surface which 

 was more largely levelled than the original floor. 



Upon this surface the Comanchean sea transgressed from the southeast, 

 depositing sediments which still show a feeble reflection of the underlying 

 geosyncline. 



COMANCHEAN 

 THICKNESS CHANGES 



Any pre-Comanchean ridges and valleys over which the Comanchean 

 seas spread would be indicated by deposition which is thicker in the valleys 

 and thinner over the crests. If therefore we identified a regional thinning 

 of the beds in two directions from a given location, this might be attributed 

 to deposition on two sides of an existing valley in the ocean bottom, whether 

 originally this valley was erosional or synclinal. This lensing is evident in 

 the Comanchean formations, and particularly in the lower ones (Glenrose, 

 Paluxy) . For example the Glenrose fills in much of the depression by lens- 

 ing; at Decatur it is about 35 feet thick but at Fort Worth about 475 feet, 



a By Professor A. W. Johnston, oral communication. 



2 Sellards: The Geology and Mineral Resources of Bexar County, Univ. Texas Bull. 

 1932, pp. 19-20. 



'These facts were discovered by Dr. J. A. Udden, who has kindly permitted their 

 use here. 



4 SelIards: On the Underground Position of the Ellenburger Formation in North 

 Central Texas, Univ. Texas Bull. 1849, 1920. 



