Washita , 



APPLICATION OF PRINCIPLE OP TRANSGRESSIVE OVERLAP 589 



sandstones, the age of which is regarded as that of the Paradoxides 

 oelandicus zone — that is, some distance up in the Middle Cambric. 



THE BASAL MESOZOIC SERIES 



The Central area. — At the beginning of Mesozoic time the North 

 American continent was mostly dry land. Transgression of the sea 

 began in Jurassic time in the Mexico area, and progressed northward 

 and westward, with some oscillations, to southern Colorado and Ne- 

 braska, and possibly to southern Dakota, when a period of extended 

 retreat was inaugurated, as recorded in the Dakota sandstone. The 

 record of the advance is embodied in the basal sands of the Comanche 

 series of Texas and the states immediately to the north. The progressive 

 advance of the sea and the resultant rising of the basal sandstone in the 

 scale have been discussed in detail by Hill, who divides the series as 

 follows :* 



Buda limestone. 



Denison formation. 



Fort Worth formation. 



Preston. 



f Edwards formation. 



Fredericksburg J Comanche Peak. 



[_ Walnut. 



f Paluxey. 



Trinity ■< Glen Rose. 



(^Travis Peak. 



In central Mexico the Comanchean series is composed mainly of lime- 

 stones which succeed the Upper Jurassic Aucella beds with perfect con- 

 formity and continuity of deposition. The Jurassic beds, however, rest 

 unconformably upon the earlier formations, with a basal sand and con- 

 glomerate, f 



On the tropic of Cancer the basal bed has risen into the base of the 

 Comanchean series, the overlying beds changing progressively through 

 arenaceous and calcareous clays to limestones (Tehuacan limestones). 

 From this point northward to Texas and into Indian Territory the basal 

 bed rises progressively in the series, but with several retreatal movements, 

 which will be referred to more fully below. The general advance, how- 

 ever, is indicated by the change in character and thickness of the forma- 

 tions. Thus, at Austin, Travis Peak beds are over 800 feet thick, and 



* R. T. Hill : Twenty-first Ann. Rept. U. S. Geological Survey, pt. 7, p. 115. 

 t R. T. Hill : Am. Journal of Science, vol. xlv, 1893, p. 311. 



