288 M. T. Hill — North American Cretaceous History. 



Disturbances and Differentiation at the close of the American 



Cretaceous. 



The uniformity of the littoral fauna at the close of the 

 Upper Cretaceous is shown by a comparison of the species of 

 the Ripley, Navarro and Pierre-Fox Hills beds. This similar- 

 ity is in remarkable contrast with the great differentiation 

 which followed it ; for at the close of the Cretaceous, that emer- 

 gence so well known to American geologists took place, which 

 lifted the western interior region permanently above oceanic 

 invasions. That the southern trans-Mississippi Gulf-Cretace- 

 ous was also slightly lifted above sea level is shown by a slight 

 but unmistakable unconformity found in Arkansas, at Elmo, 

 Texas, and elsewhere between the beds of these periods. 

 The oldest littoral beds of the marine Tertiary (Lignitic), 

 which are mostly composed of sediments derived from the 

 soft strata of the underlying Cretaceous, are distinguishable 

 from them only by a slight non- conformity sometimes accom- 

 panied by a thin sub-stratum of siliceous pebbles. This un- 

 conformity is made more certain, however, by the recent dis- 

 covery of unmistakable paleontologic evidence. As in the case 

 of the mid-Cretaceous non- conformity, the basal Tertiary rests 

 upon different horizons of the Upper Cretaceous, owing to 

 inequalities in its erosions. In Texas, southwest of Bastrop 

 and Austin to the Sabinas river in Mexico, for a distance of 800 

 miles, there is a more conspicuous and unmistakable sign of 

 the disturbances at the close of the Cretaceous than this un- 

 conformity, and this is an elevation accompanied by a line 

 of many basaltic outbursts* in close proximity to, but not im- 

 mediately connecting with the line of elevation along the 

 Austin-New Braunfels unconformity above mentioned, which 

 seems to have been a line of weakness since Jurassic times. 

 The basaltic outcrops occur at no less than fifty places in a line 

 from near Yegua Hills in Bastrop County southwest via Hays, 

 Kendall, Bandera, Kerr, Nueces and Yal Verde counties to 

 the Santa Rosa mountains in Mexico. Pilot Knob, seven 

 miles southwest of Austin, is a typical example. This is a 

 small dome-shaped protuberance of columnar basalt rising 

 through the Upper Ci'etaceous chalks which surround it on all 

 sides and producing in them a quaquaversal dip of ten degrees, 

 and metamorphosing them into saccharoidal marble at the con- 

 tacts. In decomposing, the basalt becomes amygdaloidal and 

 zeolitic. The whole exposure has the appearance of a trun- 

 cated laccolite. Throughout the whole region, as seen in fig, 

 1, there are other evidences of the presence of these igneous 

 rocks beneath, as seen in the dome-like disturbances and meta- 



* Conjointly with Mr. E. T. Dumble, the writer will publish at an early day, a 

 paper upon this remarkable igneous area. 



