across Mississippi, Louisiana, and Texas. 365 



the faulting is said to be " not less than 8000, probably 10,000 

 feet," and " the base of the adjacent Coal Measures must be at 

 least 4000 feet below sea level !"* Gen. Gibson also notes 

 that the throw of the Alabama faults is greater toward the 

 south. If the theory of the continuity of this old land 

 area into Texas is correct, the ridge must have bent westward 

 in western Alabama. 



Eruptives in Arkansas and Texas. — The distribution of the 

 eruptive rocks of Arkansas and Texas follow close and parallel 

 to the old Cretaceous-Tertiary shore line. The syenites near 

 Little Rock and those of Saline county, Arkansas, are both 

 just within the present Tertiary border, while the Magnet 

 Cove eruptives in Hot Spring county are but two miles north- 

 west of it. The age of these eruptives has never been defi- 

 nitely settled. In Pike county there is a dike of peridotite in 

 Lower Cretaceous rock. The bauxites near Little Rock are 

 interbedded with Tertiary sediments, and, believing, as I do, 

 that the bauxites were formed at the time of the extrusion of 

 the syenites 1 conclude that the syenites are of Tertiary age. 

 In any case they lie near the old Tertiary shore line, and 

 appear to offer corroborative evidence of faulting or other 

 weakness along this line. 



At Austin, Texas, are many eruptives of Upper Cretaceous 

 age;f in Uvalde county, Texas, eruptives penetrate Lower 

 Cretaceous rocks ; \ in Rockwall county east of Dallas is an 

 isolated area of Tertiary igneous rocks. Hill and Dumble 

 have pointed out the fact that there is a line of eruptives of 

 post-Eocene age at no less than fifty places from east of Austin 

 to the Rio Grande, and speak of them as being "along a line 

 of weakness in the earth's crust which has apparently existed 

 in this region. "§ These facts suggest that the line of weakness 

 referred to by Messrs. Hill and Dumble has been manifesting 

 itself since Jurassic times and that its influence reached into 

 Tertiary times. 



Hot Springs. — The hot springs of Arkansas are a little 

 northwest of the Tertiary border, and may be regarded, like 

 the eruptives, as having some possible connection with the 

 line of disturbance that extends across the state. Professor 

 Hill mentions evidences of hot springs along the line of the 

 great fault in southwest Texas. 



Thickness of the Cretaceous and Tertiary sediments. — If 

 the lapping of the Cretaceous and Tertiary sediments across 

 the Coal Measures of eastern Arkansas and Louisiana was 



*A. M. Gibson, Rep. upon the Coosa Coal Field, p. 86. 



f R. T. Hill, Amer. Geol., 1890, vi, 291. 



% A. Ossan, Jour. Geol., i, 341. 



§ Proc. A. A. A. S., vol. xxxviii, 242, 243; this Journal, vol. cxxxvii, p. 288. 



