364 Branner — Former Extension of the Appalach 



tans 



holds in the Territory, namely, that the faults about the Ozarks 

 are normal, while in the region south of the Canadian river 

 they are reversed. 



Faults in Texas. — I know of no considerable faults in the 

 Cretaceous rocks of Arkansas, but in Texas there is a remark- 

 able one. As a prominent break it begins about eight miles 

 north of Austin and passes southward through the western 

 edge of the city of Austin, through San Marcos, New Braun- 

 fels and San Antonio, and thence westward toward the Rio 

 Grande. West and north of this fault rises an escarpment 

 capped by Lower Cretaceous rocks, while on its east and south 

 side the beds forming the top of the downthrow are Upper 

 Cretaceous. This downthrow is from a thousand to fifteen 

 hundred feet and is on the south side.* The nature of the 

 contact between the upthrow and the downthrow often shows 

 that the fault is not a single slip but several faults close 

 together. The direction of this great Texas fault agrees closely 

 with the western margin of the Cretaceous-Tertiary border 

 through Arkansas. It is supposed to be of Tertiary or later 

 age.f 



The effect of such a depression upon the physiography of 

 the lower Mississippi will be realized when we remember that 

 such a movement (1500') at the present time would submerge 

 the greater part of the Appalachian mountains. 



This fault shows, moreover, that there has been more than 

 one epoch of disturbance and depression in the region under 

 discussion. The embayment began in early Cretaceous times, 

 but this fault is post-Cretaceous. 



Faults in Alabama. — In the Cahaba coal fields of Alabama 

 the faults are parallel with the Appalachian axis and the down- 

 throw is on the northwest — the embayment side.J Inasmuch 

 as the Coal Measures rocks abut against Silurian and Cambrian 

 beds on the southeast, the total displacement cannot be deter- 

 mined. From the bottom of the Carboniferous on the north- 

 west side to the surface on the southeast side is a distance of 

 5500 to 5600 feet, but this makes no allowance for erosion 

 from the uplifted surface, which must be somewhat greater 

 than the thickness of the Carboniferous beds. McCalley says 

 that the throw of some of these faults is 10,000 feet or more.§ 

 Sections across Blount Mountain, Alabama, coal field show the 

 basin tipped toward the northwest. | In the Coosa coal field 



* Communicated by N. F. Drake. 



fit. S. Tarr, Proc. Acad. Nat. Sci. Phil., 1893, p. 319. Professor Hill shows 

 six faults on the Colorado river northeast of Austin. Amer. Geol, May, 1889, 

 p. 7. 



% Map of the Cahaba Coal Field, by Squire aDd McCalley, 1896. 



§ Coal Measures of the Plateau Region of Alabama, 1891, p. 218. 



I A. M. Gibson, Report oa Blount Mountain, 1893, map. 



