336 PROCEEDINGS OF THE ACADEMY OF [1893. 



Carboniferous beds to cut through, and these higher up in its 

 course. This has consequently given to the Brazos tributaries much 

 more power than is possessed by those of the Colorado. The Colo- 

 rado flows in a channel between eleven and twelve hundred feet 

 above sea-level, just above the Silurian, whereas the Brazos east of 

 here is only half that elevation. 



(d) Adjusted Stream Courses. — The valley of the Colorado in this 

 vicinity is remarkable for its serpentine course. In one place the 

 river flows in a bend six miles long where a cut-off would reduce the 

 distance to two miles. Along the boundary of San Saba county the 

 river flows fifty miles, and, at the end of this distance, is only thirty 

 miles from the first point. The valley here is a canon, somewhat 

 degraded it is true, but nevertheless the serpentine course is one in 

 rock walls, not upou a flood plain. I have suggested in an earlier 

 part of this paper that this course may be in a large measure an 

 inherited course from a Tertiary peneplain, formed before the eleva- 

 tion recorded both by the Tertiary forested area and by the rejuven- 

 ated condition of the streams in the Cretaceous. The fact that the 

 Brazos has much the same peculiarity supports this hypothesis. 



"While I believe this to be a true cause I am equally convinced 

 that it is not the sole cause. The entire drainage system of the 

 Colorado in the denuded area is superimposed. 47 It is in conse- 

 quence not in accord with the revealed structure, for its course was 

 originally consequent on a Cretaceous surface, and this course is 

 now enforced upon it in spite of the altered condition. It is. how- 

 ever, a habit of rivers to attempt adjustment under such conditions 

 and to be influenced by the revealed structure ; and, as time goes 

 on, this adjustment becomes more complete. As I have shown in 

 the article cited many of the tributaries to the Colorado are quite in 

 accord with the Carboniferous structure, particularly in the regions 

 that have been longest exposed, and in the smaller streams. This 

 has given rise to strike valleys which, in the vicinity of the Creta- 

 ceous, change to unadjusted courses. 



Dr. Comstock 28 has called attention to the quite perfect accord of 

 the streams on the Silurian to the more complicated structure, this 

 being the result, in a large measure, of the longer exposure of this 

 region, by the early removal of the Cretaceous. 



M Tarr, Am. J. Sci., XL, 1890, pp. 359-362. 



27 For a discussion of this subject see an article by the author in Am. J. Sci., XL, 

 1890, pp. 359-362. 



28 Second Ann. Report, Geol. Survey Texas, 1890, p. 664. 



