1893.] NATURAL SCIENCES OF PHILADELPHIA. 337 



The Colorado itself shows signs of attempts at adjustment. The 

 general course of the Colorado is across the strike of the Carbonifer- 

 ous, but in several of the loops the two long sides are in the direc- 

 tion of the strike while the short side is at right angles to this. The 

 river is there locally in adjustment. While this may be accidental, 

 it seems hardly possible when we take into consideration the 

 number of instances where it may be noticed. 



(e) Summary of the History of the Colorado. — To briefly summa- 

 rize the history of the Colorado in this region, as I interpret it, it 

 may be said that the river appears to have begun upon the Creta- 

 ceous which in this region was first to be raised above the sea, at the 

 close of the Cretaceous. The long stand indicated by the Tertiary 

 deposits gave the rivers time to reduce the elevated portion approx- 

 imately to base-level, and the present serpentine courses of the 

 streams are an inheritance from this period. 



The late Tertiary elevation of seven hundred feet, and probably 

 more, rejuvenated the streams, and this accounts for the signs of 

 youthfulness now apparent. The later Quaternary elevation has pro- 

 duced no marked effect here. The Colorado, being superimposed 

 upon the Silurian, is retarded in its development and has been 

 robbed by the Brazos of much of the drainage territory which would 

 normally belong to it. It has also, above the barrier, been reduced 

 to a temporary base-level and its canon is being degraded and its 

 channel even being built up. Both the smaller tributaries and the 

 Colorado itself are adjusting themselves to the structure of the Car- 

 boniferous upon which they have been superimposed. 



6. — The Arid Plateau. 



(a) Topographic Description. — My knowledge of the plains is 

 confined to a single journey across them by wagon and this descrip- 

 tion must therefore be very general ; but the plains are very monot- 

 onous and one becomes acquainted with their general physical fea- 

 tures even in such a hasty reconnoissance. It is a great semi-arid 

 plain, truly arid in the west, with isolated buttes here and there. 

 These buttes are in part of Cretaceous strata, but there are many 

 formed of Permian beds. They are the remnants of layers which 

 were once continuous. The greater part of the area is Permian 

 and the soil is red while the water is alkaline or saline for the rea- 

 son that these beds are inland sea deposits. 



The Staked Plains cross the line of the Texas and Pacific railway 

 in the neighborhood of Marienfeld presenting here a low wall or 



