1893.] NATURAL SCIENCES OF PHILADELPHIA. 325 



feet. This change is greater than can be accounted for by the 

 recent slight submergence, and there is evidence that it began before 

 the submergence, for the flood plains are broad and deep and conse- 

 quently old, whereas the estuaries are so recent that they have 

 hardly begun to be filled. 



Whatever be the explanation it must be one whose chief factor is 

 an increase in sediment, and for this two hypotheses suggest them- 

 selves. In the normal development of a stream there comes a time 

 when, by the growth of headwaters, the supply of sediment exceeds 

 the power of the stream to transport it and the stream becomes 

 overburdened. Under different circumstances this condition ap- 

 pears at different times. In such rivers as the Platte it is a long 

 enduring condition, in others it may never occur, but in general it is 

 a stage in development. It is possible that the Texas streams have 

 reached this stage. 



From my knowledge of the Texas rivers I am inclined to believe 

 that the explanation must be modified by the introduction of the 

 second hypothesis. It is quite certain that the climate of west and 

 central Texas has not long since changed from a moderately moist to 

 an arid climate. This is indicated by the old lake basins, now 

 nearly dry, in the Trans-Pecos region ; and, in central Texas, by 

 the headwaters of the Colorado, well defined valleys now abandoned 

 through dessication, to be described later. Probably this change 

 of climate was coincident with the change registered in Lake 

 Bonneville, and consequently with the disappearance of the conti- 

 nental glaciers. During the moist period the soil was clothed with 

 vegetation and the rains were more uniform and less violent than at 

 present. Under the present conditions nearly every rain in the 

 semi-arid region comes as a heavy fall, and, striking the barren 

 plains, transports much sediment. An arid tyj)e of arroyo-like 

 drainage has replaced the more perfectly developed drainage of the 

 earlier period. 



It seems to me quite certain that the extensive flood plains of the 

 forested area are the result partly of a change in climate to more 

 arid, partly of a stage in development when the overburdened con- 

 dition of the rivers has been reached. What effect the Llano Esta- 

 cado, which Professor Hill considers Quaternary Lake beds, may 

 have in this connection I am unable to say. It would in all proba- 

 bility add one more factor to the problem and this another source of 



