1950] PALMER, CANYON FLORA OF EDWARDS PLATEAU 237 



These colonies can in no sense be considered as extensions of the Atlantic 

 forest belt: for besides being so restricted in habitat their species nowhere 

 form more tlian a small per cent of the forest growi-h even in the canyons 

 where they are found. With the exception of the Cypress, Pecan and Wild 

 Cherry none of them grow even in the wide low^er canyons; and they are 

 separate 1 from their kind or their nearest allies by more than three degrees 

 of longitude, the intervening countrj^ between the Edwards Plateau and the 

 lower Brazos River being a dry, open country where they have been unable 



to survive. 



The interesting question then naturally arises: how did they reach these 

 remote stations and what does their presence here signify? The answer is 

 probably to be found in tlie later geological history^ of w^estern Texas, to the 

 record of which they may add valuable corroborative testimony. 



That the seeds of these species have been transported either by w^ind or 

 water from the nearest stations where they are at present found in eastern 

 Texas is out of tlie question; for not only is the distance much too great but 

 both the prevailing air currents and drainage are in other directions. Nor 

 could they have been carried to these remote canyons by migratory birds 

 or other animals; for tliere is no migration in this du^ection and the fauna 

 of the Edwards Plateau is distinct in almost all of it^ species from that of 

 the luimid portion of eastern Texas, Moreover, the seeds of some of the 



be 



After 



belt ftn4 



rich 



uccumb 



tions, but one reasonable hypothesis suggests itself. 



Towards the close of the Tertiary period or early in the Pleistocene great 

 mountain making movements occurred in the continental interior, resulting 



in the elevation of the Trans-Pecos region of Texas, the obliteration of the 

 vast inland seas previously occupying portions of the plains country and 

 the diversion of drainage systems over wide areas. As a result of these 

 distur< anccs a cycle of changes was inaugurated that must have profoundly 

 affected the chinate and physiography of the region, and these reacting in 

 turn upon tlie fauna and flora, like ripples raised by a stone thrown into the 

 water, diminishing in force but widening in circumference, have been active 

 until very recent times, if indeed their influence has yet entirely subsided. 



That the Edwards Plateau existed as an outstanding physiographic 

 region before the beginning of this cycle is evidenced by the fact that the 

 great deposits of gravel brought down from the foothills of the Rocky 

 Mountains by the swollen rivers of the Lafayette stage, sweeping across 

 where now is tlie canyon of the Pecos River, passed to the north and south 

 of it, some of the streams reaching the Gulf of Mexico far to the east. 



The vast forest of the Gulf coastal plain, stretching at the present time 



from the Atlantic seaboard to eastern Texas, has beyond doubt occupied the 



region, or such parts of it as were above sea-level, from remote geological 



