1897. 



ASHLEY — GEOLOGY OF ARKANSAS. 301 



western end of the Arkansas base line. To tlie north runs the 

 Poteau river, to the northeast the Fourche la Feve, to the east the 

 Ouachita, to the southeast the Caddo and Little Missouri, to the 

 south the West Saline, Cossatot and Rolling Fork, to the southwest 

 the Mountain Fork of the Little river, and to the west the Black 

 Fork of the Kimishi river. 



We may then assume that that region was the centre of elevation 

 for the land period that gave rise to the present drainage. 



We can readily imagine a condition of things, such as at present 

 exists in the Cretaceous region, having previously existed over 

 the whole area. The creeks and rivers following the general slope 

 of the Cretaceous strata, entirely unaffected by the buried Paleozoic 

 sandstones. Then as they sink their channels until the Paleozoic 

 strata are reached, they find it easier to continue sinking their 

 beds than to force their banks and make new channels conformable 

 with the softer rocks. 



If next, we suppose that, due to shore conditions, the thickness 

 of the beds diminished to the north, we can understand how 

 erosion would first expose the Paleozoic strata at the north and the 

 line of contact would gradually move southward until it reached 

 its present position. 



As erosion began to cut deeply into the Paleozoic strata, the 

 minor streams yielded to the influence of structure and became in 

 time structural streams, and the larger streams, though maintaining 

 their first courses in a general way, became conformable to the 

 structure in minor details of their courses. 



Li which of the land epochs the present drainage originated 

 and became fixed we do not know. It must be remembered that 

 these land periods each produced a nonconformity in the Cretaceous 

 and Tertiary rocks, so that it is not impossible that all the effects 

 of erosion during one land epoch may have been erased in the 

 next subsidence, new beds being laid down and the next land 

 epoch beginning a new system of drainage. However this may 

 have been, the evidence seems to show, that, at the end of the land 

 period following the Tertiary, the topography of most of our area 

 was much as it is to-day, therefore it will be discussed at this point. 



Drainage of the Area. — The direction of the main streams 

 would be represented diagrammatically by drawing radiants from 

 the centre of drainage at the western end of the Arkansas base 

 line. 



pnOC. AMER. PHILOS. SOC. XXXVI 155 U. PRINTED AUGUST 12, 1807. 



