74 THE NAUTILUS. 



what regular curve to Helena, Arkansas, on the Mississippi 

 River, varying in width from less than a mile to more than 

 fifteen miles. Its greatest distance from the Mississippi River 

 is sufficiently indicated by the line between Nettleton and 

 Osceola, approximately forty miles. Near Greenville in Green 

 County the Ridge reaches a maximum elevation of 250 feet 

 above the valley of the Cache, and in many sections there are 

 denuded areas or " ampitheatres" of great extent, but of lim- 

 ited agricultural promise. The lowest beds are clays on which 

 are imposed great deposits of sand and gravel, and the whole 

 capped with loess. The loess is of early Pleistocene age, the 

 gravels are Tertiary, the sands are also Tertiary but of a much 

 older period, while the clays at the base are Eocene as estab- 

 lished upon paleo-botanic data. Thrice alternately this region 

 has been depressed and elevated during Tertiary times and be- 

 fore the great depositary and erosive activities of the glacial 

 period began. As Professor Call* truly observes, Crowley's 

 Ridge is " the residual product of long-continued erosion. It 

 is in no sense an upheaval, nor does it, in Arkansas, contain a 

 rock of crystalline character or of Paleozoic age. Its existence is 

 due to the resistance it has offered to erosive forces which have 

 leveled the greater part of the region. It stands now a silent 

 witness to a history so wonderful that the imagination is taxed 

 by any attempt to compass all the details." 



The fauna of all the rivers west of Crowley's Ridge and east 

 of the Paleozoic escarpment f in Arkansas possesses a peculiar 

 interest owing to the fact that they are the modern representa- 

 tives of the ancient Mississippi, which in pre-glacial times did 

 not cut through the solid rock southeast of Cape Girardeau, 

 Missouri, but swept through the lowlands of Black, White and 

 Cache rivers, uniting with the Ohio south of Helena. For the 

 same reason the fauna of St. Francis basin will claim attention 

 as this was the subsequent channel of the Mississippi when it 

 broke through Crowley's Ridge at Chalk Bluff on the Missouri- 



* An. Kep. Geol. Surv. Ark., 1889, Vol. II, p. 131. 



| This boundary is accurately enough marked for descriptive purposes by the 

 Saint Louis and Iron Mountain Railroad. 



