302 GEOLOGY. 



sented, if our interpretation be correct, in the valleys west of the Appa- 

 lachians. An analogous formation is found on the Coastal plain of 

 Texas, and, by inference at least, this is associated with analogous 

 deposits on the Great plains, and through them with the intermontane 

 deposits of the west, already mentioned. The term Lafayette has 

 been usually applied only to the formation on the slope between the 

 Appalachians and the Atlantic, to that in the Mississippi basin below 

 the junction of the Ohio, and to the Texan tract. The formation 

 thus limited has been estimated to have an area of from 200,000 to 

 250,000 square miles. It lies like a blanket over the eroded edges 

 of all the older formations of the region, from the pre-Cambrian to 

 the Miocene. It extends inland from the coast up to varying altitudes. 

 In Mississippi, its landward edge is said to reach an elevation of 500 

 or 600 feet; in Tennessee, 800 feet; at Austin, Texas, 500 feet, and 

 near the Rio Grande, 1000 feet 1 ; but on the Atlantic slope, the ele- 

 vation is generally less. 



At its mountainward edge, ragged belts of the Lafayette forma- 

 tion follow the valleys up into the mountains, and unless our identifi- 

 cations be in error, they reach back through the gaps, where they 

 are locally interrupted, into the intermontane valleys. Between the 

 valley phases, its mountainward edge recedes and is ragged, and 

 has not yet been carefully mapped. At its seaward margin, the for- 

 mation is more or less completely concealed by younger beds. It is 

 not to be doubted that the Lafayette formation or its equivalent passes 

 out to sea beneath these younger beds. Indeed, there is some reason to 

 believe that at some points it is replaced within the present land -area, 

 by marine beds, as such a formation is very liable to be where the 

 plain on which it was deposited slopes gently to the sea, But such 

 marine deposits as can be correlated, even hypothetically, with the 



Geology of Tenn. (Bluff Gravels), and Am. Jour. Sci., Vol. XXXVII, 1864; Hilgard, 

 Agriculture and Geology of Mississippi, 1860, and Am. Jour. Sci., Vol. XLI, 1866, and 

 Vol. IV, 1872; Loughridge, Kentucky Geological Survey; Jackson Purchase Region, 

 1888; Geology of Illinois, Vol. I, pp. 417 and 447; Salisbury and Call, Geol. Surv. 

 of Ark., Report on Crowley's Ridge, 1889; Hill, Am. Geol., Vol. VII, 1891, p. 368, 

 and (with Vaughan) Uvalde formation of Texas, 18th Ann. Rept. U. S. Geol. Surv., 

 Pt. II, p. 560; Dumble (Blanco Formation of Texas), Jour. Geol., Vol. II, 1894, p. 560; 

 Smith, E. A., and Johnson, L. C, Geol. Surv. of Ala., 1894. , For synonyms of the 

 formation, see Am. Geol., Vol. VIII, 1891, pp 129-131, and Bull. 84, U. S. Geol. 

 Surv., p. 328. 



1 McGee, loc. cit. 



