374 STUDIES FOR STUDENTS 



As shown by Hilgard,^ this is a widespread, flood-made formation, 

 extending along the great valley of the continent, the Mississippi, 

 south of its junction with the Missouri and on the Gulf Coast reaching 

 from Mobile Bay in Alabama to the Sabine River on the borders of 

 Texas. The formation consists in large part of orange, rust colored, 

 but sometimes purpHsh, white, or variegated sand, consisting almost 

 entirely of quartz grains much rounded and smoothly polished, and 

 very commonly encrusted with the rusty pigment. Near the great 

 river channels, notably that of the Mississippi on either side, on the 

 Tombigbee and Tennessee, as well as on the Sabine there is a steady 

 increase of gravel. The beds are irregularly stratified, sometimes 

 structureless for 20 feet of thickness, but have generally the flow-and- 

 plunge structure. Hilgard considers that the formation as a whole 

 is the outcome of fresh water in the state of violent flow. The " Orange 

 Sand" increases in thickness toward the Gulf. In an artesian well 

 near the Calcasieu River, 200 miles west of New Orleans, beds 

 referred to the Lafayette are found to extend to a depth of 450 feet, 

 beneath 160 feet of clay of the Port Hudson group; and at New 

 Orleans the gravels have been found by borings to extend to 760 

 feet below the level of the sea, Hilgard showed that it overlaid the 

 Grand Gulf beds and was in its turn covered by the Port Hudson or 

 Columbia. He considers that it is latest Tertiary or earhest Quater- 

 nary, and he correlates this increase of fresh-water action with the 

 melting and retreat of the first glacial stage. Chamberlin and SaHs- 

 bury consider, however, that glacio-fluvial work is not concerned 

 in the origin of this Mississippi portion, since upon careful search 

 neither was able to find any pebbles referable to glacial action,^ 

 this origin being restricted to the Natchez formation which overlies 

 it.3 These authors favor the hypothesis that the Appalachian por- 

 tions at least were developed by successive shiftings of river deposits 

 toward the sea during Pliocene upward bowing of the partially 

 developed Tertiary peneplain in the region of the Appalachians, but 



1 "The Age and Origin of the Lafayette Formation," Amer. Journal of Science, 

 Vol. XLIII (1892), pp. 389-402. 



2 T. C. Chamberlin, " Some Additional Evidences Bearing on the Interval between 

 the Glacial Epochs," Bulletin of the Geological Society of America, Vol. I, 1890, p. 470. 



3 Geology, Vol. Ill (1906), p. 308. 



