980 The American Naturalist. d [December, 
the ice-sheets; (2) the Glacial period, when the glacial and 
modified drift were formed ; and (3) the Postglacial, Recent, 
or Present period, bxterdias from the departure of the ice- 
sheet until now. The first and second of these periods, which 
were comparatively long, constitute the Pleistocene division, 
while the third and very brief period is the Present or Psy- 
chozoic division, of the Quaternary era. 
THE LAFAYETTE PERIOD. 
The broad lower part of the Mississippi Valley, from the 
southern boundary of the glacial drift to Louisiana, contains 
avery extensive unfossiliferous deposit of sand and gravel, 
designated formerly from its prevailing ferruginous color as 
the Orange sand, later called by McGee the Appomattox for- 
mation in its development on the costal plain of the Atlantic 
and Gulf States, but recently named the Lafayette formation, 
from Lafayette County in northern Mississippi, where it was 
earliest discriminated by Professor E. W. Hilgard in 1855 and 
1856. This formation was spread across the valley plain 50 
to 150 miles or more in width along an extent of 600 miles 
from the mouths of the Missouri and Ohio Rivers to the Gulf 
of Mexico, during the closing stage of the Tertiary era and 
the beginning of the Quaternary, to each of which it has been 
assigned. McGee, Chamberlin? and Salisbury,‘ hold that it is 
probably referable to the Pliocene period ; while Spencer,’ Hil- 
gard, E. A. Smith’ and others, as it seems to me preferably, 
have considered it asthe earliest of our Pleistocene formations. 
Its northern continuation beneath the glacial drift is recog- 
nized by Salisbury? in western Illinois to a distance of a hun- 
"Am. Journ. of Science, III, Vol. xxxv, February, April, May and June, 
1888 ; Vol. xl, July, 1890. U.S. Geol. Survey, Twelfth An. Rep., for 1890-91, 
pp. 347-521, with 10 plates, and 45 figures in the text. 
‘Bulletin Geol. Soc. of America, Vol. i, 1890, pp. 469-480. Am. Jour. Sci., 
III, Vol. xli, May, 1891. 
*Article last cited. Geol. Survey of Arkansas, An. Rep. pi 1889 (published 
1891). Vol. ii, “ The Geology of Crowley's Ridge," pp. 224-24 
*Geol. Survey of Georgia, First An. iens for 1890-91, p. 6 
*Am. Jour. Sci, II, Vol. xlii, May, 1866; Vol. xlvii, Jan., A Vol. xlviii, 
Nov. 1869; III, Vol. ii, Dec., 1871; S rtis May, 1892. Am. Geologist, Vol. 
viä, Aug., 1891, pp. 129-131. | 
m. Jour. Sci., IHI, Vol. xlvii, April, 1894. 
"Bulletin Geol. Bolar of America, Vol. iii, 1892, pp. 183--186. 
