LAFAYETTE FORMATION AND SASKATCHEWAN GRAVELS. 89 



nent? This question has led nie to review in the present essay the 

 series of formations in the Mississippi and Nelson river basins which 

 belong to the times immediately preceding, during, and following the 

 Glacial period, esj^ecially considering the changes in the altitude and 

 slopes of the land and the probable measures of time demanded by the 

 processes of drift transportation and deposition, by subsequent weather- 

 ing with soil formation, and by stream erosion. To take occasionally 

 such a view of the whole era and extended area to which one's limited 

 studies appertain seems to me the duty of every working geologist, that 

 his interpretations of observations in his own field may be guarded from 

 error, and that perchance his neighbor's field of exploration may be 

 better understood. 



The Lafayette Formation and the Saskatchew^an Gravels. 



The broad lower part of the Mississippi valley, from the southern 

 boundary of the glacial drift to Louisiana, contains a very extensive 

 deposit of sand and gravel, designated formerly from its prevailing fer- 

 ruginous color as the Orange sand, later called b}^ McGee the Appomat- 

 tox formation in its development on the coastal plain of the Atlantic 

 and Gulf states, but recently named the Lafayette formation from 

 Lafayette county in northern Mississippi, where it was earliest discrimi- 

 nated 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,t and Salisbury J hold that it is probably referable 

 to the Pliocene period; while Spencer, § Hilgard, || and others, as it 

 seems to me preferably, consider it as the earliest of our Pleistocene 

 formations. Its northern continuation beneath the glacial drift is recog- 

 nized by Salisbury T[ in western Illinois to a distance of a hundred 

 miles northward from the Missouri river and boundary of the drift, and 

 gravels believed by him to be probably of the same formation occur in 

 the Wisconsin and Minnesota driftless area, while northeastward he has 



* Am. Jour. Sci., third series, vol. xxxv, Feb., 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. 



t Bulletin, G. S. A., vol. i, 1890, pp. 4G9-480. Am. Jour. Sci., third series, vol. xli. May, 1891. 



t Article last cited. Geol. Survey of Arkansas, An. Rep. for 1889 (published 1891), vol. ii, "The 

 Geology of Crowley's Ridge," pp. 224-248. 



g Geol. Survey of Georgia, First An. Rep. for 1890-'91, p. C2. 



II Am Jour. Sci., second Jseries, vol. xlii. May, 1866; vol. xlvii, Jan., 1869; vol. xlviii, Nov., 1869 ; 

 third series, vol. ii, Dec, 1871 ; vol. xiiii, May, 1892. Am. Geologist, vol. viii, Aug., 1891, pp. 129-131. 



% Bulletin, G. S. A., vol. iii. 1892, pp. 183-186, 



