111? 



as it is. (PI. XIII.; XIV., Fi^-. 1; XV.) Frecfuently a later sta^^e 

 of this evolution is the grovvtii of large tracts of a .scrubby 

 black-jack forest ( PI. Mil.; XXI., Fig. 1. ), and this, in turn, by 

 the gradual formation of leaf-mold, approaches the character 

 of the ordinary Illinois forest. Forests of the latter cla.ss (PL 

 XXL, Fig. 2) are especially noticeable on the fixed dune ridges 

 which lie along the edge of the sand plain, next the river or its 

 bottom-lands, such as the ridge extending through the city of 

 Havana. The areas of lilow-sand and l)lack-iack are about equal, 

 that of the final stage comparatively small. 



A very different and characteristic sand fauna and Hora 

 may be found upon the constantly moist strip of sand which 

 usually occurs along the present .'^tream valleys at the margin 

 of the sand plain, twenty to forty feet itelow its surface level, 

 not only upon the present shores ( PI. XXllL). but also along 

 the line where the absorbed rainfall of the sand plain drains 

 out upon the river liottoms at the foot of the present low mar- 

 ginal bluff. 



The Localities Visited. 



The most I'eniarkable sand area known to me in Illinois is 

 in the out-of-the-way interioi' of the low sand plateau indicated 

 by Ijeverett ( "91). PL VI.) as an island in the channel of the 

 (^'hicago outlet north of Havana (see map; also p. 143 of this 

 ai'ticle ). The eastern margin of this ancient island is skirted by 

 the Chicago. L*eoria and St. jjouis Ifailway. liut from the train 

 only a suggestion of its character ap[iears. It is approximately 

 five or six miles wide and twenty miles long, having an area of 

 about one hundred .square miles. The middle third is especially 

 sandy and almost entirely waste laud. About half of this is 

 covered with black-jack, and the other half, especially the 

 south-central part, contains blow-sand to an extent not sur- 

 passed anywhere else in the state, this region being locally 

 known as the Devil's Neck (PL XII., Fig. 1: XIV.. Fig. 1; XX.. 

 Fig. 1 ). One tract of altout eighty acres is almost entirely blow- 

 .sand in successive ridges, suggesting great ocean waves in a 

 storm. Blowoutsof unusual extent sui'round it on all sides. and 



