177 



These changes go on very slowly. There are now areas of 

 black-jack covering several square miles with scarcely a trace 

 of leaf-mold. This is illustrated especially by the black-jack 

 timber south c-f Havana between Bath and Kilbourne. where, 

 in a belt five miles wide, there is nothing but pure sand without 

 any covering of humus. In the country east of Havana, where 

 the sand is mostly confined to long wooded dunes, and near the 

 Hlinois River, the formation of the soil appears to be more rapid. 

 Some of the wooded ridges back fi-om the river have a coating 

 of leaf-mold only half an inch to an inch in thickness, and the 

 cacti still growing in it show that it has lieen but a few years 

 since its formation. 



With the first traces of leaf-mold such semi-xerophytic 

 plants as AijiiiJcgin cuiKif/iiisis, Tri<i>tt<-i(tn aiiniutitirum. Silfii*' 

 sffl/(if(i. Atieniont' rirtfhiidiKi. and Ai/riinoiiid mollis begin to ap- 

 pear, together with many other species common to most up- 

 land-wood associations, although many of the sand-loving xero- 

 phytes. such as ('n/llrrhoe triaitfjidiiffi, BInis aroiiififiea. and Lfs- 

 pfthzn aipifaf'i. still pei*sist. As the soil increases in depth 

 more characteristic mesophytes appear, including Vnynero 

 mcemosa. lai/tiera sffllnfu. (jiinit nnKuh'iisf, Asilt-phis cjoltufii, 

 and Eiip((foriiiiii iii/endoides. PmilicmicissH.s ijidtiijuefoltii be- 

 comes very abundant, climbing up most of the trees, and trail- 

 ing prostrate on the sand, covering it with a dense mat. The 

 arborescent tiora is still unchanged : the two oaks and the 

 hickory constitute nearly all of the forest, and the only addi- 

 tions are small scattering trees of C'/'/^.- ainadt-nsis. Morns ru- 

 hra. and Cfltis orridf-nf'iJis. 



None of the black-jack forests observed has as yet passed 

 beyond this semi-mesophytic stage except in a narrow belt 

 along the Illinois River i PI. XXL. Fig. 2k where plants from 

 the neighboring mesophytic and hydrophytic forests may spread 

 more quickly over the sand ridges. In such places the forests 

 of the wooded dunes contain but a small proportion of black- 

 jack oak. its place being taken by bur-oak y(Jiifiri(s mucrocurp'i > 

 and white oak yQueirits albn). The leaf-mold is deep, and the 



