141 



brown sand of the middle slope. It is caused by past generations of 

 plants which occupied this surface before the dune was formed, or at 

 least before it had migrated so far inland. Since this soil blows, 

 weathers, or dries out less rapidly or easily than the pure sand, the 

 outcrop is marked by a slightly steeper slope and by dark-colored 

 patches. Digging behind shows that the stratum extends indefinitely 

 beneath the sand. For long distances the outcrop line is very dis- 

 tinct, but not perfectly level. Its elevation varies gradually, but 

 irregularly, and in some places the whole outcrop disappears, cor- 

 responding to irregularities in the original level of the sand, or to 

 locations of former blowouts. This soil stratum is on the same level 

 as the counti"y behind the dune and illustrates plainly the continued 

 inward migration of the dune. (Cf. also PI. XVIII, Fig. 2.) It 

 is characterized now by a line of Blymiis canadensis. 



The upper slope consists of fine sand piled at an average angle 

 of about 20 degrees. The vegetation is much like that of the middle 

 slope, but denser and with many additional species (PI. XVII, Fig. 2). 

 It is likewise referred to the blowsand association. Cassia Chamae- 

 christa and Diodia teres are again the most abundant species, and 

 Aristida tuberculosa is also conspicuous. Other less characteristic 

 species are Monarda punctata, Lithosperinum Gmelini, Euphorbia 

 corollata, Diodia teres, Cristatella Jamesii, Ambrosia psilostachya, 

 Oenothera rhonibipctala, Linaria canadensis, Kuhnia eupatoriodes, 

 var. coryinbulosa, Acer at es viridiflora, var. lance olata and var. line- 

 aris, Oxybaphus nyctagineiis^ Teucrimn occidentale, Tradescantia re- 

 iiexa, Lespede^a capitata, and Rumex Acetosella. In somewhat shel- 

 tered places Scropliularia leporella, Draha caroliniana, and Coryd- 

 alis luicrautha occur. 



In some places, near the top of the slope, adjacent to the thickets 

 described later, are associations of Physalis heterophylla (PL 

 XVIII, Fig. i). The individual plants grow in large patches and 

 are very loosely aggregated, w-ith a large amount of open sand be- 

 tween them, yet the patches are remarkably free from other plants. 

 Even Cassia and Diodia, so abundant on the upper slope, are almost 

 entirely absent from these patches. This peculiarity of distribution 

 leads to the inference that they are more closely related ecologically 

 to the thickets w-hich crown the dunes than to the slope below. They 

 may bear the same relation to the blowsand association that Sniilacina 

 stellata on the lee slope bears to the deposit association. 



The crest of the river dune is primarily an area of deposit, and 

 is consequently occupied at first by the regular deposit association, 

 already described in connection with the blowout fonnation. Rhus 



