102 Trans. Acad. Sci. of St. Louis. 



eral the surface is very rough and irregular, splintered 

 and jagged, or rising into innumerable small hummocks 

 and bosses and sinking into basin-like depressions, seldom 

 exceeding a few inches or at most a foot or two in verti- 

 cal measurement, but sufficient to cause the accumulation 

 of a thin layer of soil and to retain an abundance of water 

 during the rainy season. In addition to this, at several 

 places, terraces or ledges a few feet in height afford still 

 further protection to soil and plants. 



The close fine textured chert being quite impermeable 

 to water, except where flawed or fractured, pools gather 

 in the basin-like depressions, where the water is retained 

 until, as the season advances and rains become less fre- 

 quent, it is gradually evaporated by the sun ; after which 

 the region becomes temporarily a parched desert again. 

 To this unequal distribution of moisture in different parts 

 and at different seasons of the year may doubtless in 

 large measure be attributed the peculiarities of the flora. 



In spring and early summer, with a superabundance of 

 water in the local depressions, a number of brackish 

 water and moisture-loving plants spring up. Amongst 

 these are Eleocharis ovata, Fimbrystylis laxa, Stenophyl- 

 lus capillaris, Cyperus aristatus, C. acuminatus, Juncus 

 marginatus, Allium mutabile and Cynosciadium pin- 



natum. 



Just beyond the margins of these temporary pools, in 

 thin rich soil, at first saturated but soon dry, or where 

 the rock is nearly exposed, such succulent species as 

 Sedum Nuttallianum, 8. pulchellum, Portulaca pilosa, 

 Talinum calycinum and T. parviflorum flourish. In some- 

 what drier situations, but where the accumulation of soil 

 is greater and moisture is consequently conserved to some 

 extent for a considerable portion of the season, the num- 

 ber of species is much larger. Typical amongst them are 

 Saxifraga texana, Selenia aurea, Rumen hastatulus, Cro- 

 tonopsis linearis, Lathyrus pusillus, Chaerophyllum tex- 

 anum, Ptilimnium Nuttallii, Spermolepis echinata, Hyper- 

 icum pseudomaculatum, Linaria canadensis, Phacelia 



