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, S. 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 
Saaifraga texana, Selenia aurea, Rumex hastatulus, Cro- 
tonopsis linearis, Lathyrus pusillus, Chaerophyllum tex- 
anum, Ptilimnium Nuttallu, Spermolepis echinata, Hyper- 
icum pseudomaculatum, Linaria canadensis, Phacelia 
