348 
Tlir. (iKOLOfilST. 
laud. It lias been remarked lliat the shade of trees preveiils tlic 
gi'owtli of plants belonging to the family of the gi-asses, ■whilst the 
latter, in their turn, hinder the developement of the seeds of trees, and 
leave the j^oung shoots exposed to a A^ariety of accidents. But why 
and hoAv have these vast tracts of country become foi-ests and prairies 
at some very ancient period — probably Avhen the waters retired fi-om 
the surface of the soil wluch they cover, and vegetation first sprang up 
there 1 Lesquereux asked himself this question whilst ramliling 
over tlie plains of Xorth America, and he endeavoured to ans^ver it by 
direct observation. 
On the banks of the Mississippi and the Minnesota M. Lesr^uereux 
has actual!}'- seen prairies in process of formation. According to his ob- 
servations, this is v.'hat takes place : — jMud is constantly deposited at 
each side of the river, ljut vlien the latter swells in the rainy seasons, 
the water passes over its banks, and forms immense marshes or swamps, 
where it becomes stagnant and putrid, and where the sand and mud 
which it has washed along m its course are deposited. Nothing is less 
favourable or contrary to arborescent vegetation ; the roots are deprived 
of air by a stratum of water, which is never renewed ; trees, if they have 
already begun to grow in these situations, perish when such inunda- 
tion occurs, and Avlien the heat of a summer's sun lias evaporated the 
water, gi-ass, rushes, and a fcAV other plants of like structure, can alnne 
resist and accomplish their developement under such circumstances. 
In Germany, and many other parts of Europe, marshes are rarely, if 
ever, dried up in this way — the heat of the summer s sun is not 
s\ifficient in this climate to effect a complete evaporation before the 
eciuinoctial rains begin — they 2:)ersist as swamps, and, in course of time, 
by the accumidation oi Spliagmim and Conferva, peat-bogs are formed. 
Ancient marshes or swamps become either prairies or peat-lieds, 
according to circinnstances, but more e.specially according as inunda- 
tion has been followed by a longer or shorter period of dryness. As 
regards America, i\f. Lequereux brmgs forward a great number of 
examples, mostly taken from the lianks of the great lakes Erie and 
Michigan, or the neighbouring I'ivers, to prove that prairies fonued at 
different periods owe their existence to the causes above-named ; -wliilst 
forests appear to have been formed solely on hills, or in parts reuiute 
from the influences of periodical inundation. 
( T'l h( roll till I f 1. 1 
