248 



THE riEOLOGIST, 



land. It has been remarked lliat the slrtde of trees prevenis ilic 

 growth of plants belonging to the family of the grasses, whilst the 

 latter, in their turn, hinder the developement of the seeds of trees, and 

 leave the young shoots exposed to a variety of accidents. But why 

 and liow^ have these vast tracts of country become forests and prairies 

 at some very ancient period — probably when the waters retired from 

 the surface of the soil which they cover, and vegetation first sprang up 

 there ? M. Lesquereux asked himself this question Avhilst imnbling 

 over the plains of ^ortli America, and he endeavoured to answer it by 

 direct observation. 



On the banks of .the Mississippi and the Minnesota M. Lesquereux* 

 has actually seen prairies in process of formation. According to his ob- 

 servations, this is what takes place : — Mud is constantly deposited at 

 each side of the river, but wlieii the latter swells in the rainy seasons, 

 the w^ater 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 in 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 when the heat of a summer's sun has evaporated the 

 water, grass, rushes, and a few other plants of like structure, can alone 

 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 

 sufficient in this climate to effect a complete evaporation before the 

 equinoctial rains begin — they persist as swamps, and, in course of time, 

 by the accumidation oi Sphagnum and Conferva, peat-bogs are formed. 

 Ancient marshes or swamps become either prairies or peat-beds, 

 according to circumstances, but more especially according as inunda- 

 tion has been followed by a longer or shorter period of dryness. As 

 regards America, M. Lequereux brings forward a great number of 

 examples, mostly taken from the lianks of tlie great lakes Erie and 

 Michigan, or iho neighbouring rivers, to prove that prairies formed at 

 (liirerent pci i( ids owe their existence to the causes above-named ; whilst 

 lorests appear to have been formed solely on hills, or in parts remote 

 fii'Ui the iuHuenees of periodical inundation. 



{ To he conlim e. /.) 



