NN logical observatious show no lack of moisture in that district, the annual precipitation being fully equal to what It is in the wellavuoded country farther east in the same latitude. Besides, the growth of ferret trees Is rich and abundant all through the prairie region, under certain conditions of soil and position, showing that their range Is not litnited by any general climatological cause.
" Taking into consideration all the circumstances under which the peculiar vegetation of the prairie occurs, we are disposed to consider the nature of the soil as the prime cause of the absence of forests and the predominance of the grasses over this widely extended region. And although chemical composition may not be without influence In bringing about this result, which is a question for farther investigation, and one worthy of careful examination, yet we conceive that the extreme fineness of the particles of which the prairie soil is composed Is probably the principal reason why it is bettor adapted to the growth of its peculiar vegetation, than to the development of forests. It cannot fail to strike the careful observer that where the prairie occupies the surface, the soil and superficial material have been so finely comminuted as to be almost in the state of an impalpable powder. This is due partly to the peculiar nature of the underlying rocks and the facility with which they undergo complete decomposi tion, and partly to the mechanical causes which have acted during and since the accumulation of the sedimentary matter forming the prairie soil.
" If we go to • thickly-wooded region, like that of the northern peninsula of Michigan, and examine those portions of the surface which have not been invaded by the forest, we shall observe that the beds of ancient lakes, which have been filled up by the slowest possible accumulation of detrital matter, and are now perfectly dry, remain as natural prairies, and are not trespassed on by the surrounding woods. We can conceive of no other reason for this than the extreme fineness of the soil which occupies these basins, and which is the natural result of the slow and quiet mode in which they have been filled up. The sides of these depressions, which were once lakes, slope very gradually upwards ; and being covered with a thick growth of vegetation, the material brought into them must have been of the finest possible kind, as is proved to have been the case by examina tion. Consequently, when the former lake has become entirely
filled up and raised above the level of overflow, we find it covered with a most luxuriant crop of grass, forming the natural meadows from which the first settlers are supplied with their winter store of fodder." The philosophical geologist may with advantage compare this part of Mr. Hall's statement and reasoning with those of Mr. G. l'oulett Scrape and Sir C. Lyehl respecting the formation of the Limagne d'Auvergne, or valley of the Allier, which was once the largest of three or four freshwater lakes existing in the miocene or middle division of the Tertiary period, in the country which is now Central France. The deposition of the fine lacnatrine sediment which constitutes the marls and a part of the limestone of the Limagne, in an age characterised by numerous extinct mammals and reptiles, offers a curious parallel to that of the corresponding sediment of the ancient American lakes, forming the soil of the prairie basins, as described by Mr. Hall, at a much later period of the earth's history, one which may be said almost to connect the present with the latest pro-historic era. Nothing can be more different than the present geographical configure ration and aspect, and geological structure, of the two regions ; but the illustration which the comparison affords of the identity of geo logical causes in all periods, is most interesting and instructive. It suggests also the inquiry whether evidence can be obtained that at any epoch in the process of filling up of the basins of the ancient lakes of Auvergne, and formation of similar lacuatrine plains in other parts of the world, the prairie condition intervened. The discovery of organic remains indicating the former existence of the peculiar prairie soil and vegetation would of course afford that evidence. In North America itself it would be equally important to ascertain whether any pleiocene or Pleistocene formation• present similar evidence of ancient prairies. See Scrope'e 'Geology and Extinct Volcanos of Central France,' ch. ii., and Hall's' Report,' cited above.