Why are they not forests I have not seen as much of the prairies, as appears to have been the case with the correspondent in your August number. But I have drawn certain conclusions from my limited knowledge that may be worth printing.

There is a remarkable stratum of sand underlying the limestone rock, and ex-tending over a large district on the Upper Mississippi. In this sand is the remarkable cave near St. Paul, Min. The largest chamber I estimated to be 70 x 40, and 18 feet high, oval in plan, and arched like a tortoise shell. It has so little tenacity that sand swallows build their nests in it It is readily detached by the finger nail, when it resembles sea sand. But it approaches so nearly to sand-stone rock, that it supports itself in the form of an arched roof in this cave. The same stratum is found at Fort Snelling. At the falls of Minnehaha it is worn away under the projecting rock, so as to form a gallery under the falls from side to side. It is found under the rocky cap at the conical hill called "The Lone Mound," in Wabashaw County, Min., and thence dug out for making mortar. It is said to be found under the rock when digging wells at Janesville, Wis. I suppose that it can be found any where in this district along the bluffs.

The geological position of the prairie about Fort Snelling, and the Falls of St Anthony, etc., is above the upper limestone rock. The prairie in the valley of the Zumbaro, in Wabashaw Co., Min., is below the upper limestone rock. This valley is several miles in width. The soil when wet resembles black city mud. The earth is composed of much lime mixed with other drift. It is based on magnesias limestone. The few trees have large roots and small stems. The bluffs have full size trees in abundance.

I infer, that the rank vegetation on these prairies, when fired, destroys the trees, while that on the bluffs only prunes them, as there is not sufficient for destruction.

Thus, the early German settlers in the Lebanon valley, Pa., chose the gravel land on account of the timber, because in their fatherland, timber was very valuable. The limestone land adjoining and running parallel, was a treeless prairie. The relative fertility may be judged. by subsequent prices. The gravel land some years since sold for about $35, while the limestone land sold for $100 to $120 per acre.

Thus, also, about forty years since, an extensive white cedar woods on the Newark meadows was destroyed by fire, and now there remains nothing but a treeless prairie.

Hence I infer that the absence of trees in these oases indicates excessive fertility. But we must not thence infer that "prairie," although French for meadow, is synonymous with fertility; for as used at the west, it simply signifies "treeless,' and this at times arises from want of fertility. Thus on the railroad route from Madison to Prairie du Chien, there is a sand prairie of small extent, where the superincumbent limestone appears to have been washed away, leaving the substratum of sand exposed, in places, without mixture with fertilizing ingredients; and the vegetation is not greater than near the sea-shore. So, also, in the fertile rolling prairies, small hillocks are found with scanty vegetation.

It is also stated that there are large districts in the neighborhood of the Rocky Mountains, that are without trees, and almost without vegetation, in consequence of the excess of alkaline salts.

But in some of the level prairies, such as those surrounding Chicago, the absence of trees is probably due to fire, since there are some trees that will grow almost in the water, and I judge that these prairies are less wet than the cedar swamps on the Newark meadows, and other cases of beech wood.

[Any facts pertaining to the prairies can not fail to be interesting. B. A. has examined them geologically, and we are obliged to him for some of his valuable reminiscences. We attach much value to them and should be glad to have more of them. - Ed].