The bed of Erie will then form an extensive plain or valley, bounded by the distant hills, and watered by a small lake or river, which will give passage to the St. Clair and Huron, and form a prolonged channel to the river Detroit. Here the geologists of future pe riods will find a fresh water formation in successive strata upon the limestone bed. These strata will proba bly be a coarse sandstone with argillaceous marl, con taining fresh water shells; among others, some of the Uniones, so well described by Mr. Barnes. These will be sedimentary fresh water formations, produced almost entirely by mechanical means, i. e. the depo sition of earthy matters, coarse or fine, enveloping or ganized bodies. They may have a different structure from other fresh water formations. The layers may be distinct and numerous, with a coarse sandy grain, having the usual perforations to manifest the extrica tion of gas from the limestone beneath. It may be similar to the fresh water formations of Paris and Rome; or may resemble the molasse of Switzer land.
But we need not look either into ancient records, or into futurity, to know that both salt and fresh water lakes have covered much of the earth; and that they have, and do, and, from analogy, will form deposits of soils and minerals. Salt lakes still exist in many places. The tout pans, in the south of Africa, are salt lakes furnishing that country with salt. Some of them are more famous than others; but all arc situate on a plain, at a considerable elevation above the sea, none being less than one hundred feet above it. A brief account of one will suffice for the rest. The greatest part of the bottom of the lake is covered with one continued body of salt, like a sheet of ice, the crystals so united as to form one solid body as hard as rock. The shore is similar to the sandy beach of the sea coast, covered with .sandstone and quartzose pebbles. At this beach begins a thin crust of salt, increasing in thickness and solidity, as it advances to the middle of the lake. Near its margin, where it is four or five inches thick, the salt is taken out with pick-axes, and is fit for use. The thickness of this bed at the middle has never been ascertained, as the waters do not subside. In endea vouring to account for the accumulation of pure crys tallized salt at the bottom of this lake, it might be considered an explanation sufficiently satisfactory to say, the waters on the south coast of Africa contain a high proportion of salt. During the strong south-east winds of summer, the sea spray is carried a great dis tance into the country, in the shape of thick mist. The powerful and combined effects of the dry wind and sun carry on a rapid evaporation of the aqueous part of the mist, and, of course, a disengagement of the saline particles, which fall on the ground and the foliage of the shrubbery. When the rains commence, they are
dissolved and carried in solution to the salt pan, to wards which the country on every side inclines.
The quantity of salt thus taken from the sea, and borne into the country, is so very great, that at the distance of many miles from the coast, the air is per ceptibly saline when walking against it. The atmos phere is obscure, and objects at a short distance are not seen. These winds last for nearly two-thirds of the whole year, and it is easy to conceive, that in the lapse of ages an immense accumulation of salt can thus be formed. This lake is in red sandstone, and the salt is in some places tinged with the red colour of the oxide of iron.
In Mexico, the salt lake of Pennon Blanco, already noticed, yields annually 250,000 fanegas of unpurified salt of 400 lbs. each, making an aggregate of about 1,7.85,714 bushels.
Turks Islands are celebrated for salt ponds, which in some years have yielded more than 30,000 tons of salt for exportation.
The occurrence of rock salt deep under the surface of the earth, or high above the level of the sea, forms no objection to its being a deposit from water, since all geologists allow, and undoubted facts prove, that the ocean once covered all the continents now known. Whether the earth has been elevated above the sea, or the sea depressed beneath the level of the earth, the valleys must have been filled with salt water, which, upon evaporation, deposited salt. At Cardona, and other places, it seems to have been deposited in the red sandstone, or rather to be enveloped by it. Some of these valleys occur at great elevation: thus the one in which is deposited the salt mine of Tyrol is 5,000 feet above the level of the sea. In the deserts of Peru is one 10,000 feet, according to Ulloa, above the sea. Others again are at various depths beneath the surface; thus, one in England is 735 feet deep, being 420 feet beneath the level of the ocean.
The difficulty which has been supposed to exist in accounting for the formation of strata under which rock salt is found, is in a great measure obviated by the organic remains found in them; proving that each stratum was once the uppermost and last formed on the globe, and was in turn covered by others at differ ent and distant periods. In the same way there are many strata occasionally covering coal and beds of shale, abounding in vegetable impressions.