THE GREAT LIMESTONE.
Its limits follow closely the ancient boundaries of the sea, as formerly set forth, but is encircled by the rim of metamorphic strata, which was the production of volcanic agencies to the period of the lime. We may note the present outcrops of this great ocean-bed. Starting from the southern portion of Alabama, near the confluence of the Coosa with the Alabama River, its eastern margin lies along the great valley region described in Chapter II.,—forming one of the most beautiful and productive regions of the continent,—through all the Atlantic States to Nova Scotia. Its course is then through the Laurentian basins west to Montreal, and thence around the north shores of Lake Ontario and the Georgian Bay of Lake Huron; and still further west, sweeping the north and west sides of Lake Michi gan, through Green Bay and Winnebago Lake and Wisconsin, until it reaches the Mississippi. It passes through the northern portions of Iowa, and thence in an indefinite course to the Rocky Mountains,—along their eastern slopes to the northern borders of Texas, through which State it circles back towards the Mississippi River. But its southern limits are not well defined, nor can we so well determine the ancient limits of the Appad lachian Sea towards the south, whether it included the Gulf and was isolated from the Atlantic by a continuation of the granite boundary through Florida and Cuba to the Southern Continent, or was on a level and in confluence with the Atlantic.
We may safely assume that the area of this Great Limestone is not less than 1,500,000 square miles, and as safely compute its average thickness at not less than 1000 feet,—its maximum thickness being nearly 5000 feet, while its minimum we have never seen less than 1000 feet.
This entire mass contains a vast store of carbonic acid; its maximum proportion to the whole being about 40 per cent. and its minimum 20 per cent., giving an average of about 30 per cent. carbonic acid, one-half of which, or 15 per cent., is carbon; or one square mile of this limestone, at an average of 1000 feet thick, contains as much carbon as the general production of our anthracite coal-fields to the square mile.
Carbonic acid is liquefied under a pressure of 40 atmospheres, and becomes solid by evaporation.
The vapor of carbon mixing with the hydrogen of water at high tem peratures forms carburetted hydrogen, coal-oil, petroleum, naphtha, &c.
Since it has been determined that our limestone rocks have not been formed by the sedimentary deposits of solid matter, as the accompanying sandstones and slates are formed, it may be an interesting question to determine how they were formed. There appear to be only two theories available: one providing for its creation by the industrious labors of myriads of marine insects, and the other assuming that the waters con taining carbonic acid, holding the carbonates of lime and magnesia in solution, had precipitated those salts at a boiling-heat. The latter theory seems plausible, since the vapors of carbon must have been in excess in the air and in the waters. Until this period there could have been no possi bility of its return to the earth in large quantities by condensation, pre cipitation, or through animal or vegetable life. The heat that vaporized
it still prevented its condensation: therefore, both the atmosphere and the waters must have been surcharged with carbonic acid, which, during the first cessation of volcanic heat and violence, returned to the earth in the shape of lime. Such is the natural process; and such, we may observe, must have been the cause and result during the formation of the second or great carboniferous limestone.
We may not reiterate here the successive formations of the Paheozoic strata, as described in Chapter II., but we must notice the fact of a frequent recurrence of volcanic action and violence from the first to the second great limestones. It was intermittent, however, and more or less violent through a long series of formations of sandstones, slates, shales, and lime stones. In those formations we find the carbon preserved in available form in the shape of carbonic acid in lime, and hydrocarbon in petroleum. From the first great limestone to the base of the coal measures, more or less of the hydrocarbons was preserved in the limestones and bituminous shales. But it must be remarked, in explanation of its absence in certain locations, the hydrocarbons do not exist in formations which admit of their escape in gas; nor were they formed at the period of those precipitates in which they are found, but were the production of subterranean gases since.
We may now pass over the subsequent periods the formation of the great limestone to the immense sand-rocks of the "old red" and the subcarboniferous, which, again, must have been produced by heat and violence, followed, as before, by quiet and a reduction of temperature; and the result, as before, was the production of lime.
There is a singular and unaccountable circumstance connected with the carboniferous lime which claims our attention. In the east and northeast of the Alleghany basin, the subcarboniferous strata are the well-known red shales of our anthracite coal-fields, but to the west it is the carboniferous limestone. The one gradually takes the place of the other; the lime thinning to the east and enlarging to the west, while the other thins to the west and increases to the east.
Prof. Rogers accounts for this change in the assumption that the lime was peculiarly a marine production, and came in from the southwest; while the Red Shale, or "Umbra'," was a shore or land production, and came from the east. This seems plausible; but we cannot imagine why the same rule did not work during the formation of the first or Auroral limestone, which is, perhaps, thicker on the east than elsewhere! There can be no doubt but the amount of lime in the red shale is equal to the average deposit where the red shale commences to thin, or even where it does not exist; that is, the amount of lime in the red shales of the east is equal to the lime formations of the west under equal areas.
However we may account for the change from shale to limestone, or vice versd, we know that both must have been formed in a quiet sea and during a season of comparatively low temperature, since the greater por tion of the shale is a soft, red mud of almost impalpable fineness, and has been evenly deposited in the most conformable manner as an aqueous sediment.