THE CONDITIONS NECESSARY TO THE EXISTENCE OF COAL.
We will briefly notice here the undeveloped coal regions reported, and give such information regarding them as we may find available. Outside of the countries enumerated above, but little is known of the coal forma tions of the world, though it is probable that vast coal regions exist in Brazil, China, Hindostan, Africa, and Australia. We may state, however, positively, that no portion of the globe is so rich in coal as North America, or, more definitely, the United States. In no other instance do we find the Palmozoie strata so perfect and extensive, or which bear any comparison to the great Palaeozoic coal formations of the ancient Appalachian basins.
The great Carboniferous era was a fixed period of time in the early geological history of the earth. It was the acme of vegetation, which owed its superior growth and magnificence to the favorable conditions that then existed ; to the constant, unchanging tropical temperature, the genial moisture, and the superabundance of carbonic acid which then gave life and vigor to the ancient flora.
But, as we have pointed out in the foregoing chapter, it was not only necessary that the atmospheric conditions should be thus favorable, but the physical condition was of equal importance. Water in shallow seas or lakes, a soft and yielding soil, and a general basin-shape were all prime necessities; and such are the conditions required to produce coal, according to the former vegetation theory.
But if we have clearly expressed the nature and requirements of coal formations in the preceding chapters, it will appear evident that the above conditions are not of themselves sufficient to produce the mineral coal of our true Carboniferous era ; and consequently the coals of that era are con fined to certain lithological strata, generally represented by the great Carbo niferous limestone and the millstone grit, on which the true coals are invariably found, except in cases of denudation or subsidence. The ex ceptions to this general law of nature are but few ; in fact, no great and extensive beds of true coal are found in any other connection. The Carbo
niferous era closed the Palozoic day, and crowned the Paleozoic column.
The simple reason is, certain combinations are required—heat, moisture, carbonic acid—to produce vegetation; a lithological structure necessary to retain water in basins ; internal heat operating on limestones, or carbo nated rocks, to produce, in connection with water, the hydro-carbons or bitumen of our coal formations.
When coal is found under other circumstances, it is always imperfect, unreliable, and limited, deriving its carbon oils or bitumen direct from volcanic sources, or, to a limited extent, from the same causes operating to form the true coal, as the Permian coal immediately above the Carboniferous has been formed.
We have thus stated briefly the reasons why coal may not be found in all countries, since the Carboniferous era existed through a comparative lengthy period of time, and seems to have flourished cotemporaneously in all parts of the earth; and we might expect to find the conditions, as set forth in the two first propositions, viz.: vegetation and basins, of water in many portions of the world where coal does not exist. We therefore cannot expect to find extensive fields of coal, or any true coal of the Car boniferous period, where all the before-mentioned conditions do not exist.
We do not expect to find great deposits of the true coal west of the Rocky Mountains, in Mexico, Central America, or the mountainous regions of the Southern Continent, or even north of the great lakes. But coal may, and does, exist in all the regions named, as it exists in the same cha racter of rocks, and, perhaps, under the same conditions of formation in small basins of imperfect form along the granitic slopes of the Atlantic, in Massachusetts, Rhode Island, Virginia, and North Carolina; or still more recent and more imperfect deposits of Tertiary coal and lignites may exist in extensive fields, as those which occupy so large an area around the Rocky Mountains.