The magnificent formations of anthracite in Pennsylvania could not have been created in their present form. No deposit could have taken place on the perpendicular strata which accompany it, in many places, or on the antielinals which are folded back on the edges of the basins.
The basins in which the original deposit took place must have been of comparatively gentle undulation,—deeper, perhaps, and of steeper angles of dip than the great bituminous basins of the West, but having no com parison to the present inclinations.
It has been stated and demonstrated in the previous pages that the weakest portions of the earth's crust are in the vicinity of active volcanoes; but in the present instance circumstances existed to make this vicinity still more susceptible to the influences of internal violence, and the irresistible forces of condensation and contraction going on in the crust of the earth.
Of all mechanical powers, there are none so tremendous and irresistible as the two we have given in illustration of the natural processes which tend to change and modify the physical geology or the lithological structure of the earth,—that is, VACUUM and CONTRACTION.
The above illustration represents the action of lateral contraction. The amount of contraction in the earth's crust is collectively very great, but the conformation of the Appalachian strata was such as to concentrate its contraction to a single group of formations, along its weakest line of crust, which passed in the vicinity of the anthracite coal-basins.
From the centre of the Appalachian formation to the eastern escarpment of the Alleghany, the formation is very thick, and generally on a plane slightly varying from the horizon. The flexibility of this level mass of strata, which was, and is, perhaps, from 20 to 50 miles thick, could not have been in proportion to the already flexed and weakened basins of the East; consequently, the contraction or condensation towards the centre of the earth must have been exerted laterally and against the weak and already partially folded strata of the eastern basins. It must be understood, in
this connection, that we are not speaking of the local contraction of the surface strata, because they had already reached their maximum of con densation. It is now the contraction of the interior crust, or the con densation of the liquid mass of the earth, that we are treating of.
We think this illustration will sufficiently demonstrate the causes and processes which effected the violent contortions and steep angles of our eastern basins. The action was gradual, as it was irresistible, and therefore produced no great rents of fissures in the strata, but exerted its crushing influence through a vast space, and left the evidences in innumerable slides, cracks, and cleavages throughout the strata affected.* We have now reached a point where we can satisfactorily contemplate the natural processes by which our magnificent fields of coal were pro duced, having, to our own mind, clearly demonstrated the facts or the hypothesis we set forth to prove.
Nature had prepared for the closing glory of her Appalachian monument. The great Appalachian Sea had been contracted to less than one-half its original superficial area, and its present shallow depth bore no compari son to its former unfathomable abyss. Its shores were now quiet, low, and clothed in verdure. Long receding shores, without cliff or mountain, stretched gently away towards the ancient coasts, from which now came vast rivers, rolling in the mud and drift of the soft and imperfect strata which had never been condensed by pressure and not yet hardened by exposure and time; and, thus prepared, we leave the formation and origin of coal for the following chapter.