But the contraction of the interior mass of the earth still continues, and the cavities left by the expulsion of volcanic matters, and the pushing-up of the mountains, may not all have been compensated for by the sinking-in of the strata, so it remains a matter of doubt whether the crust of the earth is sufficiently strong to bear the pressure of its own and the air's gravity, or whether at some future day another contraction will break it up and destroy the whole existing order of things. The shocks of earthquakes are strictly analogous to such catastrophes, but on an immensely inferior scale.
FIG. 15.—LIMESTONE MADE UP OF ENCRINITE.
FIG. 16.—LIMESTONE MADE UP OF CORALS (Favosites polymorpha).
The effects of the last great contraction having subsided the surface of the earth assumed a new arrangement of its matter, and the order of things which caused the deposition of the secondary strata commenced. Sedimentary sandstones were deposited from the washings of the surface by the waters which had not yet absorbed all the superfluous carbonic acid of the air, but continued to do so, and as fast as it became absorbed and saturated with the calcareous matter of the soil, swarms of coral-polypi, which were then formed, began to build their habitations in the warm waters of the ocean and rob them of the carbonate of lime which they contained, leaving them capable of absorbing more from the atmosphere. These little creatures formed immense masses of coral, and together with the "Encrinites" (having stony frameworks, figs. 15 and 16), were the chief artificers of that age, and well did they show what perseverance was able to accomplish, for through the thousands of years of quiet which now succeeded, they must have filled up nearly all the existing oceans with their structures, causing the water gradually to flow from its former beds into other situations, and thus from time to time was changed the position of both sea and land. The strata of mountain limestone which now exist are the remains of what these little polypi then produced, for there is scarcely a portion of this kind of rock, but shows evidences of having been, once, coral, madrapore, or some analogous structure (fig. 17). This mountain limestone sometimes occurs thousands of feet thick, and extends over vast districts both in Europe and America.
FIG. 17.—ENCRINITIC LIMESTONE.
During this period, although there were no great disturbances, capable of displacing the strata, yet there appears to have been frequent small eruptions of volcanic matter through the crevices of the lower strata, filling up all their cracks and vacuities, and in many cases rising to the surface, overflowing with basalt, toadstone, and other volcanic formations; in other cases rising in enormous quantities through openings at the bottom of the seas, partly filling these up, driving off the waters, and thus covering many of the coral formations with coatings of sand to a great thickness, raising up others into considerable elevations, and forming hills and fresh tracts of earth.
