« AnteriorContinuar »
underlying portions of the states of New York, Pennsylvania, Ohio, Indiana, Illinois, Missouri, and the territory of Arkansas on that line. In Tennessee, Kentucky, Virginia, and Maryland, it is bounded by a line of which the Cumberland Mountains form a part. In the plains through which the Mississippi flows, and which include the Illinois prairies, it appears like a continuous floor, forming an almost unvarying flat."—Feathersionhaugffs Geological Report, 1835, pp. 27, 28.
Of the aggregate of the several layers in the carboniferous group, the following section of the upper coal series in Western Virginia may be taken as an example:
Making a total thickness of limestone in this group along the line of section of fifty feet; adding to these twenty-four in the lower shale and sandstone group, and three in the lower coal group, and we have in the whole extent of the
coal measures embraced in the section, a thickness of about seventy-seven feet of limestone."—Rodgers's Report on the Geology of Virginia, 1839, p. 93.
The inadequacy of their theory to account for this important portion of the strata, though seen and acknowledged by geologists, has not led them either to abandon or modify it. Some candidly confess themselves unable to give a satisfactory explanation of its origin; while Macculloch, Phillips, and some others, maintain, as the most probable hypothesis, that it was formed of the exuviae of testaceous animals, and was drawn originally by them from the waters of the sea. But that, besides being a mere conjecture and infinitely improbable, furnishes no indication of its original source; as it implies either that the lime was previously held in solution in the waters of the sea—which was impossible, as the quantity is such as would have thickened all the waters of the globe to a paste—or else that it was gradually introduced into them from some unknown source, which is no explanation whatever of its origin. Instead, therefore, of demonstrating their hypothesis that the whole of the materials of the strata were drawn from their fabled mountains of granite, by their own concession that large portion of them that consists of limestone was of a different derivation. Those vast formations, accordingly, interspersed through the whole mass of the strata, are so many monuments of the error of their theory.
Iron, also, which enters very largely into the composition of many of the strata, especially of the carboniferous groups, cannot have resulted from the decomposition of granite, but must have been altogether drawn from some other source. Besides, indeed, those rocks which imbed it in masses and derive from it their principal character, it exists in ordinary sandstones, and shales consisting mainly, like granite, of silica and alumine, in far greater portions than in that rock.
So, also—to say nothing of chalk—of the vast beds of salt. The nature of that mineral forbids the supposition that it can have resulted from the disintegration of granite; as there is no such element in its composition.
The theory thus fails to make any provision for the formation of at least one-third of the strata for which it professes to account by a scientific induction according to "the strictest rules of the Baconian philosophy." Can higher evidence be asked of its utter erroneousness? Yet its authors, though aware that it is thus incommensurate to the vast task which they assign to it, seem not to regard its failure on so immense a scale as a proof of its inaccuracy, or reason for its abandonment. They continue to make it the basis of their arguments for the vast age of the world, and treat the inference they found on it as a scientific induction.
Unfortunately, however, for the theory, this defect does not terminate at that point. It, in fact, fails as entirely to account for those strata of which silicious sand is the principal element, as for those which consist of limestone, iron, and salt; for though the main materials of sandstone are those of which granite consists, silica and alumine, yet the form in which they now exist demonstrates that they cannot have been derived from that rock. In granite those elements, with a slight mixture of potash, iron, and lime, are combined in three different proportions in crystals of quartz, felspar, and mica, or hornblende; but in sandstone they are not in the form of quartz, felspar, and hornblende, or mica crystals, as the first three would undoubtedly have been, at least in a chief degree, had they been drawn from granite. Nor are they crystallized; but instead, are, at least mainly, of a mere granular structure, or formed by an aggregation of particles by a law essentially unlike that of crystallization. The nature and importance of the distinction in structure and form that exists between them—the crystals of granite being geometric, though imperfect, but the particles of sandstone generally simple grains or comminuted mud—may be seen from the following passage:
"Quartz is crystallized in double six-sided pyramids in the substance of granitic, porphyritic, and other igneous rocks; in six-sided prisms terminated by six-sided pyramids in mineral veins and in cavities in granite; compact in veins; nodular in amygdaloidal traps; rolled masses in old red conglomerate millstone grit, and grauwacke; worn grains in sandstones, clays, certain quartz rocks, and coarse clay slates.
"Felspar; primary rhomboidal crystals in granite, porphyry, trachyte, and basalt; composite crystals in cavities of granite and veins; disturbed crystals in gneiss; rolled crystals in conglomerate; decomposed or porcelain clay in some granites and sandstones.
"Mica, crystallized in hexagonal plates in granite, porphyry, lava, and primary limestone; disturbed crystals in gneiss and mica schist;fragmentary scales in sandstone, sand, shale, and clay.
"Hornblende, crystallized with felspar in granite, greenstone, basalt, and lava, also in hornblende slate."—Phillips's Guide, p. 79.
In granite, quartz has thus a geometric shape, but in sandstone it is in the form of minute particles or grains. The Potsdam Calciferous and Medina sandstones, for example, of this State, generally exhibit no traces in any of their parts of a crystalline structure, but are formed by a mere aggregation of minute particles, and, on being broken, are easily reduced to the most attenuated granules or dust. They are nevertheless usually represented by geologists as drawn wholly from granite, and as owing their new shape