The Elements of Geology; Adapted to the Use of Schools and Colleges. Justin R. Loomis. Читать онлайн. Newlib. NEWLIB.NET

Автор: Justin R. Loomis
Издательство: Bookwire
Серия:
Жанр произведения: Языкознание
Год издания: 0
isbn: 4057664561756
Скачать книгу
11). The Cephalopoda are characterized by having the organs of locomotion attached to the head. The shell of several species is peculiar in being divided into distinct cells, or chambers (Fig. 12, b d), perforated by a tube (siphuncle a). These fossil shells are sometimes straight, as the Orthoceras (Fig. 13), or curved, as shown in the several forms of Fig. 14. The Trilobite was an articulated, crustaceous animal, having two lines along the back dividing it into three lobes, from which circumstance its name is derived. It is found in great numbers in the Silurian rocks (Fig. 15). In a few instances remains of fishes have been found, but they by no means characterize the system.

      

      The geographical range of this system is probably greater than that of any system of rocks above it. It is found occupying a large part of the territory west of the Alleghany Mountains, from Canada, through New York, and the other states, to Alabama; and extending westward to and beyond the Mississippi river. It occupies a large district in the west of England, and is found in great force in the north and east of Europe.

      2. The Old Red Sandstone.—This formation consists almost entirely of a sandstone of a red color. It admits of division into three parts, though the characters vary in different places. The lowest is a thin-bedded argillaceous sandstone, consisting of finely levigated material, and easily splitting into thin sheets. From this circumstance it has received the name of tilestone. The middle portion is composed of nodules or concretions of limestone imbedded in a paste of red sand and shale. This has been called by English geologists, cornstone, and though very partially developed in some regions where the system is found, it is yet a very persistent member. The highest member of this formation is a mass of red sandstone, often passing into a coarse conglomerate. In England the thickness of the Old Red Sandstone is not less than ten thousand feet. In this country it is scarcely three thousand feet.

      

      Fig. 16.

      The fossils of this system are a few shells, a small number of vegetable species, and in particular localities the remains of fishes in great abundance. The system is characterized principally by fossils of this last kind. The fishes of this system have a cartilaginous skeleton, but are covered with plates of bone, which were faced externally with enamel. The jaws, which consisted of solid bone, were not covered with integument. The exterior bony covering seems to have been the true skeleton, as is, in part, the case with the tortoise. In some of the fishes of this period there is a wing-like expansion on each side of the neck, which has given them the name of Pterychthis (Fig. 17). In others, as the Cephalaspis, the plate of bone on the back is so large as to cover nearly the whole body, and make it resemble a trilobite (Fig. 18).

      This system has an extensive geographical range. In England, it occupies a band of several miles in width, extending from the Welsh border northward through Scotland to the Orkney Islands. In this country, it forms the Catskill Mountains, in New York, and extends south and west so as to underlie the coal-fields of Pennsylvania and Virginia.

      3. The Carboniferous System.—This system consists of three parts, distinguished by lithological and fossil characters.

      The carboniferous limestone is a dark-colored, compact limestone, forming the base of the system, and reposing on the old red sandstone. Its thickness is from six hundred to one thousand feet, often with scarcely any intermixture of other rock; but it sometimes loses its character of a limestone, and becomes a sandstone, or conglomerate. It generally contains the ores of lead in considerable quantity, and from this circumstance has been called metalliferous limestone. In England it is the principal repository of these ores. In the Western States it is the upper portion of the lead-bearing strata.

      The fossils are marine, and very numerous. Corals and crinoidea are very abundant. The crinoidea, in some localities, form so large a part of the rock as to have given to it the name of encrinal limestone. The orthoceras and trilobite are found, but become extinct with this formation. Several species of bivalves, such as Delthyris and Leptæna, are also common.

      Next above the limestone lies the sandstone, sometimes called millstone grit. It is generally drab-colored, but occasionally red. Its thickness is often equal to that of the limestone. Sometimes it is fine-grained and compact; but generally it is coarse-grained, and often passes into a conglomerate. It contains but few fossils, and those of vegetable origin.

      The highest part of the system is the coal measures. They consist of beds of sandstone, limestone, shale, clay, ironstone and coal, occurring without much uniformity in their order of superposition. The coal measures have a thickness of about three thousand feet. The sandstones and limestones are not distinguishable from the sandstones and limestones in the lower part of the system. The ironstone either occurs in concretionary nodules, often formed around some organic nucleus, or it is an argillaceous ore, having a slaty structure. In either case, it consists of subordinate beds in the shale. The coal consists of several beds distributed through the measures. The beds vary in thickness from a few lines or inches to several feet. In a few cases beds have been found measuring fifty or sixty feet in thickness. The workable beds are ordinarily from three to six feet thick.

      The carboniferous formation is very much disturbed by dikes, faults (Fig. 18; see also Fig. 50), and other dislocations. The amount of change of position in the strata, by faults, is very various; frequently but a few feet. In one case in England there is a fault of nearly a thousand feet. There is a case of dislocation in Belgium where the strata are bent into the form of the letter Z, so that a perpendicular shaft would cut through the same bed of coal several times.

      The characters and order of superposition which have now been given may be regarded as the general type of the carboniferous formation. There are, however, several important modifications. 1. Beds of coal sometimes alternate with beds of millstone grit. Thus, in Scotland and in the north of England, this intermediate member of the system disappears, or, rather, is incorporated with the coal measures. The same is true, to considerable extent, in this country. 2. Sometimes the carboniferous limestone also disappears as a distinct member of the system, partly by becoming arenaceous, and partly by the intercalation of beds of coal. In this last case, the whole formation from the old to the new red sandstone becomes a series of coal measures. In this country the carboniferous limestone is found very generally to underlie the coal strata. 3. The fractures and faults, which were formerly supposed to be characteristic of the coal formation, are seldom found in the great coal-fields of this country, except in those of the anthracite coal of Pennsylvania; and even there they are much less common than in the coal-fields