The Elements of Geology; Adapted to the Use of Schools and Colleges Part 3

D : the Devonian { { } : { { Portage Group. } No. 9. } Waver : System. { { } } { { {Chemung Group. } } Sandstone.

This name, _Silurian_, was first used to designate the lowest well-characterized fossiliferous rocks in England. But it is now used to embrace the whole system as it occurs elsewhere. It is well exhibited in New York, both in consequence of its great development there, and because the whole system is only slightly acted upon by disturbing forces, so that the outcropping edge of each division extends over a large surface.

This system is of great thickness, amounting, in places where it is well developed, to twenty thousand feet.

The Champlain division commences with a quartzose sandstone, pa.s.sing gradually into limestone, which is succeeded by a very thick argillaceous deposit, the Utica slate and Hudson River group. The Ontario division in the lower part is a ma.s.s of sandstone. Above this is the Clinton group, consisting of shales and sandstones. The most important part of this group, in an economical point of view, is a fossiliferous, argillaceous iron ore, coextensive with the group in this country, and is worked to supply a large number of furnaces. The last of the division is the Niagara group, which commences with a ma.s.s of shale, and becoming at length calcareous, it terminates in a firm compact limestone. This limestone has withstood the action of denuding causes better than the shales either above or below it. It therefore presents a bold escarpment at its outcrop, and occasions waterfalls wherever streams of water cross it. The falls of Niagara are formed by this rock.

The Niagara limestone, in its extension westward, becomes the lead-bearing rock of Missouri, Iowa and Wisconsin. The Helderberg division is a succession of highly fossiliferous limestones, with the intervention of only occasional beds of grits and shales. One member of the series is the Onondaga Salt group. The water obtained from this group in New York annually furnishes immense quant.i.ties of salt. The Erie division consists of a thick ma.s.s of shales and sandstones.

[Ill.u.s.tration: Fig. 8.]

[Ill.u.s.tration: Fig. 9.]

[Ill.u.s.tration: Fig. 10.]

The fossils of this system are very numerous, but consist mostly of the lower forms of animal life. _Corals_ (Figs. 8 and 9) are abundant, and const.i.tute in some places a large proportion of the limestones. The _Crinoidea_, or lily-shaped animals, consist of a jointed stem permanently attached, and bearing at the free extremity of the stem an expanded portion, which is the pelvis, or digestive cavity. The mouth is surrounded with a series of leaf-like tentacula, which serve the purpose of seizing and holding food. Fig. 10 represents the pelvis of one of the silurian fossils. The general character of the animal is better represented by Fig. 30. The most abundant fossils of this period are the lowest orders of _bivalve mollusca_ (Fig. 11). The _Cephalopoda_ are characterized by having the organs of locomotion attached to the head.

The sh.e.l.l 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 sh.e.l.ls 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 circ.u.mstance 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.

[Ill.u.s.tration: Fig. 11.]

[Ill.u.s.tration: Fig. 12.]

[Ill.u.s.tration: Fig. 13.]

[Ill.u.s.tration: Fig. 14.]

[Ill.u.s.tration: Fig. 15.]

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 circ.u.mstance 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 ma.s.s of red sandstone, often pa.s.sing into a coa.r.s.e _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.

[Ill.u.s.tration: Fig. 16.]

The fossils of this system are a few sh.e.l.ls, a small number of vegetable species, and in particular localities the remains of fishes in great abundance. The system is characterized princ.i.p.ally 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 quant.i.ty, and from this circ.u.mstance has been called _metalliferous limestone_. In England it is the princ.i.p.al repository of these ores. In the Western States it is the upper portion of the lead-bearing strata.

[Ill.u.s.tration: Fig. 17.]

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 Leptaena, 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 coa.r.s.e-grained, and often pa.s.ses 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.

[Ill.u.s.tration: Fig. 18.]

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 of Europe.

There are three princ.i.p.al varieties of coal, distinguished by the different proportions of bitumen which they contain. The common bituminous coal kindles readily, emits much smoke, and throws out so much liquid bitumen that the whole soon cakes into a solid ma.s.s. It contains about forty per cent, of bitumen. The second kind, or cannel coal, contains twenty per cent., and inflames easily, but does not agglutinate. The stone-coal, or anthracite, contains scarcely any bitumen, ignites with difficulty, emits but little smoke, and produces a very intense heat. The bituminous varieties are always found in the least disturbed portions of the coal districts; and the anthracite is found in the more broken and convulsed portions, where we may suppose that the subterranean heat has been sufficient to drive off the volatile bituminous part, and reduce it to the anthracite form. Hence the eastern Pennsylvania coal-fields, which lie near the princ.i.p.al axes of elevation of the Appalachian Mountains, furnish only anthracite; while the same coal-seams, in their extension to the western part of the state, are bituminous.

Where coal is quarried in large quant.i.ty, a shaft is sunk through the overlying strata to the coal-beds, and the coal is raised to 'the surface by steam power. After the coal has been quarried to some distance from the shaft, pillars of unquarried coal are left to support the overlying strata. Fatal accidents have sometimes occurred by the giving away of these supports. Over a large part of the coal-fields of the United States it has not yet become necessary to sink shafts. The quarrying is commenced at the outcrop of the coal-bed; and, till the cover becomes of considerable thickness, it has been found economical to "strip" off the overlying rock, rather than to work a subterranean gallery.

Brine-springs are often found in the coal measures of sufficient strength to be used in the manufacture of salt. This is now done to considerable extent in Ohio. In the valley of the Kenhawa river, Kentucky, the rocks of which belong to the carboniferous system, the brine is nearly saturated with salt; and in some of the borings they have even discovered beds of rock-salt of great thickness and purity.

There is no other part of the geological series so obviously connected with national prosperity as the coal formation. While a country is new, the forests furnish an abundant supply of fuel; but in the course of a few years these are consumed. This country will soon be princ.i.p.ally dependent upon its coal-mines for fuel, even for domestic purposes; and, in carrying on the great branches of national industry, such as the smelting and working of iron, and in the formation of steam for the purposes of manufacture and transportation, we are already mainly dependent upon mineral coal. A nation which does not possess an abundant supply of this mineral, _or which does not use it_, cannot long maintain a high degree of national prosperity.

In these inexhaustible ma.s.ses of coal, acc.u.mulated ages before the existence of the human race, is a most obvious prospective arrangement for securing our happiness and improvement. And this arrangement embraces not only the acc.u.mulation of a combustible material in such abundance, but also its juxtaposition with an equally inexhaustible acc.u.mulation of iron ore, and the limestone which is necessary as a flux in the reduction of the ore. So bulky and heavy materials as coal and iron ore could neither of them have been transported to any considerable distance for the manufacture of iron; and without the manufacture of iron on a large scale, the present operations in manufactures and transportation could never have been entered upon. A large proportion of the iron furnaces in this country, and nearly all of them in Great Britain, employ mineral coal for fuel, and obtain their ore from the beds contained in the coal measures.

The fossils of the coal measures are almost entirely of vegetable origin, and are very abundant. They are seldom found in the coal-beds, but in the strata of shale immediately above or below the solid coal.

[Ill.u.s.tration: Fig. 19.]

The _Stigmaria_ (Fig. 19) is found most abundantly, and in a large proportion of cases to the exclusion of every other form, in the lower shales. It consisted of a large dome-shaped ma.s.s, often three or four feet in diameter, with trailing branches, or roots, spreading off horizontally to a distance of twenty feet. In a few instances tree ferns have been found, petrified in a horizontal position, and being apparently a mere continuation of the stigmaria. Hence the stigmaria has been supposed to be the base of the tall tree ferns, the leaves of which so abound in the upper shales. If this is not the case, there are no forms of the existing flora of the earth a.n.a.logous to the stigmaria. It is always found in connection with the coal-beds of the carboniferous formation, and never with the coal-beds which sometimes occur in the later formations.

[Ill.u.s.tration: Fig. 20.]

[Ill.u.s.tration: Fig. 21.]

The tree ferns (Fig. 20) attained a height of fifty or sixty feet, and a diameter of four feet. They have received the name of _Sigillaria_ in consequence of the seal-like impressions (Fig. 21) with which the surface is covered, and which are the scars left where the fronds have fallen off. These fronds (fern leaves) are the most abundant fossil of the series. They are distinguished by some peculiarity in form, as the Sphenopteris (wedge-shaped fern leaf), Pachypteris (thick fern leaf), &c. (Figs. 22 and 23.)

[Ill.u.s.tration: Fig. 22.]

[Ill.u.s.tration: Fig. 23.]

[Ill.u.s.tration: Fig. 24.]

[Ill.u.s.tration: Fig. 25.]

There was another kind of Sigillaria (Fig. 24), in which the surface was fluted, and the markings are superficial, and occur on the ridges. It reached as great a size as the tree ferns, but to what general cla.s.s of plants it belonged is still doubtful.

The _Lepidodendron_ (scale-covered tree) (Fig. 25) is the fossil which most nearly resembled in general appearance our present forest trees.

Specimens are found four feet in diameter and seventy feet in height. In botanical characters it resembled, in some respects, the trailing club-mosses, while in others it was very similar to the Norfolk Island pine.

[Ill.u.s.tration: Fig. 26.]

The _Calamite_ (Fig. 26) was a plant resembling, in its jointed and striated surface, the equisetum (rush), but was sometimes twelve inches in diameter.

The carboniferous formation exists more or less abundantly in all the great divisions of the earth. It occurs in nearly all of the countries of Europe. The largest deposits known are, however, in the United States; especially in the States of Pennsylvania and Virginia, and in Ohio.

4. _The New Red Sandstone._--The lower division of this formation, called the Permian system, consists of a thick ma.s.s of sandstones, generally of a red color, with occasional alternations of argillaceous rock, succeeded by a series of magnesian limestones. The upper division, or Tria.s.sic system, is composed of a red conglomerate, a limestone which has received the name of Muschelkalk (sh.e.l.ly limestone), and a series of variegated marls and sandstones.