The Elements of Geology; Adapted to the Use of Schools and Colleges Part 2

[Ill.u.s.tration: Fig. 4.]

Granite is itself intersected with granite veins more frequently, perhaps, than any other rocks; but the vein is a coa.r.s.er granite than the rock which it divides. It is not uncommon to find one set of dikes intercepted and cut off by a second set, and the second by a third. The substance of the dikes was, of course, in a liquid state when it was injected, and the first must have become solid before the second was thrown in; hence the dikes are of different ages. The dikes _a b c_, represented in Fig. 4, must have been injected in the order in which they are lettered.

It is probable that, by the process of cooling, the liquid ma.s.s from which these dikes have proceeded has been gradually solidifying from the surface downwards. If so, it would follow that the granite nearest the surface (1, Fig. 2) is the oldest, and the newest is that which is at the greatest distance below (4). It is possible that at great depths granite may be still forming, that is, taking the solid form, though of this there can be no direct proof. There is, however, proof that it has been liquid at periods of time very distant from each other; for the dikes sometimes reach to the top of the coal formation (for example), and then spread themselves out horizontally, as at _a_, showing that the rock above the coal had not then been deposited. Another dike will extend through the new red sandstone, as at _b_, and spread itself out horizontally as before. These horizontal layers of granite, by their position in strata whose ages are known, indicate the periods when granite has existed in a liquid state. Granite veins have been discovered in the Pyrenees as recent as the close of the cretaceous period, and in the Andes they have been found among the tertiary rocks.

There are several other rocks, of minor importance, often found in connection with granite. Hypersthene rock, in a few cases, forms the princ.i.p.al part of mountain ma.s.ses. Greenstone is more frequently a.s.sociated with the trappean rocks, but it sometimes pa.s.ses imperceptibly into syenite and common granite. Limestone is found in considerable abundance, and serpentine in small quant.i.ties, as primary rocks, and have evidently been formed like granite, by solidifying from a state of fusion.

SECTION III.--THE VOLCANIC ROCKS.

The volcanic rocks consist of materials ejected from volcanoes. They are, however, ejected in very _different states_; sometimes as dust, sand, angular fragments of rock, cinders, &c., and sometimes as lava streams. In some instances, the lava has so little fluidity that it acc.u.mulates in a dome-shaped ma.s.s over the orifice of eruption, and perhaps in a few instances it has been thrust upward in a solid state.

There are _two princ.i.p.al varieties_ of lava, the trachytic, consisting mostly of felspar, and the basaltic, consisting of hornblende. When both kinds are products of the same eruption, the trachytic lava is thrown out first, and the basaltic last. The reason of this is, that felspar is lighter than hornblende, and probably rises to the surface of the lava ma.s.s at the volcanic focus, and the basaltic lava is therefore reserved till the trachytic has been thrown off.

These, like other rocks, have been produced at different epochs. There is, however, great difficulty in determining their age; There are some differences of structure and composition observed, in comparing the older and newer lavas; but the only method that can be relied on to determine their age is their relation to other rocks. When they occur between strata whose age is determined by imbedded fossils, they must be of intermediate age between the inferior and superior strata.

1. _Modern Volcanic Rocks._--Some of the volcanic rocks are of modern origin, and are produced by volcanoes now active. The total amount of these, and of all the other volcanic rocks, is probably less than that of either of the other princ.i.p.al divisions of rocks; yet they form no inconsiderable part of the earth's crust. The number of active volcanoes is not far from three hundred, and the number of eruptions annually is estimated at about twenty. In some cases, the lava consists of only a single stream, of but a few hundred yards in extent. It extends, however, not unfrequently twenty miles in length, and two or three hundred yards in breadth. The eruption of Mount Loa, on the island of Hawaii, in 1840, from the crater of Kilauea, covered an area of fifteen square miles to the depth of twelve feet; and another eruption of the same mountain, in 1843, covered an area of at least fifty square miles.

The eruption in Iceland, in 1783, continued in almost incessant activity for a year, and sent off two streams in opposite directions, which reached a distance of fifty miles in one case, and of forty in the other, with a width varying from three to fifteen miles, and with an average depth of more than a hundred feet. The size of some of the volcanic mountains will also a.s.sist in forming an idea of the amount of volcanic rocks. Monte Nuovo, near Naples, which is a mile and a half in circ.u.mference and four hundred and forty feet high, was thrown up in a single day. aetna, which is eleven thousand feet high, and eighty-seven miles in circ.u.mference at its base, has probably been produced wholly by its own eruptions. A large part of the chain of the Andes consists of volcanic rock, but the proportion we have not the means of estimating.

2. _Tertiary Lavas._--There is another cla.s.s of volcanic products, which are so situated with reference to the tertiary strata that they must be referred to that period. The princ.i.p.al localities of these lavas, so far as yet known, are Italy, Spain, Central France, Hungary, and Germany.

They are also found in South America. Those of Central France have been studied with the most care. They occur in several groups, but they were the seats of volcanic activity during the same epoch, and formed parts of one extensive volcanic region. Each of these minor areas, embracing a circle of twenty or thirty miles in diameter, is covered with hills two or three thousand feet in height, which are composed entirely of volcanic products, like the cone of aetna. On many of them there are perfectly-formed craters still remaining. Numerous streams of lava have flowed from these craters, some of which can now be traced, throughout their whole extent, with as much certainty as if they were eruptions of the present century. Some of the lavas have acc.u.mulated around the orifices of eruption, forming rounded, dome-shaped eminences. These lavas generally consist of trachyte, and have therefore a low specific gravity, and imperfect fluidity. The basaltic lavas have often spread out over broad areas, and, when they have been confined in valleys, have reached a distance of fifteen miles or more from their source. There still remain indications of a current of lava which was thirty miles long, six broad, and in a part of its course from four to six hundred feet deep. The above sketch (Fig. 5) will give some idea of the highly volcanic aspect which the district of Auvergne, in France, presents.

[Ill.u.s.tration: Fig. 5.]

The unimpaired state of some of the cones and craters, and of the lava currents, would lead to the impression that these regions have been the theatre of intense volcanic action within a very recent period. But there is good reason to believe that this has not been the case. "The high antiquity of the most modern of these volcanoes is indeed sufficiently obvious. Had any of them been in a state of activity in the age of Julius Caesar, that general, who encamped upon the plains of Auvergne and laid siege to its princ.i.p.al city, could hardly have failed to notice them."

It is equally certain that the commencement of their activity was at a late period in the history of the earth. Lava currents are frequently found in France resting upon the early tertiary strata, but no lava current is found below them. The later tertiary strata contain pebbles of volcanic rocks, showing that lavas had been previously ejected, but none are found in the older strata of this formation. We must, therefore, conclude that these volcanic tracts a.s.sumed their volcanic character at some intermediate point in the tertiary period.

When we find that their activity commenced at so late a period and closed so long ago, we might be led to suppose that it was of very short duration. But a great number of facts, in the present condition of the country, require that we should a.s.sign to them a very prolonged activity. A single instance will be sufficient to show the nature of the evidence upon which this conclusion rests. The heavy line (Fig. 6) represents the present form of one of the valleys. A bed of lava forms the highest point of land represented, and a second bed is found in an intermediate part of the slope. The position of the upper bed must have been a valley, when the lava flowed there. We may represent this valley by the line _a b c_. The slow operation of natural denuding causes at length excavated the valley _d e h_, when another lava current flowed through it, covering its bed of pebbles, as before. The same denuding causes have at length produced the present valley, _f g h_. These remnants of lava-currents, as they have formed a very imperishable rock, have protected the subjacent strata from erosion, and furnish evidence of the position of the valley at different periods. When we consider with what extreme slowness denuding causes produce changes on the surface, and what extensive changes they have here nevertheless effected in the interval between the production of the different lava currents, we are compelled to feel that that interval was a very prolonged one.

Yet this period, however long it may have been, was evidently less than the period of activity of these volcanoes.

[Ill.u.s.tration: Fig. 6.]

3. _Volcanic rocks of an earlier date_ are also found, sometimes as distinct lavas, though generally as volcanic grits. They occur interstratified with the cretaceous rocks, and with every other formation of the fossiliferous series, showing that, from the earliest times, these rocks have been acc.u.mulating as they now are.

_The trappean rocks_ may, in a general cla.s.sification, be considered as volcanic. It will be shown, hereafter, that they are the lavas of submarine volcanoes. They do not, however, occur in the form of lava currents, but in great tabular ma.s.ses, generally between stratified rocks, or in the form of dikes. They are also entirely unconnected with cones or craters.

The trappean rocks occur more or less abundantly in all countries. One of the most noted localities of this rock is a region embracing the north of Ireland, and several of the islands on the western coast of Scotland. It contains the celebrated Giant's Causeway, which consists of a ma.s.s of columnar trap; also Fingal's Cave, which is produced by a portion of the trap being columnar, and thus disintegrating more rapidly than the rest, by the action of the waves. An immense ma.s.s of greenstone trap, which has generally been considered as a vast dike, though often a mile in thickness, is found extending from New Haven to Northampton, on the west side of the Connecticut river. It then crosses to the east side, and continues in a northerly direction to the Ma.s.sachusetts line.

Under different names, it const.i.tutes a nearly continuous and precipitous mountain range for about one hundred miles. Dr. Hitchc.o.c.k supposes this greenstone range to be, not an injected dike, but a tabular ma.s.s of ancient lava, which was spread out on the bed of the ocean during the period of the deposition of the Connecticut river sandstone. It was subsequently covered with a deposit of strata of great thickness, and then by subterranean forces thrown into its present inclined position.

There is a ma.s.s of basaltic rock in the valley of the Columbia river, in the Oregon Territory, which extends without interruption for a distance of four hundred miles. Its breadth and thickness is not known, but in some places the river has cut a channel in this rock to a depth of four hundred feet. Its age has not been determined, and it will, perhaps, be found to be a tertiary or modern production.

SECTION IV.--THE NON-FOSSILIFEROUS STRATIFIED (OR METAMORPHIC) ROCKS.

1. _Gneiss_ is the most abundant rock in this cla.s.s, and is generally found reposing on granite. Its stratification is sometimes very distinct, but it is often so imperfect that it can scarcely be recognized. This is more frequently the case in the vicinity of granite on which it rests, and into which it insensibly pa.s.ses. A large part of the material used for building purposes, under the name of granite, is obscurely marked gneiss. In all primary countries it is an abundant rock, occupying extensive districts, and sometimes forming mountain ma.s.ses.

2. _Mica slate_ lies next above gneiss, and is a very abundant rock. As it differs from gneiss only in the proportion of mica which it contains, and as the quant.i.ty of mica in it is very different in different places, it is often difficult to make the distinction between them. It also pa.s.ses by insensible degrees into the argillaceous rocks. Many of the argillaceous rocks are found, upon close examination, to contain mica in minute scales in such abundance as to make it doubtful whether they ought not to be regarded as mica slates; that is, the metamorphic action by which argillaceous slate is converted into mica slate had proceeded so far, before it was arrested, that it becomes impossible to say whether the argillaceous or micaceous characters predominate.

3. _Argillaceous slate._--The last rock of this series is a slaty rock, more or less highly argillaceous. It does not differ in lithological characters from the same rock in the higher strata. It is doubtful whether the roofing-slates should be considered as belonging to the metamorphic series or not. They have been subjected to a very high degree of metamorphic action, and yet strata intimately a.s.sociated with them have, in occasional instances, contained fossils.

It is not easy to fix the exact upper limit of this series. The fossils are few, obscure, and seldom met with in the lowest fossiliferous series; and the transition is very gradual from the distinctly metamorphic to the fossiliferous rocks. This renders it impossible always to determine accurately the line of separation.

The gneiss, mica slate and argillaceous slate, have the order of superposition in which they are here named. They differ only in the amount of metamorphic action to which they have been subjected; and the gneiss which is most highly metamorphic has, by being the lowest, been most acted upon,--the mica slate less, and the argillaceous slate least.

In a particular locality, however, the lowest rock which was subjected to these causes of change, instead of having been of such a character as to produce gneiss, may have been a limestone, and in that case the lowest metamorphic rock would be a saccharine marble. In another locality the lowest rock may have been a sandstone, which would be converted into quartz rock. Hence there may occur, in any part of the metamorphic series, crystalline limestone, quartz rock, hornblende slate, chlorite slate, and talcose slate; and any one of these rocks may be as abundant in any particular region, as gneiss, mica slate or argillaceous slate, is in another.

The metamorphic rocks occur in all countries where there has been any considerable amount of volcanic action, and their total amount is very great; but their stratification is so confused and contorted, their superposition so irregular, and denudations have been so extensive, that no estimate can be made of their thickness. They are, perhaps, equal to all the other stratified rocks.

SECTION V.--THE FOSSILIFEROUS ROCKS.

The fossiliferous rocks are divided into seven systems, which are readily distinguished by the order of superposition, lithological characters and organic remains. These systems are the Silurian, the Old Red Sandstone, the Carboniferous, the New Red Sandstone, the Oolitic, the Cretaceous, and the Tertiary systems. There is also an eighth system now in process of formation.

It is the opinion of some geologists that there is another system situated between the metamorphic rocks and the silurian system. It has been called by Dr. Emmons, who has studied it with much care, the "Taconic System," the Taconic Mountains, in the western part of Ma.s.sachusetts, being composed of these rocks. It is the lower part of what has been called, in England and Wales, the _Cambrian system_.

The strata of this system have a nearly vertical position, and consist princ.i.p.ally of black, greenish and purple slates, of great thickness.

Granular quartz rock, however, occurs in considerable quant.i.ty, and in this country two thick and important beds of limestone are found. These limestones are occasionally white and crystalline. Generally, however, as a ma.s.s, they are a dark, nearly black rock, with a network of lines of a lighter color. All the clouded marbles for architectural and ornamental purposes are from these beds, and our roofing and writing slates are all obtained from the argillaceous portion of this system.

The number of species of organic remains contained in this system is very small, and these, so far as discovered, belong to the annelida, with a few doubtful cases of mollusca. This system of rocks is found coming to the surface in a large part of New England, and the eastern part of New York, also in the western part of England and Wales.

Those geologists who deny the existence of this system consider these rocks as parts of the silurian system which have been most disturbed by subterranean forces, and most altered by proximity to igneous rocks. The annexed sketch (Fig. 7) will exhibit the relations here referred to.

Certain portions of the silurian rocks are supposed to have been thrown into folds by the upheaval of the primary rocks. The plications nearest to the intrusive granite would be most altered. That part of the figure below the line _a a_ represents the outcropping edges as they now appear, the upper portion of the folds having been removed by some abrading cause.

[Ill.u.s.tration: Fig. 7.]

As it is yet uncertain which of these views is correct, convenience will justify us in retaining the name of Cambrian system till further investigations shall settle the question.

1. _The Silurian System._--The following tabular arrangement exhibits the divisions of the system as recognized in England, in New York, in Pennsylvania and Virginia, and in Ohio.

Key to Divisions ---------------- C - Cambrian Rocks.

S - Silurian System.

D - Devonian.

Ch - Champlain Division.

On - Ontario Division.

He - Helderberg Division.

Er - Erie Division.

Divisions as recognized Divisions as recognized Pennsylvania Ohio.

by English Authors. by the New York and Virginia.

Geologists.

/------------------- /----------------------- /---------- /-------- { Upper Cambrian { { Potsdam Sandstone. } No. 1.

{ { { C { Rocks, of Sedgwick{ { Calciferous Sandrock. } { { { Birdseye Limestone. } No. 2. } { (probably). {Ch{ Trenton Limestone. } } Blue { { } Limestone { Llandeilo Flags. { { Utica Slate. } } and Marl.

{ { { } No. 3. } { { Hudson River Group. } { { { { { Gray Sandstone. } No. 4.

{ { { Oneida Conglomerate. } { {On{ { Caradoc Sandstone.{ { Medina Sandstone. } No. 5.

{ { { Clinton Group. } { { Niagara Group.

{ { } } { { { Oneida Salt Group. } : { { { Water-lime Group. } : { { { Pentamerus Limestone. } No. 6. : { { { Delthyris Shaly } : { { { Limestone. } : S { { { Encrinal Limestone. } : Cliff { Wenlock Rocks. {He{ Upper Pentamerus } : { { { Limestone. : Limestone.

{ { { Oriskany Sandstone. } : { { { } No. 7. : { { { Cauda-Galli Grit. } : { { { Schoharie Grit. } : { { { Onondaga Limestone. Wanting. : { { { Corniferous Limestone. Wanting. } { } { { { Marcellus Shales. } } Black { { { Hamilton Group. } } Slate.

{ Upper and Lower { { } No. 8.

{ { { Tully Limestone. } { Ludlow Rocks and {Er{ } : { { Genesee Slate.