Part 17

3. Original composition and structure both obliterated, and form alone preserved.—This occurs most commonly with shells, although fossil trees are also often good illustrations. The general result is accomplished in several ways: (a) The shell after being buried in the sediment may be removed by solution, leaving a mould of its external form, (b) This mould may subsequently be filled by the infiltration of finer sediment, forming a cast of the exterior of the shell. (c) The shell, before its solution, may have been filled with mud; and if the shell itself is then dissolved, we have a cast of its interior in a mould of its exterior.

TIME REQUIRED FOR THE FORMATION OF STRATIFIED ROCKS.—Many attempts have been made to determine the time required for the deposition of any given thickness of stratified rocks. Of course, only roughly approximate results can be hoped for in most cases; but these are at least sufficient to make it certain that geological time is very long. The average relative rate of growth of different kinds of sediment is, however, less open to doubt, for we have already seen that coarse sediments like gravel and sand accumulate much more rapidly than finer sediments like clay and limestone; and we are sometimes able to compare these two classes of rocks on a very large scale.

Thus, during what is known as the Paleozoic era, a sea extended from the Blue Ridge to the Rocky Mountains. Along the eastern margin of this sea, where the Alleghany Mountains now stand, sediments—chiefly conglomerate and sandstone, with some slate and less limestone—accumulated to a thickness of nearly 40,000 feet. Toward the west, away from the old shore-line, the coarse sediments gradually die out, and the formations become finer and thinner. In western Ohio and Indiana, slate and limestone predominate; while in the central part of the ancient sea, in Illinois and Missouri, the paleozoic sediments are almost wholly limestones, and have a thickness of only 4000 to 5000 feet. In other words, while one foot of limestone was forming in the Mississippi Valley, eight to ten feet of coarser sediments were deposited in Pennsylvania.

The best estimates show that coral-reefs rise—i.e., limestones are formed on them—at the rate of about one foot in two hundred years. But coral limestones grow much more rapidly than limestones in general. Sandstones sometimes accumulate so rapidly that trees are buried before they have time to decay and fall (Fig. 9). Such a buried forest, like a coal-bed, represents a land surface, and proves a subsidence of the land; and in some cases, as indicated by the section, repeated oscillations of the crust may be proved in this way.

The mud deposited by the annual overflow of the Nile is forty feet thick near the ancient city of Memphis; and the pedestal of the statue of Rameses II., believed to have been erected B.C. 1361, is buried to a depth of nine feet, four inches, indicating that 13,500 years have elapsed since the Nile began to spread its mud over the sands of the desert.

But the greatest difficulty in estimating the time required for the formation of any series of strata arises from the fact that we cannot usually even guess at the length of the periods when the deposition has been partially or wholly interrupted. Now and then, however, we find evidence that these periods may be very long. A layer of fossil shells in sandstone or slate proves an interruption of mechanical deposition. Beds of coal, fossil forests, and other indications of land surfaces are still more conclusive. The interposition of strata (Fig. 5) proves a prolonged interruption of deposition over the area not covered by the interposed bed. But the most important of all evidence is that afforded by unconformability; and the length of the lost interval between the two formations is measured approximately by the erosion of the older.

Original Structures of Eruptive Rocks.

The structures of this class are divisible into those pertaining to the volcanic rocks and those pertaining to the fissure or dike rocks. But since volcanoes are rare in this part of the world, while dikes are well developed in many sections of our country, it seems best to give our attention chiefly to the latter.

The term dike is a general name for all masses of eruptive rocks that have cooled and solidified in fissures or cavities in the earth’s crust. But the name is commonly restricted to the more regular, wall-like masses (Fig. 10), those having extremely irregular outlines, like most masses of granite (Fig. 11), being known simply as eruptive masses. The propriety of this distinction is apparent when we consider the origin of dike as a geological term. It was first used in this sense in southern Scotland, where almost any kind of a wall or barrier is called a dike. The dikes traverse the different stratified formations like gigantic walls, which are often encountered by the coal-miners, and on the surface are frequently left in relief by the erosion of the softer enclosing rock, so that in the west of Scotland, especially, they are actually made use of for enclosures. In other cases the dike has decayed faster than the enclosing rock, and its position is marked by a ditch-like depression. The narrow, straight, and perpendicular clefts or chasms observed on many coasts are usually due to the removal of the wall-like dikes by the action of the waves. Dikes are sometimes mere plates of rock, traceable for a few yards only; and they range in size from that up to those a hundred feet or more in width, and traceable for scores of miles across the country, their outcrops forming prominent ridges. The sides of dikes are often as parallel and straight of those of built walls, the resemblance to human workmanship being heightened by the numerous joints which, intersecting each other along the face of a dike, remind us of well-fitted masonry.

FORMS OF DIKES.—A dike is essentially a casting. Melted rock is forced up from the heated interior into a cavity or crack in the earth’s crust, cools and solidifies there, and, like a metallic casting, assumes the form of the fissure or mould. In other words, the form of the dike is exactly that of the fissure into which the lava was injected. Now the forms of fissures depend partly upon the nature of the force that produces them, but very largely upon the structure—and especially the joint-structure—of the enclosing rocks. Nearly all rocks are traversed by planes of division or cracks called joints, which usually run in several directions, dividing the rock into blocks. And it is probable that dike-fissures are most commonly produced, not by breaking the rocks anew, but by widening or opening the pre-existing joint-cracks. Hence the straight and regular jointing of slate, limestone and most sedimentary rocks is accompanied by wall-like dikes—the typical dikes (Fig. 10); while the more irregular jointing of granite and other massive rocks gives rise to sinuous, branching, variable dikes. The general dependence of dikes upon the joint-structure of the rocks is proved by the facts that dikes, like joints, are normally vertical or highly inclined, and that they are usually parallel with the principal systems of joints in the same district. The wall-like dikes also give off branches, but usually in a regular manner, as shown in Fig. 12.

STRUCTURE OF DIKES.—The rock traversed by a dike is called the country or wall rock. Fragments of this are often torn off by the igneous material, and become enclosed in the latter. Such enclosed fragments may sometimes form the main part of the dike, which then, since they are necessarily angular, often assumes the aspect of a breccia. This is the only important exception to the rule that dikes are homogeneous in composition; i.e., in the same dike we can usually find—from end to end, from side to side, and probably from top to bottom—no essential difference in composition. But there is often a marked contrast in texture between different parts of a dike, and especially between the sides and central portion. The liquid rock loses heat most rapidly where it is in contact with the cold walls of the fissure, and solidifies before it has time to crystallize, remaining compact and sometimes even glassy; while in the middle of the dike, unless it is very narrow, it cools so slowly as to develop a distinctly crystalline texture. There is no abrupt change in texture, but a gradual passage from the compact border to the coarsely crystalline or porphyritic middle portion. It is obvious that a similar gradation in texture must exist between the top and bottom of a dike.

CONTACT PHENOMENA.—Under this head are grouped the interesting and important phenomena observable along the contact between the dike and wall-rock. These throw light upon the conditions of formation of dikes, and are often depended upon to show whether a rock mass is a dike or not. The student will observe here:—

1. The detailed form of the contact. It may be straight and simple, or exceedingly irregular, the dike penetrating the wall, and enclosing fragments of it, as in Fig. 11, which is a typically igneous contact.

2. The alteration of the wall-rock by heat. This may consist in: (a) coloration, shales and sandstones being reddened in the same way as when clay is burnt for bricks; (b) baking and induration, sandstone being converted into quartzite and even jasper; clay, slate, etc., being not only baked to a flinty hardness, but actually vitrified, as in porcelainite; and bituminous coal being converted into natural coke or anthracite; and (c) crystallization, chalk, and other limestones being changed to marble, and crystals of pyrite, calcite, quartz, etc., being developed in slate, sandstone, and other rocks.

3. The alteration of the dike-rock by (a) more rapid cooling, and (b) the access of thermal waters.

The alteration of the wall-rock may extend only a few inches or many yards from the dike, gradually diminishing with the distance; and the cases are surprisingly numerous where there is no perceptible alteration; and, again, the alteration is usually mutual, the dike-rock being altered in texture, color, and composition.

INTRUSIVE BEDS.—We commonly think of dikes as cutting across the strata, but they often lie in planes parallel with them; and the same dike may run across the beds in some parts of its course and between them in others (Fig. 12), or the conformable dike maybe simply a lateral branch of a main vertical dike, as shown in the same figure. All dikes or portions of dikes lying conformably between the strata are called intrusive beds or sheets.

When a dike fails to reach the surface, but spreads out horizontally between the strata, forming a thick dome or oven-shaped intrusive bed, the latter is called a laccolite (Fig. 13). Laccolites are sometimes of immense volume, containing several cubic miles of rock. Fig. 14 enables us to compare the laccolite with the volcano.

In the one case a large mound of eruptive material accumulates between the strata, the overlying beds being lifted into a dome; while in the other case the fissure or vent reaches the surface, and the mound of lava is built up on top of the ground.

COTEMPORANEOUS BEDS.—When the lava emitted by a crater is sufficiently liquid, it spreads out horizontally, forming a volcanic sheet or bed. If such an eruption is submarine, or the lava flow is subsequently covered by the sea, sedimentary deposits are formed over it; and beds of lava which thus come to lie conformably between sedimentary strata are known as cotemporaneous sheets or beds, because they belong, in order of time, in the position in which we find them, being, like any member of a stratified series, newer than the underlying and older than the overlying strata. Cotemporaneous lava-flows are sometimes repeated again and again in the same district, and thus important formations are built up of alternating igneous and aqueous deposits. Evidently, the student who would read correctly the record of igneous activity in the past must be able to distinguish intrusive and cotemporaneous beds. The principal points to be considered in making this distinction are: (1) The intrusive bed is essentially a dike, dense and more or less crystalline in texture, altering, and often enclosing fragments of, both the underlying and overlying strata, and frequently jogging across or penetrating the sediments. (2) The cotemporaneous bed, on the other hand, being essentially a lava-flow, is much less dense and crystalline, being usually distinctly scoriaceous or amygdaloidal, especially at the borders, and the underlying strata alone showing heat action, or occurring as enclosures in the lava; for the overlying strata are newer than the lava, and often consist largely, at the base, of water-worn fragments of the lava.

AGES OF DIKES.—The ages of dikes may be estimated in several ways. They are necessarily newer than any stratified formation which they intersect or of which they enclose fragments; but any formation crossing the top of a dike must usually be regarded as newer than the dike, especially if it contains water-worn fragments of the dike rock.

The relative ages of different dikes are determined by their relations to the stratified formations; and still more easily by their mutual intersections, on the principle that when two dikes cross each other, the intersecting must be newer than the intersected dike. It is sometimes possible, in this way, to prove several distinct periods of eruption in the same limited district. The textures of dikes also often afford reliable indications of their ages; for, as we have already seen, the upper part of a dike, cooling rapidly and under little pressure, must be less dense and crystalline than the deep-seated portion, which cools slowly and under great pressure.

Now, the lower, coarsely crystalline part of a dike can usually be exposed on the surface only as the result of enormous erosion; and erosion is a slow process, requiring vast periods of time. Hence, when we see a coarse-grained dike outcropping on the surface, we are justified in regarding it as very old, for all the fine-grained upper part has been gradually worn away by the action of the rain, frost, etc. Other things being equal, coarse-grained must be older than fine-grained dikes; and the texture of a dike is at once a measure of its age and of the amount of erosion which the region has suffered since it was formed.

ERUPTIVE MASSES.—In striking contrast with the more or less wall-like dikes are the highly irregular, and even ragged, outlines of the eruptive masses; and it is worth while to notice the probable cause of this contrast. The true dikes are formed, for the most part, of comparatively fine-grained rocks—the typical “traps”; while the eruptive masses consist chiefly of the coarse-grained or granitic varieties. Now we have just seen that the coarse-grained rocks have been formed at great depths in the earth’s crust, while the fine-grained are comparatively superficial. But we have good reason for believing that the joint-structure, upon which the forms of dikes so largely depend, is not well developed at great depths, where the rocks are toughened, if not softened, by the high temperature. In other words, trap dikes are formed in the jointed formations, which break regularly; while the granitic masses are formed where the absence of joint-structure and a high temperature combine to cause extremely irregular rifts and cavities when the crust is broken.

VOLCANIC PIPES OR NECKS.—Every volcano and every lava-flow or volcanic sheet must be connected with the earth’s interior by a channel or fissure, which becomes a dike when the lava ceases to flow. But the converse proposition is not true, for it is probable that many dikes did not originally reach the surface, but have been exposed by subsequent denudation. This is conspicuously the case with laccolites and other forms of intrusive sheets. Volcanic sheets or beds have probably often resulted from the overflow of the lava at all points of an extensive fissure or system of fissures; but the vent of the true volcano must be more circumscribed, an approximately circular opening in the earth’s crust, although doubtless originating in a fissure or at the intersection of two or more fissures, the lava continuing to flow at the widest part of the wound in the crust long after it has congealed in the narrower parts. Such a tube is known as the neck or pipe of the volcano; and volcanic necks are a highly interesting class of dikes, since they determine the exact location of many an ancient volcano, where the volcanic pile itself has long since been swept away. Necks and dikes are the downward prolongations or roots of the volcanic cone or sheet, and cannot be exposed on the surface until the volcanic fires have gone out and the agents of erosion have removed the greater part of the ejected materials.

Hence, equally with the dikes which originally failed to reach the surface, they, wherever open to our observation, testify to extensive erosion and a vast antiquity.

Original Structures of Vein Rocks.

Many things called veins are improperly so called, such as dikes of granite and trap, and beds of coal and iron-ore. The smaller, more irregular, branching dikes, especially, are very commonly called veins, and to distinguish the true veins from these eruptive masses, they are designated as mineral veins or lodes, although the term lode is usually restricted to the metalliferous veins.