at the mouths of our great rivers, and on a smaller scale wherever there is running water.

Every stream, where it runs into

Fig. 4. Sketch of Carboniferous strata at Kinghorn, in Fife, showing stratified beds (limestone and shales) surmounted by an unstratified mass of trap. (Original.)

a lake or into the sea, carries with it a burden of mud, sand, and rounded pebbles, derived from the waste of the rocks which form its bed and banks. When these materials cease to be impelled by the force of the moving water, they sink to the bottom, the heaviest pebbles, of course, sinking first, the smaller pebbles and sand next, and the finest mud last. Ultimately, therefore, as might have been inferred upon theoretical grounds, and as is proved by practical experience, every lake becomes a receptacle for a series of stratified rocks produced by the streams flowing into it. These deposits may vary in different parts of the lake, according as one stream brought down one kind of material and another stream contributed another material; but in all cases the materials will bear ample evidence that they were produced, sorted, and deposited by running water. The finer beds of clay or sand will all be arranged in thicker or thinner layers or laminæ ; and if there are any beds of pebbles these will all be rounded or smooth, just like the water-worn pebbles of any brook-course. In all probability, also, we should find in some of the beds the re

mains of fresh-water shells or plants or other organisms which inhabited the lake at the time these beds were being deposited.

In the same way large rivers such as the Ganges or Mississippi-deposit all the materials which they bring down at their mouths, forming in this way their "deltas." Whenever such a delta is cut through, either by man or by some channel of the river altering its course, we find that it is composed of a succession of horizontal layers or strata of sand or mud, varying in mineral composition, in structure, or in grain, according to the nature of the materials brought down by the river at different periods. Such deltas, also, will contain the remains of animals which inhabit the river, with fragments of the plants which grew on its banks, or bones of the animals. which lived in its basin.

Nor is this action confined, of course, to large rivers only, though naturally most conspicuous in the greatest bodies of water. On the contrary, all streams, of whatever size, are engaged in the work of wearing down the dry land, and of transporting the materials thus derived from higher to lower levels, never resting in this work till they reach the sea.

Lastly, the sea itself-irrespective of the materials delivered into it by rivers-is constantly preparing fresh stratified de

posits by its own action. Upon every coast-line the sea is constantly eating back into the land and reducing its component rocks to form the shingle and sand which we see upon every shore. The materials thus produced are not, however, lost, but are ultimately deposited elsewhere in the form of new stratified accumulations, in which are buried the remains of animals inhabiting the sea at the time.

Whenever, then, we find anywhere in the interior of the land any series of beds having these characters-composed, that is, of distinct layers, the particles of which, both large and small, show distinct traces of the wearing action of water-whenever and wherever we find such rocks, we are justified in assuming that they have been deposited by water in the manner above mentioned. Either they were laid down in some former lake by the combined action of the streams which flowed into it; or they were deposited at the mouth of some ancient river, forming its delta; or they were laid down at the bottom of the ocean. In the first two cases, any fossils which the beds might contain would be the remains of fresh-water or terrestrial organisms. In the last case, the majority, at any rate, of the fossils would be the remains of marine animals.

The term "formation" is employed by geologists to express "any group of rocks which have some character in common, whether of origin, age, or composition" (Lyell); so that we may speak of stratified and unstratified formations, aqueous or igneous formations, fresh-water or marine formations, and

so on.

CHIEF DIVISIONS OF THE AQUEOUS ROCKS.

The Aqueous Rocks may be divided into two great sections, the Mechanically-formed and the Chemically-formed, including under the last head all rocks which owe their origin to vital action, as well as those produced by ordinary chemical agencies.

A. MECHANICALLY-FORMED ROCKS. These are all those Aqueous Rocks of which we can obtain proofs that their particles have been mechanically transported to their present situation. Thus, if we examine a piece of conglomerate or puddingstone, we find it to be composed of a number of rounded pebbles embedded in an enveloping matrix or paste, which is usually of a sandy nature, but may be composed of carbonate of lime (when the rock is said to be a "calcareous conglomerate"). The pebbles in all conglomerates are worn and rounded by the action of water in motion, and thus show

that they have been subjected to much mechanical attrition, whilst they have been mechanically transported for a greater or less distance from the rock of which they originally formed part. The analogue of the old conglomerates at the present day is to be found in the great beds of shingle and gravel which are formed by the action of the sea on every coast-line, and which are composed of water-worn and well-rounded pebbles of different sizes. A breccia is a mechanically-formed rock, very similar to a conglomerate, and consisting of larger or smaller fragments of rock embedded in a common matrix. The fragments, however, are in this case all more or less angular, and are not worn or rounded. The fragments in breccias may be of large size, or they may be comparatively small (fig. 6); and the matrix may be composed of sand (arenaceous) or of carbonate of

A

lime (calcareous). In the case of an ordinary sandstone, again, we have a rock which may be regarded as simply a very finegrained conglomerate or breccia, being composed of small grains of sand (silica), sometimes rounded, sometimes more or less angular, cemented together by some such substance as oxide of iron, silicate of iron, or carbonate of lime. sandstone, therefore, like a conglomerate, is a mechanically-formed rock, its component grains being equally the result of mechanical attrition and having equally been transported from a distance; and the same is true of the ordinary sand of the sea-shore, which is nothing more than an unconsolidated sandstone. Other so-called sands and sandstones, though equally mechanical in their origin, are truly calcareous in their nature, and are more or less entirely composed of carbonate of lime. Of this kind are the shell-sand so common on our coasts, and the coral-sand which is so largely formed in the neighbourhood of coral-reefs. In these cases the rock is composed of fragments of the skeletons of shellfish, and numerous other marine animals, together, in many instances, with the remains of certain sea-weeds (Corallines, Nullipores, &c.) which are endowed with the power of secret

The fragments are all of small size, and consist of angular pieces of transparent quartz, volcanic ashes, and limestone embedded in a matrix of crystalline limestone. (Original.)

ing carbonate of lime from the sea-water. Lastly, in certain rocks still finer in their texture than sandstones, such as the various mud-rocks and shales, we can still recognise a mechanical source and origin. If slices of any of these rocks sufficiently thin to be transparent are examined under the microscope, it will be found that they are composed of minute grains of different sizes, which are all more or less worn and rounded, and which clearly show, therefore, that they have been subjected to mechanical attrition.

All the above-mentioned rocks, then, are mechanically-formed rocks; and they are often spoken of as "Derivative Rocks," in consequence of the fact that their particles can be shown to have been mechanically derived from other pre-existent rocks. It follows from this that every bed of any mechanically-formed rock is the measure and equivalent of a corresponding amount of destruction of some older rock. It is not necessary to enter here into a minute account of the subdivisions of these rocks, but it may be mentioned that they may be divided into two principal groups, according to their chemical composition. In the one group we have the so-called Arenaceous (Lat. arena, sand) or Siliceous Rocks, which are essentially composed of larger or smaller grains of flint or silica. In this group are comprised ordinary sand, the varieties of sandstone and grit, and most conglomerates and breccias. We shall, however, afterwards see that some siliceous rocks are of organic origin. In the second group are the so-called Argillaceous (Lat. argilla, clay) Rocks, which contain a larger or smaller amount of clay or hydrated silicate of alumina in their composition. Under this head come clays, shales, marls, marl-slate, clay-slates, and most flags and flagstones.

B. CHEMICALLY-FORMED ROCKS.-In this section are comprised all those Aqueous or Sedimentary Rocks which have been formed by chemical agencies. As many of these chemical agencies, however, are exerted through the medium of living beings, whether animals or plants, we get into this section a number of what may be called "organically-formed rocks." These are of the greatest possible importance to the palæontologist, as being to a greater or less extent composed of the actual remains of animals or vegetables, and it will therefore be necessary to consider their character and structure in some detail.

By far the most important of the chemically-formed rocks are the so-called Calcareous Rocks (Lat. calx, lime), comprising all those which contain a large proportion of carbonate of lime, or are wholly composed of this substance. Carbonate