or less ancient sediments, formed and accumulated in the same way as the gravel, sand, and mud of existing rivers, lakes, and seas. Another common rock of aqueous origin is limestone, of which there are countless varieties—some formed in lakes, like the shellmarls of our own day; others representing the calcareous ooze and coral-reefs of ancient seas; while yet others are obviously chemical precipitates from water surcharged with carbonate of lime. Now and again, also, we meet with rocks of terrestrial origin, such, for example, as many beds and seams of peat, lignite, and coal, which are simply the vegetable débris of old land-surfaces. To these land-formed beds we may add certain sandstones of wind-blown origin— indurated sand-dunes, in short.

The igneous rocks consist partly of lavas and fragmental materials which have been ejected at the surface, as in modern volcanoes, and partly of formerly molten masses which have cooled and consolidated below ground. The former, therefore, are spoken of as volcanic, the latter as plutonic or hypogene rocks. As it is useful to have some general name for the rocks which owe their origin to the action of epigene agents (i. e., the atmosphere, terrestrial water, ice, the sea, and life), we may term these derivative, since they have been built up chiefly out of the relics of pre-existing rocks and the débris of plants and animals. Byand-by we shall learn that igneous and derivative rocks have in certain regions been subjected to many remarkable changes, and are in consequence so

altered that it is often hard to detect their original character. These altered masses form what are called

the metamorphic rocks. They are typically represented by such rocks as gneiss, mica-schist, clay-slate,

etc.

The derivative rocks, with which in many regions igneous rocks are associated, occupy by far the larger portion of the land-surface, entering abundantly into the composition of low grounds and mountains alike. Most of these derivatives are sedimentary accumulations, and very many are charged with the remains of animals and plants. By noting the order in which such stratified deposits occur, and by comparing and correlating their fossils, geologists have been able to group them into a series of successive systems, the oldest being that which occurs at the bottom of the series.1 The united thickness of the several systems probably exceeds twenty miles, but it must not be supposed that all these occur together in any one region. Many broad acres of the earth's surface are occupied by the rocks belonging to one system only. In other countries two or more systems may be present. Again, each individual system is of very variable thickness—swelling out here, thinning off there: in some lands being represented by strata many thousands of feet in thickness, in others dwindling down to a few yards. In short, we may picture to ourselves each system as consisting of a series of larger and smaller lenticular sheets, irregularly distributed over 1 See Appendix for Table of Geological Systems.

the earth's surface. The various systems thus fre quently overlap, the younger stealing over the surface of the older so as often to bury these out of sight.

The metamorphic rocks do not appear at the surface over such extensive areas as those just referred to. Nevertheless, they are widely distributed, and now and again overspread continuously vast regions. The enormous tract that extends from the Great Lakes of North America to the shores of the Arctic Ocean is almost entirely occupied by them. Another im mense area of crystalline schistose rocks is met with in Brazil. The Highlands of Scotland, the Scandinavian Peninsula, and North Finland are in like manner largely composed of them, and the same is the case with many parts of Africa, Asia, and Australia. It is further noteworthy that similar rocks form the backbones of most of the great mountain chains of the globe. As already indicated, metamorphic rocks are of various origin, some of them being primarily of igneous and others of aqueous formation. Those which form the nuclei of the youngest mountain chains are sometimes of relatively recent age, while those occupying such broad tracts as Brazil, the Canadian uplands, etc., are of vast antiquity. Crystalline schistose rocks, with associated granites and other igneous rocks, seem everywhere to underlie the sedimentary fossiliferous formations. Very often. the latter are separated by a broadly marked line of demarcation from the schists, granites, etc., upon which they repose. In other cases the sedimentary rocks

become gradually altered as they are traced downwards, until eventually they themselves assume the aspect of crystalline schists, penetrated here and there by granitoid igneous rocks.

The origin of those ancient crystalline schists has been much discussed, but does not concern us here. Some geologists have maintained that the rocks in question represent the original cooled crust of the globe, while the majority consider them to be all metamorphic. It is enough for our present purpose to know that a pavement of such rocks appears everywhere to underlie the sedimentary fossiliferous formations.

The upper continuous line, A-B, = surface of ground; the lower continuous line, C-D, level; 7, limestone; s, sandstones and shales.

= sea

The great bulk of the derivative rocks being of sedimentary origin, it is obvious that they must have been at the time of their formation spread out in approximately horizontal layers upon the beds of ancient lakes and seas. This we are justified in believing by what we know of the accumulation of similar sediments in our own day. The wide flats of our rivervalleys, the broad plains that occupy the sites of siltedup lakes, the extensive deltas of such rivers as the Nile, the Po, the Amazon, the Mississippi, the narrow

or wide belts of low-lying land which within a recent period have been gained from the sea, are all made up of various kinds of sediment arranged in gently inclined or approximately horizontal layers. Now, over considerable areas of the earth's surface the derivative rocks show the same horizontal arrangement, a structure which is obviously original. And this is frequently the case with younger and older sedimentary strata alike. Here, for example (Fig. 1), is a section across a country, the superficial rock-masses of which are horizontally arranged.

The upper line of the section (A-B) represents, of course, the surface of the ground, while the lower we shall take to be the level of the sea. The section thus shows the geological structure or arrangement of the rocks from the surface down to the level of the The strata represented consist of a great series of sandstones and shales with one prominent bed of limestone () at the top. In this case we cannot doubt that the horizontal bedding is original-that the strata were accumulated one above the other in the same order as we see them.

sea.

Although such horizontal arrangements are of common enough occurrence, and now and again characterise the sedimentary systems over wide areas, yet, as a general rule, strata tend to be inclined. In many regions the inclination, or dip, as it is termed, is sometimes very high-not seldom indeed the beds are seen standing on end, like rows of books in a library. This last appearance of extreme disturbance is not confined