altered, forming a metamorphic ring round the eruptive rock. (2) Sheets or masses which have been thrust between the bedding-planes of strata. These resemble truly interstratified beds, but the difference between the two kinds of structure can be readily appreciated. Interstratified beds mark the occurrence of volcanic phenomena at the surface during the time of the formation of the strata among which they occur. Intrusive sheets, on the other hand, are always subsequent in date to the rocks between which they lie. (3) Veins and dykes, consisting of eruptive rock which has been thrust between the walls of irregular rents or straight fissures. (4) Necks, or the filled-up pipes of former volcanic vents. II. Mineral veins are masses of mineral matter which has been deposited, probably from aqueous solution, between the walls of fissures in the earth's crust, and consists of bands of veinstones (quartz, calcite, barytes, etc.) and ores (native metals, or oxides, sulphides, etc., of metals).

CHAPTER XV.

HOW FOSSILS HAVE BEEN ENTOMBED AND PRESERVED, AND HOW THEY ARE USED IN INVESTIGATING THE STRUC

TURE OF THE EARTH'S CRUST, AND IN STUDYING

GEOLOGICAL HISTORY.

In an earlier part of this volume (chapter viii.) attention was called to the various circumstances under which the remains of plants and animals may be entombed and preserved in sedimentary accumulations. When these remains have thus been buried they are known as Fossils. The word "fossil," meaning literally "dug up," was originally applied to all kinds of mineral substances taken out of the earth; but it is now exclusively used for the remains or traces of plants and animals imbedded by natural causes in any kind of rock, whether loose and incoherent, like blown sand, or solid, like the most compact limestone. It includes not only the actual remains of the organisms. The empty mould of a shell which has decayed out of the stone that once enveloped it, or the cast of the shell which has been entirely replaced by inorganic sand, mud, calcite, silica, etc., are fossils. The very impressions left by organisms, such as the burrow or trail of a worm in hardened mud, and the footprints of birds and quadrupeds upon what is now sandstone, are undoubted fossils. In short, under this general term is included whatever bears traces of the form, structure, or presence of organisms preserved in the sedimentary accumulations of the surface, or in the rocks underneath.

In geological history fossils are of fundamental importance. They enable us to investigate conditions of geography, of climate, and of life in ancient times, when these conditions were very different from those which now prevail on the earth's surface. They likewise furnish the ground on which the several epochs of geological history can be determined, and on which the stages of that history in one country can be compared with those in another. So valuable and varied is the evidence supplied by fossils to the geologist, that he regards them as among the most precious documents accessible to him for unravelling the past history of the earth. Some knowledge of the structure and classification of plants and animals is essential for an intelligent appreciation of the use of fossils in geological inquiry. Tο aid the learner, a synopsis of the Vegetable and Animal Kingdom is given in the Appendix, with especial reference to the fossil forms; but it must be understood that for adequate information on this subject recourse should be had to text-books of Botany and Zoology.

It is obvious that all kinds of plants and animals have not the same chances of being preserved as fossils. In the first place, only those, as a rule, are likely to become fossils, whose remains can be kept from decay and dissolution, by being entombed in some kind of deposit. Hence land-animals and plants, on the whole, have much less chance of preservation than those living in the sea, because deposits capable of receiving and securing their remains are exceptional on land, but are generally distributed over the floor of the sea. We should expect, therefore, that among the records of past time, traces of marine should largely preponderate over traces of terrestrial life. Now this is everywhere the case. We know relatively little of the assemblages of plants and animals which in successive epochs have lived upon the dry land, but we have a comparatively large amount of information regarding those which have tenanted the sea. For this reason, marine fossils are more valuable than terrestrial, in comparing the records of the successive epochs of geological history in different parts of the globe.

In the second place, from their own chemical composition and structure, plants and animals present extraordinary differences in their aptitude for preservation as fossils. Where they possess no hard parts, and are liable to speedy decay, we can hardly expect that they should leave behind them any enduring relic of their existence. Hence a large proportion, both of the vegetable and animal kingdoms, may at once be excluded as inherently unlikely to occur in the fossil condition. Of course, under exceptional circumstances, traces of almost any organism may be preserved, and, therefore, we should probably not be justified in saying that by no chance might some recognisable vestige of it be found fossil. Nothing seems more perishable than the tiny gnats and other forms of insect life that fill the air on a summer evening. Yet many of these short-lived flies have been sealed up within the resin of trees (amber), and their structure has been admirably preserved. Such exceptional instances, however, only bring out more distinctly how large a proportion of the living tribes of the land must utterly perish, and leave no recognisable record of their ever having existed. But, where there are hard parts in an organism, and especially where, from their chemical composition, they can, for some time, resist decay, they may, under favourable conditions, be buried in sedimentary deposits, and may remain for indefinite ages locked up there. It is obvious, therefore, that animals possessing hard parts are much the most likely to leave permanent relics of their presence, and ought to occur most frequently as fossils. It is these animals whose remains are preserved in peat-mosses, river-gravels, lake-marls, and on the sea-floor at the present time. Yet, if we were to judge of the extent of the whole existing animal kingdom from these fragmentary remains alone, what an utterly inadequate conception of it we should form! So, too, if we estimate the variety of the living creatures of past time merely from the evidence of the fossils that have chanced to be preserved among the rocks, we shall probably form quite as erroneous a conclusion. There can be no doubt that from the earliest time only an insignificant fraction of the varied life of each period has been preserved in the fossil state, as is unquestionably the case at the present day.

The essential parts of the solid framework of plants consist of the substances known as cellulose and vasculose, which, when kept in dry air, or when waterlogged and buried in stiff mud, may remain undecomposed for long periods. The timber beams in the roofs and floors of old buildings are evidence that, under favourable conditions, wood may last for many centuries. Some plants eliminate carbonate of lime from solution in water, and form with it a solid substance which requires no further treatment to enable it to endure for an indefinite period, when screened from the action of water. Still more durable are the remains of those plants which abstract silica and build it up