history of the fossils he dealt with. But he learnt to recognise them, and to judge accurately of their position in the geological series, and he made as admirable use of them in tracing the outlines of the development of the Secondary rocks across England as if he had been able to name and describe each species. Geology has made vast strides since his time. Though the field-geologist may use the fossils without any scientific knowledge of them, the sooner he obtains that knowledge the better for his work. The broad outlines of William Smith's days have to be filled in by more minute and exhaustive work now.

In fine, the field-geologist will find in all quarters of the world that an acquaintance with fossils can be turned to profitable account. It enables him at the outset to fix more or less definitely the relative age of the rocks among which he is engaged and thus affords means of comparison with the corresponding rocks of other countries. Where his labours are of no ambitious kind, but where he works for the quiet pleasure and open-air life of the pursuit, the study of organic remains affords him an endless fund for delightful meditation. They show him at one place evidence of an old sea-bottom, in the strata where marine remains are crowded together. At another locality they bring before him, in fresh-water shells and other forms, the traces of long-vanished lakes and rivers. At a third spot they reveal, by successive layers of compressed vegetation and hardened loam, the gradual depression and submergence of old forest-covered lands. In such cases they suggest the lines along which his further search should be prosecuted for additional corroborative testimony as to the ancient aspects of the district in which

he is at work. The land-plants, for example, lead him to look for fresh-water forms of life, for sun-cracked and rain-pitted surfaces of rock; while the occurrence of marine forms of life prompts to search for other proofs of the ancient encroachments of the sea.

CHAPTER VIII.

THE TRACING OF GEOLOGICAL BOUNDARY-LINES.

WHETHER or not the observer sets about the construction of a map, he can form but a limited notion of the geology of a country if he confines his attention merely to a few quarries or lines of natural section. Having learned in such openings what is the nature and order of succession of the rocks, he ought to try to follow them out, from where they are clearly seen into other parts of the country, and in so doing, endeavour to note as he goes any variation in character which they may present, and every feature which serves to indicate what must be the disposition of the rocks below.

A very short experience of geological work in the field suffices to show the observer that over wide spaces he cannot actually see what rock lies beneath him. He may get an admirable section laid bare in some ravine or brook, or by the shore of the sea; but beyond the limits of this section the ground may be deeply buried under vegetation, soil, sand, gravel, clay, or other superficial formation, and no other section may occur for an interval of, it may be, several miles. Yet he must form some

conclusion as to the nature of the rocks between these places.

In cases of this kind information may often be obtained from an examination of the soil. What we call vegetable soil is merely the upper stratum of decayed rock mixed with vegetable and animal remains. (Fig. 21.) It commonly betrays its origin by the still undecomposed fragments of stone mixed through its mass. In one tract, for instance, we may find it full of pieces of sandstone, to

FIG 21.-Section to show the superficial covering of soil (1), subsoil (2) derived from the disintegration of the underlying rock (3).

the exclusion perhaps of every other kind of rock. If the land has been under cultivation, the sandstone may be in large pieces, where it has been turned up by the plough. We should there infer with some confidence that sandstone lay underneath in situ. If again the soil were a stiff red loam or clay, with few or no stones, it would indicate the existence of some red marl or clay immediately underneath. A sandy soil full of well-rounded, water-worn stones, would show the presence of some gravelly deposit below. A calcareous soil full of blocks

of flint would probably indicate the existence of chalk. A stiff argillaceous soil, abounding in smoothed stones, many of them well-striated, would prove that a boulderclay or till lay below. A profusion of fragments of some peculiar rock, a basalt, for example, or a diorite, or a porphyrite, extending in a definite band across a field or hill-side, would probably show us that a rock of that character existed, in situ, somewhere in the immediate neighbourhood of the fragments. We require, of course, in all these cases, to go carefully over the ground, and draw our conclusion only after we have exhausted all the evidence procurable.

But it may be remarked that, except on freshly-ploughed land, the soil is not bare and exposed to our scrutiny; that, on the contrary, it is commonly just as much concealed by its coating of vegetation as the hard rocks are by their covering of soil. Even under the most unfavourable circumstances, however, the geologist may often be able to learn not a little of the information he needs. Where the ground has a slope he will probably have no great trouble in finding some little rut or trench which has been cut, or at least deepened, by rain, and where he will obtain access to the underlying soil, or even, it may be, to the subsoil and the still undecomposed rock below it. Where, on the other hand, the ground is too flat to hope for assistance from rain-action, he will look for traces of burrowing animals, by which the soil may have been thrown up to the surface. In Britain the common earth-worm, the mole, and the rabbit, are excellent coadjutors in his work. The fine castings of the earthworm give him at least the colour and general constitution of the soil, whether sandy or clayey. The heaps of the