roll in again and form another basin beneath the newer rocks. As a matter of fact the coal-measures have been reached at a depth of about 1,000 feet below the surface at Burford in Oxfordshire, and it is quite possible that several such buried coal-fields may exist under the country between Bristol and London.

Recent borings in London, and at other places in Middlesex and Hertfordshire, have shown that the newer beds (chalk, etc.) rest unconformably upon a broad ridge of older rocks, comprising the Silurian, Devonian, and Carboniferous series, which appear to have a general dip to the south, and that by this southerly dip coal-measures which overlie the carboniferous limestone may be brought in to the south of London, and may underlie parts of Surrey.

The considerations above mentioned will be sufficient to' demonstrate the great practical importance of an unconformity between two series of rocks.

THE

PART III.

PHYSIOGRAPHICAL GEOLOGY.

HE first chapter of this book treated of the earth as a whole, and the opinions entertained with regard to the general structure of its mass and the condition of its interior were there given. In the chapters of Part II. the structure and arrangement of the rock-masses which compose the superficial portion of the earth's crust were described. We are now in a position to deal with the problems of Physiographical Geology, or that branch of the science which seeks to account for the existence of the varied physical features of the earth's surface, and to explain the manner in which hills and valleys, plains and table-lands, continents and mountain chains, have been gradually developed by the operation of the physical forces which act within or upon the crust of the earth.

CHAPTER I.

LAND-SCULPTURE, OR THE EVOLUTION OF SURFACEFEATURES BY THE PROCESS OF EROSION.

IN

N Part I., Chapters VI., VII., VIII., we described the action of the various agencies which are continually engaged in the work of erosion and denudation; but their operations were treated rather from a detritive point of view, as resulting in the collection of materials for the formation of new rocks, than as leading to the production of new physical features.

It is clear, however, that agencies which sweep away so much rock from the land must in time cause very great changes in its physical features and contours. An elevated table-land, for instance, by the unequal disintegration of its surface, and by the erosion of valleys out of its mass, may gradually be converted into a series of hill ranges more or less isolated from one another. Such a process is fitly termed Land-sculpture, as analogous to the work of a sculptor, who carves effigies out of solid blocks of stone.

Many persons express surprise that the formation of such prominent features as lofty hills and profound valleys should be attributed to such comparatively feeble agencies as those above mentioned; but it must be borne in mind that we are naturally apt to underrate the amount of work done by these erosive agencies, because we see that in any period of time during which we can observe their action the results produced are very small. On the other hand, when we look at the magnitude of the results which have been produced in past time, we are liable to suppose that the agencies which have operated in former times were much

more powerful and destructive than those which are now in action around us. "When, however, we come to reason on the matter, we find it very difficult to imagine what these agencies could have been if they were altogether different from existing causes'; and equally difficult to suppose that existing agencies have ever acted with much greater intensity than at present, unless we assume the general physical laws of the world to have been different from what they are now" (Jukes).

We shall therefore take it for granted, in accordance with the tenets of the Lyellian philosophy, that all the geological phenomena observable among stratified rocks are due to the same causes as those now acting in some part of the world, or to some modification and combination of these causes, such as we may reasonably suppose to have occurred in the course of the earth's history.

To such seemingly insignificant and slowly acting causes, operating continually through long periods of time, must be attributed all the erosion of rock which gives to elevated land its cliffs and precipices, its hills and valleys, and all the varied features of the earth's surface.

Share taken by different Agencies.-The detritive and erosive agencies already described may be grouped under two heads, according to differences in their mode of operation, and in the results of their action.

1. Marine agencies, which act along the margin of the land, and tend to produce an approximate level surface or plain.

2. Subaerial agencies, which act over the whole surface of the land, and tend to produce a system of valleys and watersheds, hollows and relative eminences.

Few parts of the sea-bottom could be raised into dry land without passing through the destructive plane of the sea-level, and no part of the land could be submerged without passing through the same plane, and being exposed for a shorter or longer period to the erosive action of the waves. These agents would wear down the inequalities of its surface and would produce an inclined. plane, the extent and inclination of which would depend on the slope of the sinking or rising land, and the rate at which the movement took place. It must be remembered,