they may be traced in places underneath those at a lower level. Indeed we find, as a rule, that the older the rocks the more wild, rugged, and mountainous is the nature of the ground they occupy, for these rocks have of course uudergone more induration and elevation from heat and pressure than the rocks formed subsequently to them; and they have often been elevated to form land for long periods, while newer deposits were forming around them, and which, indeed, were made up to a large extent from their destruction. Some of our rucked layers of cloth should have the summits of their folds cut off to represent denudation or wearing away which took place before the layers above were deposited. This is called an unconformity, an indication of a lapse of time. Then, when we have our pile of cloths thus arranged, we may cut imaginary valleys out of the even layers that were deposited last; and then the different coloured pieces that are exposed, on looking at the surface of the whole mass, would give a very good idea of the phenomena exhibited by a geological map. geological map. Illustrations of this kind in wood have indeed been prepared by Mr. Sopwith, and they form admirable models for the student.

The object of a geological map is therefore to indicate the areas where the various rocks of which the earth's crust is formed appear at the surface. And we must bear in mind that, although there was a regularity in succession, there was much irregularity in the area over which each series was deposited, so that some may be absent in places; and that wherever we find one deposit resting on the upturned edges of another, we know that between the two there was a great interval of time when some other deposits were forming which we shall find elsewhere, where no disturbance took place to prevent a regular succession.

Mere soils are omitted in our maps, and in general also those irregular superficial deposits of gravel, sand and boulder-clay, known as drifts; but their importance is becoming more and more apparent the better they are understood, so that before. many years they will probably be represented on most of our geological maps.

We may now turn our attention to the way in which our geological maps are constructed; how the lines are drawn which separate the different rocks which are exposed at the surface. To anyone walking over a flat grass-covered or well-wooded country, probably the difficulties attending a minute survey would appear insurmountable, and they might conclude that a great deal is done by conjecture. It is true that the geologist is in a great measure forced to be guided by inference when there is no direct evidence; but when he has surveyed all round any obscure tract of country, the inferences which guide him in

determining the boundaries of the rock or rocks of which it is composed, and of drawing his lines to join with others in the surrounding area, would probably be safe enough, and something more than mere conjecture. Moreover, with experience one notices many little facts that afford valuable hints in mapping, which by a person not accustomed to the work would pass unnoticed: such as a slight feature in the ground, a spring, or some peculiarity of the soil.

In mapping a country, therefore, the first thing is to get acquainted with the best sections; to observe whether the rocks be of the stratified or unstratified kind, or both; to observe their relations one to another, and also to the form of the ground. In sections of the stratified rocks we must examine each bed, and particularly notice the junction between any two formations, which if conformable may require some assistance from a study of the fossils to fix. We must carefully note any features such junctions may make in the ground, whether in the form of a terrace due to the resistance of some hard bed to denuding agents, or in the wetness or dryness of the soil, due to springs and the pervious or impervious nature of the rocks. We must also endeavour to ascertain to what extent the country is covered with drift deposits, and how far the soils may be an indication of the rocks beneath. The best sections are seen in a railway-cutting, a sea-cliff, a quarry, a deep roadcutting, or a foundation-pit. The sections recorded in a notebook are useful for comparison with other sections, so that some notion of the general character and thickness of each formation, and of the beds at different horizons in it, may be arrived at. Then, when one has a general notion of the rocks and their relations to one another, the boundaries between them may be traced out on the ground. The character of the mapping here, however, depends very largely upon the nature of the rocks; if they be the older rocks, much disturbed and contorted, or penetrated by igneous dykes, or if they be secondary rocks maintaining great parallelism and conformity, or tertiary rocks covered irregularly by drift deposits. Having fixed all the clear junctions that may be observed in a preliminary survey of the best sections, one may then draw the lines which mark the junctions across the intermediate ground, noticing every section on the way, in the banks of a stream, or in ditches, which usually afford some indications of the strata.

In the igneous and older stratified rocks one usually gets some bosses of the rock projecting here and there on the hillsides, and even on the high roads. Attention must be paid to the general direction or dip of the older stratified rocks, great care being taken to discriminate between this and the "slaty cleavage" which cuts up the beds at all angles, and is a pheno

menon produced by pressure subsequent to the consolidation of the beds. So difficult is this sometimes, that the late Professor Jukes has remarked, "You may sometimes toss up' which is cleavage and bedding and jointing." The dip of the beds, the direction of which is marked by arrows on the map, is of the greatest importance in mapping conformable rocks such as the carboniferous, or the secondary strata; for frequently this and the feature together, with some notion of the thickness of the beds, will be our sole guides in tracing a boundary for some distance. In conformable beds, where the dip is at a low angle of 3° or 4°, a very small irregularity in the ground, a gentle hollow, may cause the boundary line to run a long way from the strike or general line of outcrop of the beds along a level surface; whereas, with a high dip, the lower bed would run but a short distance even in a deep valley. Care must be taken to avoid mistaking false or current bedding for the dip. In the tertiary strata, from the general absence of hard beds, few dips can be taken; but they usually maintain great regularity and horizontality. In tracing the boundaries along escarpments, the ground is often obscured by rain-wash and small land-slips.

The superficial deposits are usually well shown in ditch sections, besides the numerous brick-yards or gravel-pits.

Of course it is impossible here to do more than give a rough idea of the nature of the evidence which guides the geologist in his survey. The work is not to be learnt from books, but can only be gained by experience in the field. The object of the Government geological survey is to obtain all the information on the geology of the country. The coal-crops that are laid down on the maps are, to a great extent, obtained from colliery sections and data. Much information about mining has to be gleaned from private individuals; and the late Professor Jukes tells how he has had to spend many days in search of some old fellow who had left the district, but who was said to be able to "tell him all about it."

The accompanying Plate (XCIII.), upon which the principal classes of stratified rocks are represented, will enable us to point out more readily the objects of a geological survey. A glance at the index will give the order of succession of the rocks; and it is very important to bear in mind the succession of all the British strata, the unconformities, overlaps, and faults which affect their arrangement, in order to thoroughly understand the phenomena exhibited even in such a small area as this. Thus the newer deposits, the alluvium and the gravel, rest irregularly on the rocks beneath; the oolites, lias, and trias-rocks of Secondary age-are all conformable, and they rest indifferently on the upturned edges of the older (Palæozoic) rocks,

from the coal-measures to the mountain limestone. These latter are themselves conformable, and have in this area been together upheaved and bent into a fold or "anticlinal," and the summit of it worn away before the newer secondary rocks were deposited. The accompanying horizontal section, taken along the line marked on the map, will show the general arrangement of the rocks. It is the knowledge of an anticlinal like this which leads to the inference that coal occurs on the south of the Mendips, where it is not actually seen nor has yet been proved. The coal comes to the surface on the north of this range of hills, and is there largely worked; while, on the south of the anticlinal, as represented in the section, the coal-measures occur again, though they are entirely concealed at the surface by the overlying Secondary deposits. Such a conclusion shows us one of the practical bearings of geology.

A study of the science will enable us to point out not only where the coal-measures exist at the surface, but generally with great confidence those areas where coal cannot exist, and also to estimate the probability of its existence beneath those rocks which were formed subsequently to it. Thus it is that coal is considered as likely to occur in the south-eastern counties of England, beneath the tertiary and secondary rocks of that area; it being considered that there is a thinning out of the rocks which normally occur between the cretaceous beds and the coal, so that it might be reached at a reasonable depth.

In our section we find that a slight "fault" or disturbance affects the older rocks, and has shifted and let down those on its southern side. This shows more plainly on the map, as in consequence of it the outcrop of the Millstone Grit has been considerably modified. The phenomena exhibited in the section show that there are many important considerations to affect our ideas of the rocks at some depth below those coloured on our maps. Though the series has a regular arrangement, many rocks may be absent in places, and the older ones affected by denudation and disturbance before the newer ones were deposited upon them; so that in mining and well-sinking the opinion of the geologist is very necessary. It is astonishing that even nowa-days so many attempts are made to find coal in places where a geologist would at once discern there could be no chance of getting it. Two years ago we came across a shaft which was being sunk in the lower limestone shales of the Mendips, and had indeed just reached the old red sandstone, two or three thousand feet below the coal-measures, which had been denuded off!

The chief economic uses of geological maps are to point out the localities where may be found limestone for building purposes and to be burnt for lime; sandstones and grits, fit for

building and paving, for millstones, &c.; sands and gravels, for making paths and mending roads; clays and loams, for the manufacture of tiles and bricks; slates, for roofing purposes and for school-boys to draw upon; marbles, for the sculptor and for ornamental purposes; granites, for building and for road-metal.

Of course the economic value of each rock cannot be made out from the map alone; this is a point on which reference must be made to a published memoir or explanation of the map, upon which alone the boundaries and extent of the rocks can be depicted.

The relation between health and geology is also a point which has in recent years received a good deal of attention; and maps have been published and memoirs written to show the relations between certain forms of disease and geological structure-even between geology and lunatics! It is well known, indeed, that a gravelly, sandy, or chalky soil is more healthy than a clay foundation, because the former are pervious to water, and the latter is impervious. On the former there is less consumption than on the latter, as Mr. Whitaker and Dr. Buchanan have clearly demonstrated: the artificial removal of sub-soil water has, however, largely decreased it. Again, the water-supply is a most important subject, for in some small country villages and towns the inhabitants suffer very much from this cause. Situated perhaps on elevated ground, with a good porous soil, they yet suffer because of the disgraceful state of the drainage, the wells being shallow and the sewage, even the churchyards, draining into them. The cause of teetotalism will not find many admirers when it is known that women and children suffer most from drinking impure water, while the men who take their beer are less subject to disease.

Enough has been said to show the many practical uses of geology, and the importance of geological maps. To the agriculturist their value is of an indirect character; for although the soils are not laid down on the maps, yet in almost every case they bear a direct relation to the sub-soil beneath, being generally to a great extent formed out of it.

We need hardly dwell upon the interest imparted to a tour or journey from an acquaintance with geology, it has been so frequently remarked upon. Undoubtedly, a good geological map is to the traveller the readiest, if not the best, mode of obtaining information. It furnishes a sort of index to the geology of a country, and is for most purposes to be preferred to any written description or guide, though if possible both should be taken together. In the British isles, as we have pointed out, we are well provided with geological maps, and there are many excellent ones of the greater part of Europe,