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,

besides many illustrating the geology of other countries, particularly North America, India, and Australia, the results of private labour, and of our colonial or foreign geological

surveys.

Such is the intimate connection between physical features and geological structure, that to the geologist a glance at the geological map of a country would enable him to obtain a very good idea of the character of its scenery. Denudation and the origin of scenery are most interesting subjects; how hard and soft rocks are acted upon by rain, river, and sea; how our bays and gulfs, islands and straits, cliffs and gorges, hills and valleys, were formed, are subjects now much discussed: they link geology with physical geography. Elevation gave the plan; denudation did the work. Geology is but the "physical geography of past ages;" we must interpret the past by the light of the present. Everything betokens change, and we cannot but sigh when we think of the vast amount of denudation that is going on. At this rate, by atmospheric agencies alone, a mass of land as large as Europe must, according to a rough estimate of Professor Geikie, disappear in about 4,000,000 years! What use, then, will our geological maps be? However, if we have thrown any light upon them, and pointed out some of their uses, so that they may be more fully appreciated at the present day, we may feel happy that our labour has not been lost.

35

THE BATTLE OF LIFE AMONG PLANTS.

BY MAXWELL T. MASTERS, M.D., F.R.S.

EVERY

VERY day, every hour, there is going on around us a veritable death-struggle. It excites little attention. People would be in no hurry to read the telegraphic despatches concerning it from the seat of war, even if there were any to read. Special correspondents there are, but their letters are appreciated but by a few. Nevertheless, it cannot be said that mankind in general is not interested in the result of the struggle. On the contrary, little as the affair is heeded, it is of very serious import to the human race. Our food-supplies depend on it; the well-being of our flocks and herds is essentially dependent on it; the building of our houses, the fabrication of our raiment, are to a large extent contingent on it; nay, the soil beneath our feet, and the very sky above our heads, are materially, very materially, influenced by the result of the contest of which we are about to speak. Edward Forbes was wont to say that the movement of a periwinkle over a rock might be of greater consequence to the human race than the progress of an Alexander; and the results of the wars of the plants are assuredly of no less importance, seeing that the very existence of an Alexander depends in no slight degree upon them. The campaigns we speak of are real; they are not mental figments, or allegorical illustrations. Success in the practice of horticulture, of agriculture, of forestry, depends on the action we men take towards the combatants. If we remain neutral, the weakest goes to the wall, overpowered by the stronger; if we interfere, we exert a very powerful influence for the time; but immediately we cease to exert our power, the combat begins again, and with enhanced violence. The essence of successful cultivation often consists almost entirely in the removal of the plant from the influence of that hostile "environment" to which, under natural circumstances, it would be subjected. It is this that accounts, in a great measure, though of course not wholly, for the oft-observed fact