VII. GEOLOGICAL MAPS: THEIR RELATION TO AGRICULTURE AND COAL SUPPLY.

1. A Geological Map of England and Wales. By G. B. Greenough, Esq., F.R.S. (on the basis of the Original Map of William Smith, 1815). New edition, revised and improved, under the superintendence of a Committee of the Geological Society of London, from the Maps of the Geological Survey of Great Britain, 1836-63, and Maps and Documents contributed by Sir R. I. Murchison, Professor Phillips, Joseph Prestwich, R. A. C. Godwin-Austen, and others. Six sheets. Scale, nearly six miles to one inch. Published by the Geological Society, July, 1865.

2. Geological Map of England and Wales. By Andrew C. Ramsay. Third edition. One sheet. Scale, twelve miles to one inch. London: Stanford, 1866.

3. First Sketch of a new Geological Map of Scotland. By Sir R. I. Murchison, Bart., K.C.B., F.R.S., and Archibald Geikie, F.R.S.E., F.G.S. One sheet. Scale, twenty-five miles to one inch. London, 1861.

4. General Map of Ireland, to accompany the Report of the Railway Commissioners, showing the principal Physical Features and Geological Structure. Geologically Coloured by Sir R. J. Griffith, Bart., LL.D., in 1855. Six sheets. Scale, four miles to one inch. London and Dublin.

5. Geological Map of Ireland, to accompany the Instructions to Valuators appointed under the 15th and 16th Vic., cap. 63. Reduced from the large map of 1855. Scale, about seventeen miles to one inch.

6. The Geology of the Country round Stockport, Macclesfield, Congleton, and Leek. Explanatory of Quarter Sheets, 81 N.W. and S. W. of the Map of the Geological Survey of Great Britain. By Edward Hull, B.A., F.Ğ.S., and A. H. Green, M.A., F.G.S. London, 1866.

THAT geological maps are often puzzles to the public is not wonderful; but that they are sometimes a source of discussion to men of science is much to be deprecated. If, at the outset, we endeavour to answer comprehensively the question, What is a geological map? we are met with some considerable difficulties, and we can only pretend to do so in accordance with the practice of our geological surveyors, and not with what we conceive to be the fundamental principle of the subject.

* See Reader,' March 31, 1866.

A geological map of any given district ought to be as nearly as possible a faithful representation of the outlines and superficial extent of those rock-formations which occur nearest to the surface in that area the vegetable soil being disregarded. But in practice we generally find that the geologists disregard, not only the vegetable soil, but an important series of "detrital" or "superficial" deposits which overlie and mask the more regularly stratified formations. It therefore becomes interesting to ascertain why this should be the case, and for what reason these so-called "superficial deposits" should be treated as something differing in kind from the rest of the rock-masses which form the crust of the globe.

From a scientific point of view, it is easy to draw one great distinction between the "drift" or "superficial" deposits and other formations-namely, that whereas the latter generally conform to certain general rules of dip, strike, &c., and bear a more or less definite relation to the strata above and below them, the latter are altogether irregular in their occurrence, distribution, and inclination. Doubtless exceptions may be quoted which would appear to disprove the correctness of even this wide distinction; but we are inclined to think that it is, nevertheless, correct in so far as it expresses a broad general fact. This being the case, it is certainly much easier to map the formations older than the glacial period than the glacial deposits, and the sands and gravels of more recent date. There is also no question that the distribution of the regularly stratified formations possesses a much higher scientific interest than that of gravel and other drift-deposits, which are of more or less uncertain age, and which are chiefly characterized by occurring anyhow.

From an economic point of view it is not quite so easy to pronounce in favour of one plan to the exclusion of another; but we think that the farmers of England have as great a right to the consideration of the Director-General of the Geological Survey as the miners and the geologists; in other words, it is expedient that the gravels and drifts should be mapped with the same care as the older formations, and more especially in districts which do not yield valuable minerals. A farmer who knows that some of his land has a gravelly subsoil, and the rest a clayey, wishes to ascertain the boundary-lines of the two deposits. To tell him that the clay is London clay, and is only irregularly covered by the postglacial gravel, is to give him information that he neither requires nor understands, and to ascertain which he would never spend a sixpence. We believe that this fact has of late been recognized by the directors of the Geological Survey, and the more recently published sheets of the survey-map of Great Britain contain either delineations or indications of the outcrops of the superficial deposits.

Fortunately for the farmers, they have a powerful ally in the opacity of even superficial formations; and the reflecting geologist is no doubt often staggered at the hardihood of surveyors who draw a wriggling line of outcrop to a stratum which is known to be buried fifty, a hundred, or more feet beneath drift-deposits, and beyond the ken of mortal men.

Much may be said, no doubt, on the part of the surveyors, who, to do them justice, are really not in the habit of mapping the outcrops of strata without some tangible warrant for so doing. If a given series of beds is observed where properly exposed to "behave" in a particular manner towards those above and below them, and towards the "form of the ground," it is reasonable to infer that they will do the same at a short distance, although they may not there be seen at the surface. Thus the geological surveyor has to take note of the most trivial circumstances that may be within the reach of observation, so as to decide for future guidance what phenomena are general and what accidental. In this manner, a roadside bank speaks volumes, and a ditch reveals wonders, while a railway-cutting affords an opportunity of luxurious surveying, which ought to stimulate the surveyor to take a sanguine view of the prospects of all projected lines.

After all, the fact remains, that perhaps the greatest desideratum in British geology is a good map of the surface-deposits of the United Kingdom. We have several first-rate geological maps (in the ordinary sense) of England and Wales, and of them all, the most generally useful is no doubt Professor Ramsay's beautiful sheet, the geology of the new edition of which is much improved, while the actual and ideal sections have required no change. The new edition of the Greenough Geological Map is probably the most accurate yet published, so far as the wretched topography of most of it will allow of geological precision; indeed, it embodies the whole of the information in the possession of the Geological Survey, and the Council of the Geological Society, almost up to the date of publication; at any rate, that portion of it which was considered sufficiently authentic to be published.

The only reliable geological map of Scotland, at present in existence, is the small one by Sir R. I. Murchison and Mr. Geikie; that is to say, if the views held by the great majority of geologists, in opposition to Professor Nicol, are correct. This map leaves very little to be desired except surface-geology and a larger scale; and we hope that when a new edition is called for, the scale will be increased. Of Ireland, the old map of Sir Richard Griffith and the small reduction of it still remain the best; but we hope they will soon be superseded by a map compiled by one of the officers of the Irish Survey, for that survey must necessarily have modified our knowledge of the distribution and limits of the several geological formations in that island.

Returning to the geological maps of England and Wales, it is noticeable that if we compare the more recent with the older editions, we shall find Sir Roderick Murchison's view of the Permian age of the St. Bee's and Corby sandstones adopted both by the Council of the Geological Society and by Professor Ramsay. This is a matter of practical as well as scientific importance; for if these sandstones be of Permian instead of Triassic age, the chance of coal occurring at a reasonable depth beneath them is very much increased, more especially as the Coal-measures themselves crop out and are extensively worked at Whitehaven, immediately to the north.

The question of coal-supply, by the way, appears at last to be attracting the attention of public men; but whether any practical result will flow from this revived interest appears more than doubtful. There are many considerations respecting the supply and consumption of coal, to which Mr. Jevons did not attach sufficient weight, and which would cause us to measure England's period of commercial supremacy more favourably than that author. Nevertheless, there is sufficient cause for our striving to answer the question-By what means can we put off the day when our manufactures will cease to be able to compete with those of cotton- and iron-producing countries? That things must come to that pass sooner or later is logically certain, and American manufactures may be able to compete with ours much sooner than we expect.

Closely connected with this inquiry, though not a matter of such vital importance to the nation, is the probable duration of our coal-fields; and in this view it is that the determination of those new districts where coal may still be wrought at moderate depths becomes such an important element in the solution of the problem. That the latent abundance of coal will not of itself ensure our commercial supremacy is sufficiently demonstrated by the present condition of America. The consideration of most consequence is the price at which the coal can be produced, and the relative abundance or scarcity is only one out of several elements which together determine the price.

Facts are not wanting which tend to show that the area of our coal-fields will probably be very much increased within a few years. We have already mentioned one case of such a probable extension, and it may be worth while to give another, which is the present result of an attempt to ascertain whether workable coal-seams occur at moderate depths beneath the Lower Permian and Triassic sandstones bordering the Poynton coal-field. This attempt is described by Mr. Hull in the memoir illustrating the quarter-sheets 81 N.W. and 81 S.W. of the Geological Survey-map of Great Britain; and he states that a seam of coal three feet in thickness, and unknown in the Poynton district, was passed through by driving

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a horizontal tunnel from the Park pit at a depth of 200 yards from the surface; "beyond this, Coal-measure shales and clays with Stigmaria were proved, and the tunnel was left off in a reddish grit, similar to those found in the Coal-measures of Denton and Hyde."

It is from private enterprizes of this nature that the area of our workable coal-seams will become extended as the geological surveyors make us acquainted with the exact structure of the districts bordering our known coal-fields; but there is a more speculative kind of "prospecting" which we venture to think is a more fit, though a much more novel, subject for national expenditure than a great many others, including even expeditions to the North Pole. We refer to the probability of coal existing at no enormous depth beneath the Tertiary and Cretaceous deposits of the south-east of England, just as it is known to do beneath the chalk of Valenciennes, where it has been extensively worked; and of Calais, where it has been reached by borings. Geologists have even mapped the probable direction and extent of the subterranean prolongation of one of our great coal-fields; but opinions are divided as to the particular English coal-field with which the French (and) therefore also the hypothetical London) field is connected. Mr. Godwin-Austen, who first started the notion of a London coal-field, considers it to be connected with the South Welsh and Midland coal-basins; but others hold the balance of probabilities to be in favour of the Northern coal-field. It would cost the nation comparatively little to demonstrate, by boring, the correctness or error of one or both of these views; and if coal were won, the expense might easily be defrayed, and a large source of revenue created, by a tax on the consequent yield.

VIII. ON A TEMPORARY OUTBURST OF LIGHT IN A STAR IN CORONA BOREALIS.

BY WILLIAM HUGGINS, F.R.S.

OCCASIONALLY, but at rather long intervals, men have been startled by the extraordinary spectacle of the sudden appearance of a brilliant star in a part of the heavens where before no star was to be seen.

During the first seventeen centuries of the present era, perhaps eight or nine of these strange visitants astonished the world. It is probable that many of the objects in the Chinese catalogue of Matuan-lin are not stars, but, as their observed motion appears to show,