be enabled to fix something like absolute dates to the recurrence of these great cosmical phenomena.

As we seek to explain the breccias and conglomerates of the stratified formation, so we endeavour to account for those of igneous origin—namely, by appealing to what is now taking place in centres and areas of volcanic activity. By so doing there is no great difficulty in understanding how, in some instances, volcanic ejections-such as sand and lapilli, and other triturated fragments-might form a conglomerate or "agglomerate," while in others irregular fragments and bombs would be converted by the cementing of dust and ashes into a veritable breccia. Such variable and fragmentary compounds occur among the igneous rocks of all ages, from the Silurian up to those of the present day, and much that is puzzling in their composition depends upon whether they have been aggregated on dry land near and around the centres of eruption, or whether they have been showered abroad and been deposited in water along with pebbly fragments of true aqueous origin. It is this circumstance that renders the history of many trap conglomerates, breccias, and ash-beds so obscure; and geologists can never hope to arrive at any satisfactory solution of their origin unless through a consideration of the means by which similar compounds are aggregated and consolidated at the present day. As we see scoriæ, lapilli, and bombs cemented by ashes and by overflows of volcanic mud on land, or consolidated by aqueous debris when deposited in adjacent waters, so we may rely they were aggregated, cemented, and consolidated in former ages-each case bearing, when minutely examined, some evidence of its own special origin and history.

Such is a slight endeavour to indicate the history of the breccias and conglomerates that occur in every formation,

and which constitute no inconsiderable portion of the masonry of the rocky crust. As existing operations vary with the latitude-tropical, temperate, or polar-and act with greater or less intensity according to local conditions, so we may rely they varied in former ages; and it is only by taking a wide survey of nature, and believing in the uniformity of her agencies, that we can ever hope to arrive at a satisfactory explanation of the rocks and rock-formations of our planet. To appeal, in the case of breccias and conglomerates, however vast in mass or heterogeneous in composition, to cataclysmal and revolutionary forces, when existing forces are sufficient to the explanation, were to shrink from that labour of investigation and patient deduction upon which have ever been founded all that is true and lasting in philosophy, and to substitute instead a mere mass of conjecture as variable and incoherent as the fancies of their suggestors. The masses of the olden time may in some localities be more gigantic and heterogeneous than anything we witness in the same localities at the present day, but they are the same in kind, and are merely the result of similar forces acting through longer periods and under more favourable conditions of local accumulation. We know that sea and land are ever shifting places, and that with every change in the position, contour, and relief of the land, new climatic conditions are engendered; and there is no difficulty in conceiving how, by such terraqueous shiftings, the glacier and iceberg may have once played their part over the latitudes of Britain, as they now operate over the latitudes of Greenland and the islands of the Arctic Ocean.

MAPPING OF SUPERFICIAL ACCUMULATIONS.

EVERY one is aware of the value of a good reliable geological map, exhibiting by distinctive colours the position and extent of the various rock-formations, and by appropriate marks and signs the outcrops, inclinations, and dislocations of their respective strata. With such a map, and a few well-chosen sections, the geological reader is in possession. of a fund of information which volumes might fail to convey; and where the contour-lines are marked, or a graduated system of hill-shading is adopted, he has at command all, or nearly all, that is necessary to complete a conception of the physical framework of a country. But this framework of rocks, of heights and hollows, however accurately delineated, can afford him but little idea of the superficial aspects of the land-its soils and subsoils, its mosses, moorlands, sand-dunes, and carse-clays, upon which its amenity and agricultural value are so intimately dependent. What is needed for this purpose is the laying down of these surface-formations on a separate sheet in distinctive colours, and with the same precision as the older rock-systems. We have long advocated the necessity for two sets of geological maps one exhibiting the underlying rock-formations as is ordinarily done, and another the superficial accumulations

which mask these formations, and frequently have no lithological connection with nor characteristic dependence upon them. In the present paper we renew this advocacy, and give some of the reasons upon which it is founded.

Were soils and subsoils always derived by disintegration from the subjacent rocks, there might be no great necessity for a set of superficial maps; but in nine cases out of ten these soils and subsoils have no connection with the rocks on which they rest, but have been brought from a distance, and in many instances of an altogether opposite character. The sands and gravels, for example, which occupy so much of Strathmore, Strathearn, and Stratheden, in Scotland, have little or no connection with the underlying Old Red Sandstone; and the geological map that indicates these places merely as Old Red Sandstone, conveys no idea whatever of the immense masses of glacial drift which constitute their surface. Again, a vast extent of estuary silts constitutes the Carse of Gowrie and the Carses of Falkirk and Stirling; but no one looking at an ordinary geological map could learn anything of this fact; or if these carseclays were indicated by distinctive colours, these would obscure the Middle Old Red that lies beneath the former, and the Coal-formation on which the latter reposes. Further, no one could know from our ordinary geological maps that the sand-dunes which extend so largely between the Tay and the Eden reposed on boulder-clay, and this again on the Old Red Sandstone; or that the recent marine silts of the Lincolnshire fens were underlaid by drifts and clays, and these again by the Chalk and Greensand. Or, still further, who could know from a map of the old rock-formation where peat-mosses, lake-silts, or boulder-clays occurred; could understand where the old rocks came hard and bare to the surface, or where masked by superficial accumulations many fathoms in thickness ?

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It is true that on the maps of the Geological Survey an attempt is made to indicate "alluvium, peat-moss, &c.," by a distinctive colour, but this attempt more frequently misleads than instructs. Lithologically speaking, peat-moss has no connection with alluvium; they are formed by totally different agencies; and the former may be thousands years old, while the latter is still in course of formation. Not only so, but the same pale tint that indicates "alluvium" includes not only recent river and lake silts, but old lake-silts, glacial sands, and gravels and clays, thus confounding things not only of totally different composition and character, but of totally different chronology. What is wanted is a separate map, indicating by different colours the position and extent of recent river alluvia, of lake-silts, marls, peat-mosses, ancient estuary deposits or carse-clays, boulder-clays, glacial sands, and gravels-of all those accumulations, in fine, which overlie the old rock formations, and which has each its own geological history in time as well as in mode of formation. Such a mapping could not fail to be of use to the farmer, to the land-agent, landscapegardener, civil engineer, road-maker, and all those whose business leads them to deal with the soils, subsoils, and general superficies of a country.

Many years ago we constructed such a map of Fifeshire, and the late Professor Johnston, while chemist to the Highland Society, invariably exhibited it along with the ordinary geological map when illustrating to the farmers of that county the intimate connection between geology and agriculture. Had he exhibited the mere rock-formations, the connection would have been apparent only in the case of the trap-soils, which constitute but a small fraction of the county, the greater portion being composed of old glacial clays and gravels, of lake-silts, peat-mosses, and sand-drifts, which have no lithological relationship what

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