Fragments of Earth Lore: Sketches & Addresses Geological and Geographical. Geikie James

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СКАЧАТЬ first section to which reference may be made is exposed in the course of the River Jed, at Allars Mill, a little above Jedburgh. This section is famous in its way as having been described and figured by Dr. Hutton, who may be said to have founded the present system of physical geology. In the bed of the stream are seen certain confused ridges of a greyish blue rock running right across the river course – that is, in a direction a little north of east and south of west. These ridges are the exposed edges of beds of greywacké and shale, which are here standing on end. The beds are somewhat irregular, being inclined from the vertical, now in one direction and now in another, or, as a geologist would say, the “dip” changes rapidly, sometimes being up the valley and sometimes down. The same beds continue up the steep bank of the river for a yard or two, and are there capped by another set of rocks altogether, namely, by soft red sandy beds which at the bottom become conglomeratic– that is to say, they are charged with water-worn stones. The annexed diagram will show the general appearances presented: g represents the vertical greywacké and shale, and c the overlying deposits of conglomerate and red sandy beds. Now let us see what this section means. What, in the first place, is greywacké? The term itself has really no meaning, being a name given by the miners in the Harz Mountains to the unproductive rocks associated with the vein-stones which they work. When we break the rock we may observe that it is a granular mixture of small particles of quartz, to which sometimes felspar and other minerals are added. The grains are bound together in a hardened matrix of argillaceous or clayey and silicious matter, blue, or grey, or green, or brown and yellow, as the case may be. At Allars Mill, and generally throughout the Cheviot district, the prevailing colour is a pale greyish blue or bluish grey; but shades of green and brown often occur. The component particles of the rock are usually rounded or water-worn. Again, we notice that the ridges and bands of rock that traverse the course of the Jed at Allars Mill are merely the outcrops of successive strata or beds. It is clear then that greywacké and the grey shales that accompany it are aqueous rocks – that is to say, they consist of hardened sediment, which has undoubtedly been deposited in successive layers of variable thickness by water in motion. But since the sediments of rivers and currents are laid down in approximately horizontal planes, it is evident that if the greywacké and shale be sedimentary deposits they have suffered considerable disturbance since the time of their formation; for, as we have seen, the beds, instead of being horizontal or only gently inclined, actually approach the vertical. The fact is, that the outcrops which we see are only the truncated portions of what were formerly rapid undulations or folds of the strata, the tops of the folds or arches having been cut away by geological agencies, to which I shall refer by-and-by. What were at one time horizontal strata have been crumpled up into great folds, the folds being squeezed tightly together, and their upper portions planed away before the overlying red sandy beds were laid down. The accompanying diagram may serve to make all this clearer. Let A A represent the present surface of the ground, and B B a depth of say fifty feet or a hundred feet from the surface. The continuous lines between A and B represent the greywacké beds as we now see them in section; the dotted lines above A A indicate the former extension of the strata, and the dotted lines below B B their continuation below that datum line. Hence it is obvious that in a succession of vertical or highly inclined beds, we may have the same strata repeated many times, the same beds coming again and again to the surface. Thus the stratum at S is evidently the same bed as that at W, X, Y, and Z.

      Such great foldings or redoublings of strata are most probably originated during subsidence of a portion of the earth’s crust. While the ground is slowly sinking down, the strata underneath are perforce compelled to occupy less space laterally, and this they can only do by yielding amongst themselves. All folding or contortion on the large scale – that, namely, which has affected areas of strata extending over whole countries – seems to have taken place under great pressure; in other words, to have been produced at considerable depths from the earth’s surface. We can conceive, therefore, of a wide tract of land sinking down for hundreds of feet, and producing at the surface comparatively little change. But a depression of a few hundred feet at the surface implies a considerably greater depression at a depth of several thousand feet from the surface, and it is at great depths, therefore, that the most violent folding must take place. Consequently considerable contortion, and much folding, and lateral crushing and reduplication of strata may occur, and yet no trace of this be observable at the surface, save only a gentle depression. For example, in Greenland, a movement of subsidence has been going on for many years – the land has been slowly sinking down. The rocks at the surface are of course quite undisturbed by this widely-extended movement, but the strata at great depths may be undergoing much compression and contortion. It follows from such considerations, that if we now get highly contorted strata covering wide areas at the surface, we suspect that very considerable denudation has taken place. That is to say, large masses of rock have been removed by the geological agents of change, so as to expose the once deeply-buried tops of the arched or curved and folded strata. We may therefore infer from a study of the phenomena in the Jed at Allars Mill, first, that the red sandy beds are younger than the greywacké and shale, seeing that they rest upon them; and, second, that a very long period of time must have elapsed between the deposition of the older and the accumulation of the younger set of strata; for it is obvious that considerable time was required for the consolidation and folding of the greywacké, and an incalculable lapse of ages was also necessary to allow of the gradual wearing away by rain, frost, and running water of the great thickness of rocks underneath which the greywacké was crumpled. And all this took place before the horizontally-bedded red sandstone and conglomerate gathered over the upturned ends of the underlying strata. The succession of rocks at Allars Mill is seen in many other places in the Cheviot district, but enough has been said to prove that the greywacké beds are the older of the two sets of strata.

      There is another class of rocks, the relative position of which we must now ascertain, for no one shall wander much or far among the Cheviots without becoming aware of the existence of other kinds of rock than greywacké and sandstone. Many of the hills east of Oxnam and Jed Waters, for example, are composed of igneous masses – of rocks which have had a volcanic origin. As we shall afterwards see, the whole north-eastern section of the Cheviots is built up of such rocks. At present, however, we are only concerned with the relation which these bear to the greywacké and the red sandy beds. Now at various localities – for example, in Edgerston Burn, on the hill-face south of Plenderleith, and again along the steep front of Hindhope and Blackball Hills, which are on the crest of the Cheviots – we find that the igneous rocks rest upon the greywacké and shale precisely in the same way as do the red sandy beds. They therefore belong to a later date than the greywacké. In other places, again, we meet with the conglomerates and red sandstones (c, resting upon and wrapping round the igneous rocks, i, and thus it becomes quite obvious that the latter occupy an intermediate position between the greywacké and shale on the one hand, and the conglomerate and red sandstone upon the other.

      We have now cleared the way so far, preparatory to an attempt to trace the geological history of the Cheviots. The three sets of rocks, whose mutual relations we have been studying, are those of which the district is chiefly composed; but, as we shall see in the sequel, there are others, not certainly of much extent, but nevertheless having an interesting story to tell us. Nor shall we omit to notice the superficial accumulations of clay, gravel, sand, silt, alluvium, and peat; monuments as they are of certain great changes, climatic and geographical, which have characterised not the Cheviots only, but a much wider area.

      II

      If we draw a somewhat straight line from Girvan, on the coast of Ayrshire, in a north-east direction to the shores of the North Sea, near Dunbar, we shall find that south of that line, up to the English border, nearly the whole country is composed of various kinds of greywacké and shale like the basement beds of the Cheviot district. Here and there, however, especially in certain of the valleys and some of the low-lying portions of this southern section of Scotland, one comes upon small isolated patches and occasional wider areas of younger strata, which rest upon and conceal the greywackés and shales. Such is the case in Teviotdale, the Cheviot district, and the country watered by the lower reaches of the Tweed, in which regions the bottom beds are hidden for several hundreds of square miles underneath younger rocks. Indeed, СКАЧАТЬ