Imagens das páginas

whole action of free-flowing tidal seas is that of an assorting one; and notably, even to the eye of the common observer, there is nothing like assortment among the materials of these Old Red Sandstone conglomerates.

The only competent agency with which we are acquainted is that of shore - ice acting for ages in conjunction with glacier transport from the adjacent lands. We can account for the piling up of such heterogeneous masses of shore-ice; but the power that kept up the supply must have been a terrestrial one, fracturing, wearing, and grinding down the old hills, and bearing the eroded material in its various stages of attrition to the sea-margin. Had it been a sudden and violent power, the angular blocks would have preponderated ; had it been of short duration, schistose and slaty fragments would have exceeded the quartzes and porphyries. As it is, angular blocks are the exception, and quartz-pebbles constitute the greater portion of the mass. And how well these smooth and rounded quartz-pebbles tell the tale of ages of waste and attrition ! In the hills of the Scottish Highlands from which these conglomerates were largely derived, quartz holds but a subordinate position in veins and pocket-like masses, the great bulk being gneisses, micaschists, chlorite schists, and other schistose rocks. How many hundreds of feet must have been worn down-what glens and corries must have been scooped out to supply such an enormous amount of quartz-pebbles as these Old Red Sandstone conglomerates contain ! And here, while the schists from which they were originally derived have been for the most part pounded to impalpable sediments, these quartz-pebbles remain as evidence of long-continued and enormous denudation—a testimony as convincing to the geologist as the steps of a demonstration in Euclid is to the mind of the mathematician.

Everything, therefore, in the composition and arrange

ment of the great conglomerates of the Old Red Sandstone points to the action of ice—tear and wear, and transport from inland by glacier, and piling and accumulation by icefloe on shore. Nor does it seem otherwise with the breccias and brecciated conglomerates of the Permian formation. Such masses of angular fragments as those of South Devon could never have been severed from their parent rocks save through the agency of ice, and no known power save floating ice could have carried them and piled them together without rounding and smoothing the chips and blocks of which they are composed. We have chiselled from these breccias chips of slate and fragments of Devonian limestone as fresh in their fractures and sharp in their angles as if they had been newly broken from their parent rocks, and which a few days' attrition in moving water would have sufficed to convert into pebbles or shingle. And when we turn to the Abberley and Malvern hills described by Professor Ramsay, and find blocks and boulders of all sizes indiscriminately huddled together—some rounded, some sharp and angular, and some boulders even grooved and striated like those of the “ boulder clay”—the evidence amounts to a demonstration that such brecciated masses are largely due to the fracturing, transporting, and aggregating agency of ice.* Nor is there anything in geological history—that is, in the slow shiftings of sea and land, the uprise and depression of continents, the change of ocean - currents, and the consequent alterations of climate—at variance with this conclusion. We witness at the present day large circumpolar areas subjected to ice and ice-action; and we know also that at the close of the Tertiary period a much larger area of the northern hemisphere was similarly subjected to boreal influences—forming "boulder clays," collecting "kaimes and osars," and grinding, smoothing, and striating all exposed rock-surfaces. What is taking place now, and was enacted during a former period, may have occurred during many former periods ; and nothing in the physical history of the globe forbids the conclusion that the aggregation of these Permian breccias and Old Red conglomerates was mainly due to the long continuance of glacial agencies over the regions in which they respectively occur. Dr Hooker, in his • Himalayan Journal,' makes the significant remark : " The further we travel, and the longer we study, the more positive becomes the conviction that the part played by these great agents (the glacier and iceberg) in sculpturing the surface of our planet is as yet but half recognised"— remark not less applicable to the phenomena of the older formations than to those of the Upper Himalaya by which it was originally suggested. How or by what means these alternating epochs of glacial intensity are brought about

* Professor Ramsay, who was the first to advocate, in a decided man. ner, the glacial origin of these breccias, founds his belief on the following evidences : 1. The great size of many of the fragments—the largest observed weighing (by a rough estimate) from a half to three-quarters of a ton. 2. Their forms. Rounded pebbles are exceedingly rare. They are angular or subangular, and have those flattened sides so peculiarly characteristic of many glacier-fragments in existing moraines, and also of many of the stones of the pleistocene drifts, and the moraine matter of the Welsh, Highland, Irish, and Vosges glaciers. 3. Many of them are highly polished, and others are grooved and finely striated, like the stones of existing Alpine glaciers, and like those of the ancient glaciers of the Vosges, Wales, Ireland, and the Highlands of Scotland ; or like many stones in the pleistocene drifts. 4. A hardened cementing mass of red marl, in which the stones are very thickly scattered, and which in some respects may be compared to a red boulder clay, in so far that both contain angular, flat-sided, and striated stones, such as form the breccias wherever they occur. -—'Journal of Geological Society,' vol. xi.

-whether by astronomical causes at fixed periods, or by mere variation in the distribution of land and water at irregular times -- science in the mean time is unable to determine. The question, however, is under investigation, and many years may not pass by when the astronomer will 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.

« AnteriorContinuar »