Besides this action in winter, which goes on more or less in every country wherever the temperature sinks sufficiently low to permit of the freezing of water, ice effects many changes on the surfaces of rocks when it takes the form of glaciers and icebergs. We have already noted the operation of a glacier during its slow progress in crushing down large fragments of stone, scratching and abrading the rocks over which it passes, and eventually producing a vast quantity of mud, which is carried down by streams to form new accumulations either in lakes or seas. We have also marked the effects of the drifting iceberg in materially modifying the contour of submarine hills, and depositing over the ocean-bottom mud, gravel, and boulders. Nothing further, therefore, need be done here than simply to keep these agencies in view, as playing an important part in the disintegration of rocks.
Another highly interesting aqueous action is that of streams and rivers, in scooping out for themselves channels through sometimes the hardest and most solid rock. Such effects may be seen all over the globe, in the old world and in the new, in the bed of the tiniest rivulet, as well as in the course of the mightiest river. And accordingly, in all the long list of geological agents, we find none so well known and so often described alike by poets, historians, and scientific writers, as well in ancient as in modern times. What a delightful volume might be written about the geology of rivers! It would, perhaps, begin with that "great river," the Euphrates, along whose green banks lay the birthplace of the human race, tracing out the features of its progress from the ravines and cataracts of Armenia, with all their surrounding relics of ancient art, down into the plains of Assyria, amid date-palms and Arab villages, onwards to the mounds of Nineveh and Babylon, and thence to the waters of the Persian Gulf. Well-nigh as remote, and perhaps still more interesting in its human history, would be the story of the Nile. We should have to follow that river from the mystic region of its birth,[53] marking the character of the rocks through which winds its earlier channel, and the effects upon them of the floods of untold centuries; it would be needful, too, to note the influence of the waters on the lower grounds, from where the stream flows over the cataracts of Syene, down through the alluvial plains of Egypt; and lastly, the concluding and perhaps most onerous part of our labour would be the investigation of the delta, marking its origin and progress, its features in ancient times, as made known to us in the graphic chapters of Herodotus, and the changes which the lapse of more than twenty centuries has since wrought in its configuration. The rivers of Europe would detain us long, not less perhaps by their historic interest than by the variety and attractiveness of their physical phenomena. One could scarce help lingering over the Rhine, with its source among Alpine glaciers, its lakes and gorges, its castles and antique towns; and when once the narrative entered the classic ground of Italy, it would perhaps become more antiquarian than geological. The ravine of Tivoli, for instance, would certainly lay claim to a whole chapter for itself, with its long-continued river action, its ancient travertin, its beautiful calcareous incrustations, and above all its exquisite scenery.
[53] "Fontium qui celat origines Nilus" a description not less true now than in the days of the Sabine bard.
"Domus Albuneæ resonantis,
Et præceps Anio, ac Tiburni lucus, et uda
Mobilibus pomaria rivis."[54]
"Albuna's grey re-echoing home,
And Anio, headlong in his foam,
And grove of Tivoli,