Sand and gravel of the boulder--What they suggested--Their consideration leads us among the more mechanical operations of Nature--An endless succession of mutations in the economy of the universe--Exhibited in plants In animals--In the action of winds and oceanic currents--Beautifully shown by the ceaseless passage of water from land to sea, and sea to land--This interchange not an isolated phenomenon--How aided in its effects by a universal process of decay going on wherever a land surface is exposed to the air--Complex mode of Nature's operations--Interlacing of different causes in the production of an apparently single and simple effect--Decay of rocks--Chemical changes--Underground and surface decomposition--Carbonated springs--The Spar Cave--Action of rain-water--Decay of granite--Scene in Skye--Trap-dykes--Weathered cliffs of sandstone--Of conglomerate--Of shale--Of limestone--Caverns of Raasay--Incident--Causes of this waste of calcareous rocks--Tombstones, 138 CHAPTER IX. Mechanical forces at work in the disintegration of rocks--Rains--Landslips--Effects of frosts--Glaciers and icebergs--Abrading power of rivers--Suggested volume on the geology of rivers--Some of its probable contents--Scene in a woody ravine--First idea of the origin of the ravine one of primeval cataclysms--Proved to be incorrect--Love of the marvellous long the bane of geology--More careful examination shows the operations of Nature to be singularly uniform and gradual--The doctrine of slow and gradual change not less poetic than that of sudden paroxysms--The origin of the ravine may be sought among some of the quieter processes of Nature--Features of the ravine Lessons of the waterfall--Course of the stream through level ground--True history of the ravine--Waves and currents--What becomes of the waste of the land--The Rhone and the Leman Lake--Deltas on the sea-margin--Reproductive effects of currents and waves--Usual belief in the stability of the land and the mutability of the ocean--The reverse true--Continual interchange of land and sea part of the economy of Nature--The continuance of such a condition of things in future ages rendered probable by its continuance during the past, 157 CHAPTER X.