In our own country, and in many other parts of the world, we find fragments of stone distributed over the surface, sometimes in the form of enormous blocks, bearing in their fresh angles evidence that they have been little disturbed since their disruption, but sometimes much rubbed and worn, and broken into smaller pieces, till they form what is known as gravel. In many cases the original rock from which these masses have been separated does not exist in the vicinity of their locality; and it is not till we reach a distance, often of hundreds of miles, that we find the formation of which they are a component part. Various causes have been suggested for the transport of these erratic blocks, of which the most satisfactory is the agency of ice, either as slow-moving glaciers, or as oceanic icebergs. "The common form of a glacier," says Professor J. Forbes, "is a river of ice filling a valley, and pouring down its mass into other valleys yet lower. It is not a frozen ocean, but a frozen torrent. Its origin or fountain is in the ramifications of the higher valleys and gorges, which descend amongst the mountains perpetually snow-clad. But what gives to a glacier its most peculiar and characteristic feature is, that it does not belong exclusively or necessarily to the snowy region already mentioned. The snow disappears from its surface in summer as regularly as from that of the rocks which sustain its mass. It is the prolongation or outlet of the winter-world above; its gelid mass is protruded into the midst of warm and pine-clad slopes and green-sward, and sometimes reaches even to the borders of cultivation."[37] The glacier moves onward with a slow but steady march towards the mouth of its valley. Its lowest stratum carries with it numerous fragments of rock, which, pressed by the weight of the mighty mass, scratch and indent the surfaces over which they move, and sometimes polish them. These marks are seen on many rock-surfaces now exposed, and they are difficult to explain on any other hypothesis than that of glacial action. But the alternate influence of summer and winter--the percolation of rain into the mountain fissures, and the expansion of freezing--dislodge great angular fragments of rock, which fall on the glacier beneath. Slowly but surely these then ride away towards the mouth of the valley, till they reach a point where the warmth of the climate does not permit the ice to proceed; the blocks then are deposited as the mass melts. But if the climate itself were elevated, or if the surface were lowered so as to immerse the glacier in the sea, it would melt throughout its course, and then the blocks would be found arranged in long lines or _moraines_, such as we see now in many places.