But besides the differences in solidity and in external appearance, there are also many other important changes taking place in the ice of these different regions, to which we shall return presently. Such modifications arise chiefly from the pressure to which it is subjected in its downward progress, and to the alterations, in consequence of this displacement, in the relative position of the snow- and ice-beds, as well as to the influence exerted by the form of the valleys themselves, not only upon the external aspect of the glaciers, but upon their internal structure also. The surface of a glacier varies greatly in character in these different regions. The uniform even surfaces of the upper snow-fields gradually pass into a more undulating outline, the pure white fields become strewn with dust and sand in the lower levels, while broken bits of stone and larger fragments of rock collect upon them, which assume a regular arrangement, and produce a variety of features most startling and incomprehensible at first sight, but more easily understood when studied in connection with the whole series of glacial phenomena. They are then seen to be the consequence of the general movement of the glacier, and of certain effects which the course of the seasons, the action of the sun, the rain, the reflected heat from the sides of the valley, or the disintegration of its rocky walls, may produce upon the surface of the ice. In the next article we shall consider in detail all these phenomena, and trace them in their natural connection. Once familiar with these facts, it will not be difficult correctly to appreciate the movement of the glacier and the cause of its inequalities. We shall see, that, in consequence of the greater or less rapidity in the movement of certain portions of the mass, its centre progressing faster than its sides, and the upper, middle, and lower regions of the same glacier advancing at different rates, the strata which in the higher ranges of the snow-fields were evenly spread over wide expanses, become bent and folded to such a degree that the primitive stratification is nearly obliterated, while the internal mass of the ice has also assumed new features under these new circumstances. There is, indeed, as much difference between the newly formed beds of snow in the upper region and the condition of the ice at the lower end of a glacier as between a recent deposit of coral sand or a mud-bed in an estuary and the metamorphic limestone or clay slate twisted and broken as they are seen in the very chains of mountains from which the glaciers descend. A geologist, familiar with all the changes to which a bed of rock may be subjected from the time it was deposited in horizontal layers up to the time when it was raised by Plutonic agencies along the sides of a mountain-ridge, bent and distorted in a thousand directions, broken through the thickness of its mass, and traversed by