In this view may also be found an explanation of the movement of glaciers for considerable distances on upward slopes, even when the _surface_ as well as the base is inclined backwards. So far does this go that superglacial streams sometimes run for some distance _backwards_, i.e. toward the heads of the glaciers, while in other places surface-waters are collected into ponds and lakelets. Such a slope of the surface of ice is not difficult to understand if the movement be due to thrust from behind, or if it be occasioned by internal crystalline changes acting upon a rigid body; but it must be regarded as very remarkable if the movement be that of a fluid body, no matter how viscous, for the length of the acclivity is sometimes several times the thickness of the ice. Crevassing and other evidences of brittleness and rigidity find a ready elucidation under the view that the ice is a really solid body at all times, and that its apparent fluency is due to the momentary fluidity of small portions of the mass assumed in succession as compression demands. In addition to the considerations already adduced, it may be urged that a glacier does not flow as a stiff liquid because its granules are not habitually drawn out into elongated forms, as are cavities in lavas and plastic lumps in viscous bodies. Flowage lines comparable to those in lavas are unknown in glaciers. All this is strictly consistent with our primary thesis, that a glacier is a crystalline rock of the purest and simplest type, and that it never has other than the crystalline state. This strictly crystalline character is incompatible with viscous liquidity. _Other Views of Glacier Motion._[137] While these views of glacial motion seem to us to best accord with the known facts, they are not to be regarded as established in scientific opinion, or as the views most commonly held. The mode of glacial motion has long been a mooted question, and is still so regarded. The main alternative interpretations that have been entertained are the following: (1) In the early days of glacial studies De Saussure thought that glaciers slid bodily on their beds; (2) Charpentier and Agassiz referred the movement to the expansion of descending water freezing within the glacier; (3) Rendu and Forbes, followed by many, perhaps most, modern writers, believed ice to be viscous, and that in sufficiently large masses it flows under the influence of its own weight, like pitch or asphalt; (4) Others, realizing the fundamental difference between crystalline ice and a true viscous body, have fallen back on a vague notion of plasticity which scarcely amounts to a definite hypothesis at all; (5) Tyndall urged that the movement was accomplished by minute repeated fracturing and regelation, appealing to the fact that broken pieces of ice slightly pressed together at melting temperatures freeze together, but neglecting the fact that this would destroy the integrity of the crystals;