This paper concludes the earlier researches of the author and his colleagues on the development of glacier ice from the snow crystal. It traces the crystal growth down to the end of the glacier and records the distribution of large crystals in glacier tongues. Various agencies are suggested which might help in the proposed investigation of the mechanisms of growth of the crystals of glacier ice.

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IN the middle of last century, Agassiz, Tyndall and others showed that a glacier flowed faster at its centre than at its margin. Partly from experiments and partly from the assumption that it behaved like a river, it became generally accepted that it also flowed faster at the surface than lower down. This belief was held until ten or fifteen years ago although search through earlier literature shows that evidence was accumulating which might disprove this. During the thirtit;s of this century several writers began actively to reexamine the question. I In 1937 the late Dr. M. Demorest,2 reporting on a West Greenland glacier, wrote that owing to plastic flow the ice was, as it were, forced out from under an ice cap at places where accumulation of snow lay deepest. He cited well-known laboratory experiments showing that ice tended to flow when under differential pressure. In the following year, Dr. R. Streiff-Becker published his first paper 3 asserting that in many parts of a glacier, flow was faster below than above. Demorest used the evidence of glacial striae in old glacier beds. On the other hand Streiff-Becker developed his hypothesis by certain measurements and this paper has been written from material submitted by him. He had been measuring snow accumulation on the Claridenfirn (Canton Glarus) yearly since 1916 and it occurred to him that if the accumulqted firn was not to raise the level of the glacier, it must be carried away faster than appeared from the rate of flow measured at the surface. His measurements of the glacier level showed that the firn moved in waves but that the mean level of the surface was no higher in 1937 than it had been in 1916. Fig. I, p. 13, shows the position of a post, Point 2900, through which is drawn a line EDC recorded in the 1 See for instance Hollingworth. S. E. Q.

1. Introduction ( a ) The transition of firn into glacier ice; glacier structure Glaciers are divided into two main parts: the accumulation area, firn region or névé where the annual accumulation in the form of snow exceeds the loss by melting, evaporation and wind erosion, and the ablation area or glacier tongue. The dividing line between the two regions is called the Firn Line. Granular, compacted snow called firn covers the accumulation area. Its crystals are rarely larger than 2 mm. in diameter and are mixed with a considerable volume of air, so that the specific gravity is much lower than that of ice. The surface of the tongue consists of blue or glassy ice, more or less covered with rock debris; here the diameter of the ice crystals varies between 1 and 10 cm. or even more; the specific gravity of the ice is never far below 0.90. In summer the tongue has a bluish or grey appearance, while the firn region retains its white or whitish hue.

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* M. Lagally, Mechanik und Thermodynamik des Stationaren Gletschers, Leipzig, 1934.t The melting-point of ice is lowered by a pressure acting on either one or on both phases, Hence, when a block of ice is compressed adiabatically, it becomes cooled by a melting process to the melting temperature corresponding to the acting hydrostatic pressure. If a large mass of ice, insulated thermally from its surroundings, is initially at oo C., then the temperature at any particular depth is finally the " pressure melting temperature " corresponding to the acting pressure.

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