He carefully examined iron which had passed under the steam-hammer, or through the rolling-mill; clay and wax were subjected to the hydraulic press. In all cases he detected signs of cleavage; and hence we are justified in the inference that the phenomenon is invariably produced by pressure in all bodies of irregular internal structure. Such is the result with glacier-ice, from whose mass the air-bubbles introduced by the snow are gradually expelled. At first of brilliant whiteness, it assumes, in the parallel layers corresponding to the planes of cleavage, those beautiful azure tints which characterize the veined structure. So little has it to do with stratification, that in places where this is apparent it has given rise to a series of horizontal lines, while the parallel veinings, in the same masses of ice, are all inclined at an angle of about 60°. The tendency to cleavage in compact ice would seem to explain the regular form of those fragments or detached pieces with which some parts of the glaciers are covered. Usually they occur as cubes, or as rectangular parallelopipeds. The Alpine mountaineers name them _séracs_,--in allusion to their resemblance to certain cheeses which bear this name, and which are manufactured in rectangular boxes. They have been found in many parts of a really colossal size, measuring fifty feet in length, breadth, and depth, and as regular in shape as if they had been hewn with a chisel. There are many interesting points connected with the formation and constitution of glaciers which we should gladly discuss, but we are confined by our limits to remarks of a general character, and we must now pass on to speak of the phenomena attendant upon their motion. No doubt, the traveller who for the first time comes in sight of one of these huge ice-rivers, and sees the mighty mass apparently rooted to its valley-bed, solid, unchangeable, adamantine, finds it hard to believe that it moves onward with a certain and an unresting, though a gradual progress. It looks like a noble river, suddenly petrified by some overwhelming force: congealed, as it flowed, in a moment, by some irresistible spell! Such, indeed, is the conception of the poet:-- “Ye ice-falls! ye that from the mountain’s brow Adown enormous ravines slope amain.... Torrents, methinks, that heard a mighty voice, And stopped at once amid their maddest plunge! Motionless torrents! silent cataracts!”