Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

618 ADDENDA. other facts (p. 381), are quite intelligible to me from the briefness, with which they are alluded to. M. Agassiz says (p. 375), " The erratic blocks of the Jura every where repose on polished surfaces, all those at least which have not been carried beyond the crest of our mountains, and which have not fallen to the bottom of our longitudinal valleys, as may be seen throughout the valleys of the Creux du Vent. But they do not repose immediately upon these polished surfaces. Wherever the rounded pebbles which accompany the great blocks have not been removed by subsequent influences, it is remarked that small blocks, in other words pebbles of different sizes, form a bed of some inches, and sometimes even of many feet, upon which the great angular blocks repose. These pebbles are also much rounded, even polished, and are heaped up in such a way that the larger are above the smaller, and that the last often pass below into a fine sand, lying immediately over the polished surfaces. Tliis order of superposition, which is constant, is opposed to all idea of a transport by currents ; for in this latter case the order of the super- position of the pebbles would have been precisely reversed." Further on (p. 379) he remarks that the action of the glaciers is immense ; " for these masses, continually moving upon each other, and on the surface, bruise and grind down every thing moveable, and polish the solid surfaces on which they repose ; at the same time that they push before them all that they encounter, with a force which is irresistible. It is to these movements (of the great stratum of ice) we must attribute the strange superposition of the rolled pebbles, and of the sand, which immediately reposes upon the polished surfaces ; and it is unquestionably to the grating of this sand upon these surfaces that the fine lines which we find (previously compared to the scratches made by a diamond on glass) are owing, and which would never have existed, if the sand had been acted upon by a current of water." Now it may be demanded, by what pos- sible means can such violent action arrange the large pebbles above the smaller ones, and these again above the sand? The fact appears to me utterly inexplicable on this view. Again it is said, that the surface of the rock is marked by furrows and gibbosities, as well as by scratches, and that these " never follow the direction of the slope of the mountain, but are oblique and longitudinal (that is, in the line of the mountain, and therefore nearly horizontal), a direction which excludes every idea of a stream of water being the cause of these erosions." What explanation will it be believed is offered for this fact? — It is, that the fine lines and furrows " must have resulted from the much greater facility which the ice had in dilating itself in the direction of the great Swiss valley, than trans- versely, confined as it were between the Jura and the Alps."