130 Charles Maclaren, Esq., on the Existence of 



axis of the loch. They are most numerous and distinct at 

 low levels, and lose this character as we ascend. In those 

 which are well marked and have a very fresh appearance, the 

 veins of quartz are cut away to the level of the slate, but in 

 others the wearing of the softer substance makes the veins 

 project. 



At one time the scratches and groovings on rocks were 

 supposed to be produced by great ciu'rents of water, bearing 

 stones and gravel with them. Butcareful observation has shewn 

 that the mightiest floods in rivers, and the greatest storms 

 on the shores of the sea, though they transport vast masses 

 of stone, produce no such effects. They do, indeed, often 

 polish the rocks, but they never groove them ; and if they 

 sometimes produce a few scratches, they are of trifling extent, 

 and entirely destitute of that rectilineal persistency and uni- 

 formitif of hearing^ which characterize so remarkably the striae 

 I have been describing. 



The only agent known at present in nature which produces 

 efi^ects analogous to those mentioned, is the glacier. The 

 vast masses of ice in the higher valleys of Switzerland, called 

 glaciers, glide downwards at the rate of a few inches or a 

 few feet per day, bearing with them fragments of rock, gravel, 

 and sand. These adhere to the ice, or are imbedded in it, 

 and as the mass moves slowly along, they abrade, groove, and 

 polish the rock underneath, while the larger fragments are 

 reciprocally abraded, grooved, and polished on their lower 

 sides. Pressed down by an enormous weight, the sand serves 

 as emery to polish the surface ; the pebbles, like coarse gra- 

 vers, scratch and furrow it ; and the large stones scoop out 

 grooves in it. When the valley is tolerably straight, these 

 scratches, furrows, and grooves, whether on the sides or 

 bottom, all point in the direction of its length, and (with 

 slight deviations easily accounted for) are parallel to each 

 other. In the course of the polishing and smoothing, small 

 protuberances are ground down and disappear, while the 

 larger are rounded off into those dome-shaped eminences 

 called roches montonnees.* 



* See Chap. xiv. of Agassiz's Etudes sur les Glaciers, 



