148 ALPINE ERRATICS. [Ch. XIL 



sides. As the forces both of pressure and propulsion are enormous, the sand, 

 acting hke emery, polishes the surface ; the pebbles, like coarse gravers, 

 scratch and furrow it ; and the large stones scoop out grooves in it. 

 Another effect also of this action, not yet adverted to, is called " roches 

 moutonnees.'* Projecting eminences of rock are smoothed and worn into 

 the shape of flattened domes, where the glaciers have passed over them. 



Although the surface of almost every kind of rock, when exposed in the 

 open air, wastes away by decomposition, yet some retain for ages their 

 polished and furrowed exterior ; and, if they are well protected by a cov- 

 ering of clay or turf, these marks of abrasion seem capable of enduring 

 forever. They have been traced in the Alps to great heights above the 

 present glaciers, and to great horizontal distances beyond them. 



There are also found, on the sides of the Swiss valleys, round and deep 

 holes, with polished sides, such holes as waterfalls make in the solid rock, 

 but in places remote from running waters, and where the form of the 

 surface will not permit us to suppose that any cascade could ever have 

 existed. Similar cavities are common in hard rocks, such as gneiss, in 

 Sweden, where they are called giant caldrons^ and are sometimes 10 

 feet and more in depth ; but in the Alps and Jura they often pass into 

 spoon-shaped excavations and prolonged gutters. We learn from M. 

 Agassiz that hollows of this form are now cut out by streams of water, 

 which, after flowing along the surface of a glacier, fall into open fissures in 

 the ice and form a cascade. Here the falling water, causing the gravel 

 and sand at the bottom to rotate, cuts out a round cavity in the rock. But 

 as the glacier moves on, the cascade becomes locomotive, and what would 

 otherwise have been a circular hole is prolonged into a deep groove. The 

 form of the rocky bottom of the valley down which the glacier is moving 

 causes the rents in the ice and these locomotive cascades to be formed 

 again and again, year after year, in exactly the same spots. 



Another effect of a glacier is to lodge a ring of stones round the sum- 

 mit of a conical peak which may happen to project through the ice. If 

 the glacier is lowered greatly by melting, these circles of large angular 

 fragments, which are called "perched blocks," are left in a singular situa- 

 tion near the top of a steep hill or pinnacle, the lower parts of which 

 may be destitute of boulders. 



Alpine blocks on the Jura. — Xow some or all the marks above enu- 

 merated, — the moraines, erratics, polished surfaces, domes, strise, cal- 

 drons, and perched rocks, are observed in the Alps at great heights 

 above the present glaciers, and far below their actual extremities ; also 

 in the great valley of Switzerland, 50 miles broad ; and almost every- 

 where on the Jura, a chain which lies to the north of this valley. The 

 average height of the Jura is about one-third that of the Alps, and it is 

 now entirely destitute of glaciers, yet it presents almost everywhere 

 similar moraines, and the same polished and gTooved surfaces, and water- 

 worn cavities. The eri'atics, moreover, which cover it, present a phenom- 

 enon which has astonished and perplexed the geologist for more than 

 half a century. No conclusion can be more incontestable than that these 



