ALPINE BLOCKS ON THE JUBA. 
169 
thus laden, descends so far as to reach a region about 3500 
feet above the level of the sea, the warmth of the air is such 
that it melts rapidly in summer, and all the mud, sand, and 
pieces of rock are slowly deposited at its lower end, forming 
a confused heap of unstratified rubbish called a moraine^ and 
resembling the till before described (p. 166). 
Besides the blocks thus carried down on the top of the 
glacier, many fall through fissures in the ice to the bottom, 
where some of them become firmly frozen into the mass, and 
are pushed along the base of the glacier, abrading, polishing, 
and grooving the rocky floor below, as a diamond cuts glass, 
or as emery-powder polishes steel. The striae which are 
made, and the deep grooves which are scooped out by this 
action, are rectilinear and parallel to an extent never seen 
in those produced on loose stones or rocks, where shingle is 
hurried along by a torrent, or by the waves on a sea-beach. 
In addition to these polished, striated, and grooved surfaces 
of rock, another mark of the former action of a glacier is 
the “roche moutonnee.” Projecting eminences of rock so 
called have been smoothed and worn into the shape of flat¬ 
tened domes by the glacier as it passed over them. They 
have been traced in the Alps to great heights above the pres¬ 
ent glaciers, and to great horizontal distances beyond them. 
Alpine Blocks on the Jura. —The moraines, erratics, pol¬ 
ished surfaces, domes, and striae, above described, are ob¬ 
served in the great valley of Switzerland, fifty miles broad; 
and almost everywhere 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 des¬ 
titute of glaciers; yet it presents almost everywhere similar 
moraines, and the same polished and grooved surfaces. The 
erratics, moreover, which cover it, present a phenomenon 
which has astonished and perplexed the geologist for more 
than half a century. No conclusion can be more incontest¬ 
able than that these angular blocks of granite, gneiss, and 
other crystalline** formations came from the Alps, and that 
they have been brought for a distance of fifty miles and up¬ 
ward across one of the widest and deepest valleys in the 
world; so that they are now lodged on a chain composed 
of limestone and other formations, altogether distinct from 
those of the Alps. Their great size and angularity, after a 
journey of so many leagues, has justly excited wonder; for 
hundreds of them are as large as cottages; and one in par¬ 
ticular, composed of gneiss, celebrated under the name of 
Pierre a Bot, rests on the side of a hill about 900 feet above 
the lake of Neufchatel, and is no less than 40 feet in diameter. 
8 
