294 ALPINE ERRATICS ON THE JURA. 



CHAP. XY. 



these marks of abrasion seem capable of enduring foi-ever. 

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

 present glaciers, and to great horizontal distances beyond 

 them. 



Another effect of a glacier is to lodge a ring of stones 

 round the summit 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 situation near 

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

 may be destitute of boulders. 



Alpine erratic Blocks on the Jura. 



Now, some or all of the marks above enumerated — the mo- 

 raines, erratics, polished surfaces, domes, strias, 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 valle}^ 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 destitute 

 of glaciers; yet it presents almost everywhere moraines, and 

 polished and grooved surfaces of rocks. The erratics, more- 

 over, which cover it present a phenomenon which has as- 

 tonished and perplexed the geologist for more than half a 

 centur}^. No conclusion can be more incontestable than that 

 these angular blocks of granite, gneiss, and other ciystal- 

 line formations, came from the Alps, and that they have been 

 brought for a distance of fifty miles and upwards across one 

 of the widest and deejiest valleys of the world; so that they 

 are now lodged on the hills and valleys of a chain composed of 

 limestone and other formations, altogether distinct from those 

 of the Alps. Their great size and angularity, after a journey 



