294 ALPINE ERRATICS ON THE JURA. CHAP. xv. 



these marks of abrasion seem capable of enduring for 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 the marks above enumerated, the mo 

 raines, erratics, polished surfaces, domes, striae, 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, 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 

 century. No conclusion can be more incontestable 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 upwards across one of 

 the widest and deepest 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 



