360 The Glacial Theory of Prof. Agassiz. 



Von Buch, Escher, and Studer, have shown, from an examination of 

 the mineral composition of the boulders, that those on Western Jura, 

 1 1, have come from the region of Mont Blanc, M, and the Valais, V ; 

 those on the middle parts of Jura, 2 2, from the Bernese Oberland, B ; 

 and those on Eastern Jura, 3, towai'ds Aargau and Zurich, from the 

 Alps of the Petits Cantons, P. The blocks have thus been derived from 

 the parts of the Alps nearest, generally speaking, to the localities where 

 we now find them, as if they had passed across the valley in a direction 

 at right angles to its length. 



The blocks are generally angular, and therefore had not been exposed 

 to much attrition, either from agitation amidst gravel, or from mutual 

 action. Many of them are of prodigious magnitude. The famous mass 

 of Pierre a Bot, containing 50,000 cubic feet, and weighing probably 

 4,000 tons, equals a goodly mansion in size, namely, one of 30 feet in 

 front, 40 in depth, and 40 in height. It rests on a part of Jura 2,177 

 feet above the sea, and about 900 feet above the level of the lake of Neu- 

 chatel, N. Near Chaumont there is a group of granite blocks, which, 

 ■from their magnitude, their number, and their juxtaposition, look like a 

 hamlet of cottages. The large Alpine boulders of Jura, in short, may 

 be counted by hundreds, and the small ones by thousands. 



The boulders are distributed in zones on the terraces, which, like the 

 steps of a stair, form the out-goings of the different formations. The 

 highest are disposed in rings, as in figure 7, round the lower summits 

 of Jura, at a height between 3,000 and 3,300 feet above the sea. The 

 other zones occur on the terraces below this ; the first at elevations 

 from 1,900 to 2,400 feet ; the next at 1,600 to 1,800 feet ; and the last 

 descends to the level of the lake of Neuchatel, 1,324 feet above the 

 sea. Moreover, these travelled blocks penetrate into the transverse 

 and into the interior valleys of Jura, and some are even found at the 

 back of the chain, near the Doubs.* 



Saussure attributed the transportation of these boulders to a dehacle, 

 or great current, rushing from the Alps ; and Von Buch, finding that one 

 current would not account for the phenomena, assumed the existence 

 of several. But the inadequacy of such explanations is obvious. 



A, The Alps ; J, Mount 

 Jura, with the great valley, 

 fifty miles wide, between 

 them. 



e, The southern declivi- 

 ties of Jura, upon which 

 most of the erratic blocks rest. 



* Etudes, p. 278-280. 



