142 COLOSSAL SIZE OF ANCIENT GLACIERS. [Ch. XL 



Alpine blocks on the Jura. — Some or all the marks above enumera- 

 ted — the moraines, erratics, polished surfaces, domes, striae, .caldrons, 

 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 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 simular moraines, and the same polished and grooved sur- 

 faces and water-worn cavities. The erratics, moreover, which cover 

 it, presents a phenomenon which has astonished and perplexed the 

 geologist for more than a 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 upward across 

 one of the widest and deepest valleys in 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 of so many leagues, 

 has justly excited wonder ; for hundreds of them are as large as cot- 

 tages ; and one in particular, 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 di- 

 ameter. 



In the year 1821, M. Venetz first announced his opinion that the 

 Alpine glaciers must formerly have extended far beyond their present 

 limits, and the proofs appealed to by him in confirmation of this doc- 

 trine were afterward acknowledged by M. Charpentier, who strength- 

 ened them by new observations and arguments, and declared, in 

 1836, his conviction that the glaciers of the Alps must once have 

 reached as far as the Jura, and have carried thither their moraines 

 across the great valley of Switzerland. M. Agassiz, after several ex- 

 cursions in the Alps with M. Charpentier, and after devoting himself 

 for some years to the study of glaciers, published, in 1840, an admira- 

 ble description of them and of the marks which attest the former ac- 

 tion of great masses of ice over the entire surface of the Alps and the 

 surrounding country.* 



M. Charpentier conceived that the Alps, at the time when the gla- 

 ciers extended continuously from them to the Jura, and conveyed to 

 them so many Alpine erratics, where 2000 or 3000 feet higher than 

 now. Professor James D. Forbes, in his excellent work on the Alps, 

 published in 1843, came in like manner to the conclusion that the 

 ancient glaciers were of colossal size, and had once stretched from the 

 principal chain to the Jura. The original theory of Saussure, that the 

 erratics were all whirled along to great distances by a rapid current of 



* Agassiz, Etudes sur les Glaciers, and Systerne Glaciere. 



