Erratics of the A $M 301 



The boulders attain their greatest elevation on Jura, at the hill of 

 Chasseron (/ in the map), precisely opposite the valley (<?), through 

 which they must have passed. At this spot, granite blocks from the 

 east shoulder of Mont Blanc (u) are found 2000 feet above the plain. 

 From /, the boulders continue to present themselves on the declivities 

 eastward, but at lower and lower elevations, all the way to A, where 

 they descend to the level of the plain. They continue also westward 

 to g, with a similar change in elevation, so that, if a section were 

 made along the south face of Jura, it would form an arc, of which 

 the middle would be probably 1500 feet higher then the extremities. 

 Supposing the fact to be well ascertained, Charpentier justly con- 

 siders it as strong evidence to shew that the boulders were transport- 

 ed by glaciers. For, in this case, the primary movement of the ice 

 (a semifluid mass, be it remembered) would be in the direction of 

 the valley (<?) from which it issued — that is, right to /. It would 

 indeed tend to diffuse itself laterally as soon as it reached the low 

 country, but the dynamic pressure, so far as it acted on the mass, 

 would operate most strongly in the direction ef. The progress of 

 the glacier would be stopped by the mountain at /, which would then 

 become a secondary centre of dynamic pressure, and accelerate the 

 motion towards g and h ; and the semifluid ice thus acted on, and 

 obeying the law of gravitation, would have its surface gently inclined 

 to these two points. The boulders resting on Jura would thus be 

 deposited at lower and lower levels, in proportion as the places they 

 rested on were farther and farther from /. If, on the contrary, we 

 assume that the boulders were transported on floating ice, is it not 

 evident that they would have been found at as high a level at g and 

 h as at/? for water, unlike a viscous fluid, would find a true level 

 over a space like this in a few hours or days, while the great number 

 of the boulders shew that their distribution must have extended over 

 thousands of years. (Charpentier, Essai, sect. 53^ i - Boi 



M. Guyot, states, that on the declivities of Jura, on the Bernese 

 Oberland, from k to i, and even in the intermediate plain, a linear 

 arrangement of the blocks and fragments may be traced, such as exists 

 in moraines, and a distribution in harmony with the laws of glacier 

 motion. Thus, fragments derived from the right bank of the Great 

 Valley (a c q), are found on the right side of the basin (k i), while 

 those derived from the left bank (y np w), are found on the left side 

 of the basin (/ and towards h), and those derived from the upper left 

 bank (6 t), occupy the middle of the basin, and are found in the space 

 between i and h, where they constitute what M. Guyot calls the fron- 

 tal or terminal moraine of the eastern glacier. There is a huge 

 boulder of talcose granite resting on a low eminence at Steinhoff, 

 ten miles south-east from Seleure (near h), containing 61,000 cubic 

 feet (French), and weighing more than 5000 tons. It will give a 

 better idea of its magnitude to state that it is equal in bulk to a mass 

 measuring forty feet in length, breadth, and depth — as large, in short, 



VOL. LIII. NO. CVI. — OCTOBER 1852. X 



