Erratics of the Alps. 313 



the requisite extension to carry the Alpine blocks to Jura, but the 

 conclusion rests on hypothetical grounds. Charpentier holds that no 

 unexampled intensity of cold is necessary, that 700 or 800 cold and 

 wet years like those from 1812 to 1818, would be sufficient — an 

 opinion in which probably few will join him. To determine the 

 duration of the glacial period, it would be necessary to know the 

 number of transported blocks, the distance which each travelled, and 

 the velocity with which it moved — elements all beyond our reach. 

 We are, however, able to say, that many of the blocks have travelled 

 a hundred miles or more, and we may form a rude idea of the rate 

 of their motion from data furnished by Forbes and Agassiz. The 

 velocity depends mainly on the slope of the glacier, and its magnitude 

 (or the area of its section), a greater slope compensating for a smaller 

 sectional area, and a greater area for a smaller slope. On the most 

 level part of the Glacier de Bois the motion at the middle was com- 

 puted to be about 676 feet per annum. On the Glacier of the 

 Lower Aar, Agassiz found that his tent travelled 64 metres (210 

 feet) in one year, and a comparison of older with recent observations 

 shewed that a block had travelled 8 kilometres in 133 years, or 197 

 feet per annum (Martin's "Revue de deux Mondes" Mars 1847.) 

 Anything like precision is unattainable, but 500 feet may be taken 

 hypothetically as a rough average of the annual motion of the middle 

 part of the ice (for the sides move much more slowly) in these two 

 great glaciers. Now, in a great glacier extending from the Pennine 

 Alps to Jura (P to/, fig. 10), the slope would be ten times less, and 

 would go to diminish the rate of motion in some such proportion ; 

 but the sectional area, or measure of size, being ten times greater, 

 would have precisely an opposite effect. Assuming, therefore, that 

 the one would counterbalance the other, let us suppose that the blocks 

 travelled on the great ancient glacier at the rate of 500 feet per an- 

 num. Then, by measuring the distances on Keller's map, we find that 

 a block carried from the east shoulder of Mont Blanc to Chasseron 

 (u or p to / in the map) would spend 740 years on its journey ; 

 one from the valley of Saas or Nicholas (y) to the same spot would 

 spend 1000 years ; and the huge boulder which made the w grand 

 tour" from t to Steinhoff, near A, must have been 1600 years upon 

 its travels ! It is true, the medial and lateral moraines do not consist 

 of isolated blocks, but of rows or trains of blocks, generally pretty close 

 behind one another. The first boulder dropped on Jura would there- 

 fore have many others near and behind it, but still the depositation 

 would be slow, for at the speed supposed, only as many as could lie 

 on a length of 500 feet would be deposited in a year, and these 

 spread over a considerable surface. We must remember, too, that 

 the blocks from the Alps are scattered not only over the long line of 

 Jura, but over all the western Swiss Plain, an area of nearly 3000 

 square miles. Many thousands of years would evidently be required 



