Erratics of the Alps. 311 



warm and wet weather. At a distance of 100 yards from the side, 

 it was found to be 483 feet per annum, and in the middle it was 

 estimated at two-fifths more, or 676. Now, if a slope of 4| degrees 

 gives motion to a glacier 250 feet deep, it follows from the rule laid 

 down, that a slope only one-tenth of this — viz., of 27 minutes (or 1 

 foot in 127), would suffice for a glacier ten times as deep (and wide 

 in proportion), such as the one which has left traces of its existence 

 3100 feet above Martigny, and 2780 feet above the Lake of Geneva 

 at the Chalet of Playau. The question put was, ivhether a slope of 

 22 minutes would be adequate ? and the result obtained (27'), is suf- 

 ficiently near to shew that there is little force in the objection drawn 

 from the small difference of level between the point P, whence the 

 blocks come, and the point/, to which they were carried. It must 

 be kept in mind, too, that the relative heights of P and / have not 

 yet been determined geometrically. Professor Forbes spoke from 

 careful consideration when he said — " We cannot admit it to be any 

 sufficient argument against the extension of ancient glaciers to the 

 Jura, that they must have moved with a superficial slope of one degree, 

 or in some parts even of a half or a quarter of that amount, whilst in 

 existing glaciers the slope is seldom or never under three degrees. 1 ' 



There is no other large glacier whose mechanical constitution and 

 motions have been studied with the same care as those of the Glacier 

 de Bois, but some interesting facts are given by Agassiz and Desor 

 respecting the great glacier of the Lower Aar. In a length of 7000 

 metres from the Hotel Neuchatelois (a cave) to the top of the ter- 

 minal declivity, the surface falls 486^ metres (Desor, " Excursions" 

 p. 242) indicating an angle of 3° 58', or a slope of one foot in 14^ 

 nearly. According to M. Martins, the lower part of the glacier in 

 1842 advanced 128 English feet per annum, the middle 233 feet, 

 and the upper part 246. The more rapid motion of the Glacier de 

 Bois is, no doubt, chiefly the consequence of its greater inclination, 

 especially at the lower end. Of the depth of the glacier of the Lower 

 Aar, we have no very satisfactory account ; for Agassiz was unable 

 to bore to a greater depth than 150 feet, and the depth obtained by 

 sounding through a natural opening (" moulin") cannot be entirely 

 depended on. The vertical height above the bed of the stream at 

 the lower end (from d' to b, fig 8) was found by Agassiz to be 96 

 metres, or 315 English feet. 



It has been shewn that various phenomena connected with the 

 transportation and depositation of boulders are better explained by 

 the glacier, than by the iceberg, hypothesis. The latter seems 

 equally deficient when applied to the case of striated and grooved 

 rocks. Granting that in certain circumstances floating ice might 

 produce horizontal striae or groovings on even surfaces, how shall 

 we account for the very common case of those groovings which in- 

 cline downward, following the slope of the valley; or those occa- 

 sionally seen which point upward ; or those traced on the curved 



