598 Mr. Hopkins's Remarks on Prof. Forbes's Reply. 



much the greater part of the motion of a glacier being due to 

 its plasticity (no part, or a very small part of the motion 

 being due to its sliding) as inadmissible, because if that hy- 

 pothesis were true, it would follow that the general state of a 

 glacier would be that of longitudinal compression, especially 

 during those seasons of the year 'when its motion is greatest. 

 This I consider inconsistent with the gejieral existence of 

 transverse fissures in the central portion of a glacier. With 

 respect to those in the lateral portions, I have distinctly 

 shown (Art. 20. Second Letter), that they are not inconsistent 

 with the existence of longitudinal compression. Local causes 

 may doubtless produce transverse fissures in the central por- 

 tion, but they must so act as to produce extension there, for 

 the formation or continued existence of a transverse fissure 

 under longitudinal compression is a mechanical contradiction. 

 Professor Forbes has endeavoured to meet this argument by 

 a reference to M. Agassiz's admeasurements on the glacier 

 of the Aar, which undoubtedly give a smaller velocity at the 

 lower extremity compared with that at higher points than I 

 should have anticipated. But still let us take the evidence of 

 this glacier in conjunction with that of the Mer de Glace. In 

 the former, it appears, the mean^ state is that of compression, 

 and if there be not an entire absence of transverse fissures in 

 its central portion, they are I believe fewer in number and 

 smaller than those in any other large glacier, and must be 

 referred to local causes. But who would venture to refer the 

 crevasses of the Mer de Glace to local causes ? And how will 

 Professor Forbes's theory account for them by any general 

 cause, or for the elongation of that glacier during the sum- 

 mer, which his measurements and observations have so clearly 

 established t ? He has in fact assigned no general cause 

 whatever for the very general phaenomena, in most glaciers, 

 of transverse fissures in their central portion. 



Prof. Forbes asserts that " plasticity is a quality of glaciers 

 without which they would remain stationary, or descend in 

 avalanches." I shall not renew any argument on this point. I 

 will merely state, that whether we consider the question with 



* The evidence actually required is the relative motions of the upper 

 and lower extremities of the glacier when the absolute motion is greatest. 



t I have never been able to understand how Professor Forbes reconciles 

 his theory, which always implies that the mass is urged by a pushing force, 

 with his impressions respecting the elongation of glaciers, as expressed in 



the following and other passages: " a circumstance mentioned by 



M. Elie de Beaumont, which is so true that one wonders it has not been 

 more insisted on, viz. that a glacier, where it descends into a valley, is 

 like a body pulled asunder or stretched, and not like a body forced on by 

 superior pressure alone." 



