374 Professors Tyndall and Huxley on the Structure 



Fig. 6. 



we therefore called this disposition of 

 the veins " the lenticular structure." 

 In other cases, however, the sharpness 

 of outline did not exist, but still the 

 tendency to the lenticular form could 

 be discerned, the veins in some cases 

 terminating in washy streaks of blue. 

 This structure is probably the same 

 as that observed by Professor Forbes 

 on the Glacier des Bossons, and de- 

 scribed in the following words : — 

 " The veins and bands .... are not 

 formed in this glacier by a simple 

 alternation of parallel layers, but the 



icy bands have all the appearance of posterior infiltration, occa- 

 sioned by fissures, tldnning off both ways*. 



In 1842 Professor Forbes undertook the survey and examina- 

 tion of the Mer de Glace, and finally arrived at a theory of gla- 

 cier lamination, which both in his 'Travels' and in a series of 

 letters, extending over a period of several years, he has expounded 

 and illustrated with great skill. The theory is summed up in 

 the following words : — "The whole phsenomena in the case of any 

 of the semifluids I have mentioned (treacle, tar, &c.), are such 

 as, combined with the evidence which I have given, that the 

 motion of a glacier is actually such as I have described that of a 

 viscid fluid to be, can leave, I think no reasonable doubt, that the 

 crevices formed bt/ the forced separation of a half rigid mass, whose 

 parts are compelled to move with different velocities, becoming in- 

 filtrated with water, and frozen during winter, produce the bands 

 which we have desc7'ibedf ." 



This theory has been opposed by Mr. Hopkins, whose excel- 

 lent papers, published in the 26th volume of the Philosophical 

 Magazine, are replete with insti-uction as to the mechanical con- 

 ditions of glaciers. On the other hand, the theory of Professor 

 Forbes is defended in the same Journal by Dr. WhewellJ. We 

 will leave the points discussed in their communications for the 

 present untouched, and confine ourselves to stating a few of the 

 circumstances which appear to us to render the theory doubtful. 



1 . It is not certain that the colds of winter penetrate to depths 



* Travels, p.J81. 



t Ibid. p. 377- M. Agassiz also seems disposed to regard the blue bands 

 as the result of the freezing up of fissures, which, however, are supposed 

 to be formed in a manner different from that assumed by Professor Forbes. 

 But M. Agassiz calls the attention of future observers to some of the related 

 phaenomena ; and gives it as his opinion, " qu'il n'est aucune phenomena 

 dont I'explication offre plus de difficultes." See his important work, Systeme 

 G/acJ^re, which, until quite recently, we had not theop))ortunity of examining. 



+ Phil. Mag. S. 3. vol. xxvi. i)p. 171. 217. 



