92 VISCOUS THEORY OF GLACIER MOTION. [1845. 



second volcanic focus, are curious proofs of how long a palpable 

 cause may be rejected as insufficient to explain a phenomenon, 

 and a totally imaginary one superadded.* 



I may add, that lava streams sometimes push .their extre- 

 mities up hill ft glaciers do the same.| 



In addition to the considerations already stated, which 

 illustrate the viscous theory of glaciers, I am glad to avail 

 myself of two which have reached me from independent and 

 impartial sources. 



The first is by Mr. Darwin, who in a small book on 

 " Volcanic Islands," published about the time that I was en- 

 gaged in making the preceding observations on Etna and Vesu- 

 vius, pointed out in a very clear manner the explanation which 

 the veined structure of glaciers lends to that of volcanic rocks 

 belonging to the Trachytic arid Obsidian Series, where the 

 lamination, instead of being obscure and rare, as it generally is 

 in the Augitic lavas, owing perhaps to their greater fluidity, 

 and more viscid and homogeneous texture, is the general rule. 

 " The most probable explanation," says Mr. Darwin, " of the 

 laminated structure of these felspathic rocks appears to be that 

 they have been stretched whilst flowing slowly onwards in a 

 pasty condition, in precisely the same manner as Professor 

 Forbes believes that the ice of moving glaciers is stretched and 

 fissured. In both cases the zones may be compared to the 

 finest agates ; in both they extend in the direction in which the 

 mass has flowed, and those exposed on the surface are generally 

 vertical. " 



The other illustration is contained in a communication with 

 which I have been favoured by Mr. Gordon, Professor of Civil 



* See the view of the termination of a lava stream in Auldjo's Sketches of 

 Vesuvius, facing p. 92. The reader may also compare the view of a gi-otto in the 

 lava, in the same work, with that of the source of the Arveiron in my Travels, p. 387. 



f Hamilton, Campi Phlegraei, folio, vol. i. p. 40, note. 



[As in the Allalein glacier, described in my Travels, p. 352, showing the 

 remarkable development of the frontal dip under these circumstances. See also the 

 Section in a 6 in the Ninth Topographical Sketch in the same work.) 



Darwin on Volcanic Islands, 1844. The whole passage, pp. 65-72, illustrates 

 this analogy. 



