Professor Forbes's Sixth Letter on Glaciers. 237 



VI. There is a circumstance attendant on the motion of 

 lava streams, which has struck several geologists, before the 

 viscous theory of glaciers had been proposed — I mean the ex- 

 istence of moraines. The moraines of lava are best seen in 

 the more defined and united lava streams on rather a small 

 scale, — those, in short, which have the unity and character of 

 a proper stream, moving at once in its various parts. The 

 moraine is composed of stranded masses of lava crust, thrown 

 aside by the liquid fiery stream, and partly, perhaps, of the 

 yielding matter of the bed of the stream pressed outwards and 

 upwards by the hydrostatic pressure of the centre. The for- 

 mer is chiefly, perhaps, the case when streams of tolerably 

 fluid lava flow down a steep inclination, as on the exterior of 

 the cone of Vesuvius ; the latter, when the inclination is small 

 and the weight of accumulated lava great. The igneous mo- 

 raines, though noticed by various geologists, are most em- 

 phatically described by M. Elie de Beaumont, in his masterly 

 memoir on Etna, in the following w^ords : — '' Une des circon- 

 stances que les coulees de lave presentent le plus invariable- 

 ment toutes les fois qu'elles ont parcouru des talus o^ elles 

 pouvaient acquerir une certaine vitesse, caracteres que j'ai ob- 

 serves sur toutes sortes de pentes depuis 33° jusqu'^ 2° et quo 

 je n'ai cesse d' observer que 1^ o\i les coulees se sont arreteea 

 faute de pente, consiste en ce que chaque coulee est flanqu^e 

 de part et d'autre par une digue de scories accumulees qui 

 rappelle par sa forme la moraine d' un glacier ; digue qui s'eleve 

 constamment ^ une hauteur sup^rieure a celle ^ laquelle la coulee 

 est reduite a la fin du mouvement, et qui marque le maximum 

 de hauteur qu'*elle a atteint dans le moment de son plus grand 

 gonflement. Souvent aussi les coulees presentent de pareilles 

 digues vers leur milieu, lorsqu' elles sont partagees en plusieurs 

 courants distincts coulant Tun ^ cote de Fautre."* 



VII. The termination of a lava stream on a level or slightly 

 inclined surface due to its increasing viscidity, presents ap- 

 pearances almost identical with those of a glacier. The same 

 protuberant convexity of surface, the same steeply-inclined 



* E. de Beaumont Recherches sur le Mont Etna, p. 184. 



