48 SIXTH LETTER ON GLACIEIiS. [1844. 



Paris, mingled with water, whose consistence may be varied at 

 pleasure, and a stream of which may be made either concave or 

 convex, or concave at its origin and convex at its termination, 

 as is the case with a glacier. The evidence on this subject, 

 afforded by the models formerly laid before the Royal Society, 

 is so complete and conclusive, that, however interesting it might 

 be to put into a mathematical form the relations of the constants 

 of the effect of gravity, the viscosity of the body, and the retar- 

 dation of the sides, as affecting the form of the surface, it is 

 sufficient for my present purpose to appeal to facts so familiar, 

 and experiments so easy, that their evidence may well be pre- 

 ferred to the more casual and embarrassed case of lava streams, 

 which, as I have already observed, are seldom or never to be 

 regarded, on a great scale, as simple moving masses. I may, 

 however, add, that when the inclination is small, the surface is 

 convex, at a certain distance from the origin. 



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 former 

 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 moraines, 

 though noticed by various geologists, are most emphatically 

 described by M. Elie de Beaumont, in his masterly memoir on 

 Etna, in the following words : " Une des circonstances que les 

 coulees de lave presentent le plus invariablement toutes les fois 

 qu'elles ont parcouru des talus ou elles pouvaient acquerir une 



