1844.] VEINED STRUCTURE IN LAVAS. 47 



ductility or tenacity of their parts. It is that fragility precisely, 

 which, yielding to the hydrostatic pressure of the unfrozen water 

 contained in the countless capillaries of the glacier, produces the 

 crushing action which shoves the ice over its neighbour particles 

 and leaves a bruise, within which the infiltrated water finally 

 freezes and forms a blue vein. In the lava, on the other hand, 

 where the tenacity is great, the discontinuity, if produced at all, 

 is soldered up by the plasticity of the parts, whose small crystal- 

 line structure farther tends to obliterate the separation. The 

 layers just mentioned, parallel to the bed, are perhaps produced 

 by the successive adhesion of warmer streams of lava to the 

 colder parts already deposited, and, consequently, their analogy 

 to the glacier structure must not be pushed farther than as 

 showing the directions of the tendency to separation of a very 

 viscid stream, powerfully retarded by its bed. It is the con- 

 gealing of the lava which makes its adhesion to the sides great 

 enough, and its own fluidity small enough, to bear a comparison 

 with the far less ductile body of a glacier. In the heart of the 

 mass where the same intestinal motions take place (as I have 

 shown conclusively by using coloured layers of plastic matter in 

 the models formerly exhibited to the Royal Society), the dis- 

 placed particles reunite and consolidate into a homogeneous 

 mass without any trace of dislocation.* 



V. The convexity or concavity of a semifluid stream like a 

 current of lava or of a glacier, depends entirely upon the rela- 

 tions or conditions in which it is placed. Upon the same slope, 

 a fluid of one degree of consistence will run off in a concave 

 stream, whilst a more viscid one, which must accumulate in 

 thickness, in order to overcome the resistance in front (just as 

 water which meets a sudden obstacle), rises into a convex curve. 

 This is perfectly seen in the case of a substance like plaster of 



* The following passage from M. Dufrenoy's Account of Vesuvius, is interesting, 

 if it were only as recording his remark, that the variation of velocity in different 

 parts of a stream must produce longitudinal striae. " La plupart des coulees pre- 

 sentent des bandes longitudinales assez paralleles entre elles ; ces larges stries sail- 

 lantes sur la surface sont les traces du mouvement de la lave qui ne s'avance pas 

 d'une seule piece, mais par bandes paralleles." Sur les Environs de Naples, 

 p. 324. 



