Professor Forbes's Sixth Letter on Glaciers. 235 



IV. But there is a more striking analogy to the ribboned 

 structure of glacier ice, to be found in lava currents at a dis- 

 tance from their origin, and where by any circumstance their 

 surface has been broken up, and their internal structure ex- 

 posed. In the Fossa della Vetrana, for instance, and other 

 places, I have found the lava divided into thin layers parallel 

 to the interior of the surface of the channel through which it 

 flowed, evidently produced by the adhesion or retardation 

 which the soil exerted upon its adjoining film of lava, and the 

 successive portions of lava upon one another, in proportion as 

 the semifluid mass, rolling upon its own particles (or rather 

 sliding imperfectly over them), produced a solution of con- 

 tinuity and a series of shells, parallel in direction to the bed 

 upon which the whole rests. The thickness of these shells 

 varies from one-third of an inch upwards. I have never, how- 

 ever, observed a structure in the interior of the lava except 

 that parallel to the sides and bottom of the canal in which it 

 moves; nothing, in short, corresponding to the fro?ital dip in 

 glaciers. But this is quite natural and conformable to the 

 very different constitution of a glacier ; and, in particular, it 

 corresponds to the fact so often urged as a difficulty to the 

 semifluid theory of glaciers, namely, the want of ductility or 

 tenacity of their parts. It is that fragility precisely, which, 

 yielding to the hydrostatic pressure of the unfrozen water con- 

 tained in the countless capillaries of the glacier, produces the 

 crushing action which shoves the ice over its neighbour par- 

 ticles 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 pro- 

 duced at all, is soldered up by the plasticity of the parts, whose 

 small crystalHne structure farther tends to obliterate the se- 

 paration. 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, con- 

 sequently, their analogy to the glacier structure must not be 

 pushed farther than as shewing the directions of the tendency 

 to separation of a very viscid stream, powerfully retarded by 

 its bed. It is the congealing of the lava which makes its 

 adhesion to the- sides great enough, and its own fluidity small 



