46 SIXTH LETTER ON GLACIEKS. [1844. 



" ribboned structure " of glacier ice, i. e. 9 inclining slightly from 

 the sides towards the centre of the current, in the direction in 

 which the current is moving. These striae, or ripple-marks, 

 which have a striking analogy in certain cases of the retarded 

 movement of rivers, are carefully to be distinguished, on the 

 one hand, from the cracks or flaws, and, on the other, from the 

 direction of motion of the fluid particles.* 



III. When, at some distance from the source, the lava 

 became viscid and tenaceous, and forced itself, in streamlets of 

 a pasty consistence; through the interstices of its slag, thence it 

 became streaky and drawn out, in the direction last mentioned, 

 as molten glass does in the hands of the workman. 



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

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

 from their origin, and where by any circumstance their surface 

 has been broken up, and their internal structure exposed. 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 continuity 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, however, 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 frontal 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 diffi- 

 culty to the semifluid theory of glaciers, namely, the want of 



* A long accidental delay in the printing of this letter enables me to add, that 

 I have found in the lavas of Etna a yet far more perfect analogy to the veined 

 structure of glaciers than that described in the text. It is, indeed, so completely 

 developed as to leave no doubt as to the identity of origin. 



