364 
THE GEOLOGIST. 
of its neighbour. The result being the production, on the surface of 
the fluid, of an arrangement which very closely resembles that of 
sections of the ribboned lavas and trachytes upon the table from 
Ponza, Ischia, the Ascension Isles, &c., as well as those of gneiss and 
mica-schist. The irregularities of movement caused by variations of 
pressure occasion evidently those accidental wavings and re-plications 
which are seen here, and which are also so frequent in the laminated, 
or schistose rocks to which I have compared them. 
It may be said that the ribboned veiuing in this case is merely 
superficial, whereas in the rocks it penetrates their substance through- 
out. But so it equally would in the first case if the colours, instead 
of merely floating on the surface of the liquid, were suspended in it 
throughout. The internal patches would then be similarly drawn out 
in the direction of the movement, as we see the superficial ones to be, 
and the same ribboned appearance would be produced in a vertical 
plane or section which we here see produced in a horizontal one. An 
example of this result may, indeed, be observed in the fractured edges 
of many coarse kinds of pottery or pantile. The squeeze to which the 
clay has been subjected when pressed into the mould having, in some 
placesj dragged or pulled out such portions as happened to be of difi'erent 
consistency from the rest into stripes or veins, which give a ribboned 
grain to the material. 
The same ribboned structure may often be seen in the slags of iron 
and of glass furnaces, as well as in some peculiar varieties of coloured 
or enamelled glass, especially among the old "\^enetian enamels. 
Even where the substance is composed entirely of solid particles, if 
they arc of difi'erent sizes and shapes, their subjection to pressure tends 
to produce a similar structural arrangement, as when a heavy roller is 
passed over gravel consisting of fragments of rocks or pebbles and sand 
of difi'erent shapes and sizes. The finer and more globular particles 
yield most readily to the pressure, and are squeezed out, in a manner, 
from among the coarsei', which themselves slide more or less over each 
other, and are turned round so as to lie with their flatter faces in the 
plane of the surface, that is, perpendicular to the direction of the pres- 
sure. If a certain amount of moisture be present (as when gravel is 
rolled after a shower of rain) this internal mobility of the materials is 
increased, and the laminar arrangement facilitated by the lubricating 
cficct of the liquid. 
