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 w^hich are also so frequent in the laminated 

 or schistose rocks to which I have compared them. 



It may be said that the ribboned vcining 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 6am.e 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 

 places, dragged or pulled out such portions as happened to be of different 

 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 Yenetian enamels. 



Even where the substance is composed entirely of solid particles, if 

 they are of difl'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 difl'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 coarser, which themselves slide more or less over each 

 other, and arc 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 

 clloct of the liquid. 



