Ch. xxxyl] condensation of slate kocks. 605 



dip of the cleavage, or they may have yielded in a plane perpendicular 

 to that dip, or they may have undergone both these movements. By 

 microscopic examination of minute ciystals, and by other observations 

 too minute to be detailed here, Mr. Sorby comes to the conclusion that 

 the absolute condensation of the slate rocks amounts upon an average 

 to about one half their original volume. This must have resulted chiefly 

 from the forcing of the particles more closely together, so as to fill 

 up the spaces left between them, when they only touched each other. 

 The rest of the change has been due to elongation, which has produced 

 slaty cleavage. 



Most of the scales of mica occurring in certain slates examined by 

 Mr. Sorby, lie in the plane of cleavage ; whereas in a similar rock not 

 exhibiting cleavage, they lie with their longer axes in all directions. 

 May not their position in the slates have been determined by the 

 movement of elongation before alluded to ? To illustrate this theory, 

 some scales of oxide of iron were mixed with soft pipe-clay, in such a 

 manner that they inclined in all directions. The dimensions of the mass 

 were then changed artificially to a similar extent to what has occmTed 

 in slate rocks, and the pipe-clay was then dried and baked. When it 

 was afterwards rubbed to a flat surface perpendicular to the pressure and 

 in the line of elongation, or in a plane corresponding to that of the dip 

 of cleavage, the particles were found to have become arranged in the 

 same manner as in natural slates, and the mass admitted of easy fracture 

 into thin flat pieces in the plane alluded to, whereas it would not yield 

 in that perpendicular to the cleavage.* 



This experiment may lend countenance to the opinion that the lamina- 

 tion of basalt and tiachyte, and even of some kinds of gneiss, aud the 

 grain of certain granites, may all have been determined by a mechanical 

 cause, a movement having taken place after the development of crystals 

 in the pasty mass. 



Mr. Scrope, in his description of the Ponza Islands, ascribed " the 

 zoned structure of the Hungarian perlite (a semi-vitreous trachyte) 

 to its having subsided, in obedience to the impulse of its own gravity, 

 down a slightly inclined plane, while possessed of an imperfect fluidity. 

 In the islands of Ponza and Palmarola, the direction of the zones is more 

 frequently vertical than horizontal, because the mass was impelled from 

 below upwards. f In hke manner, Mr. Darwin attributes the lamination 

 and fissile structure of volcanic rocks of the trachytic series, including 

 some obsidians in Ascension, Mexico, and elsewhere, to their having 

 moved, when liquid, in the direction of the laminae. The zones consist 

 sometimes of layers of air-cells drawn out and lengthened in the supposed 

 direction of the moving mass. He compares this division into parallel 

 zones, thus caused by the stretching of a pasty mass as it flowed slowly 

 onwards, to the zoned or ribboned structure of ice, which Professor 



* Sorby, as cited above, p. 610, note. f GeoL Trans, 2d ser. vol. ii. p. 227. 



