486 R. MALLET ON THE MECHANISM OF 



diagonal or "wiping" local thrust. These facts present striking 

 evidence of the immense power of the internal movements which 

 take place within the mass of volcanic cones at periods long sub- 

 sequent to original deposition. 



Beferring now to the disposition of the air-bubbles, these are 

 comparatively rare in the thin and more compact dykes. They are 

 not uniformly diffused, generally existing in greater numbers near 

 the sides of the dyke, although more or less parsemes throughout ; in 

 some instances they thickly congregate in zones towards the centre 

 of the mass, running nearly parallel with its sides. Some large 

 and generally longish irregular cavities of considerable size, with 

 very rough internal surfaces, and often a foot or more in length, 

 occasionally occur, the direction of their longest axes being very 

 indeterminate. But these must not be confounded with the air- 

 bubbles, as they are obviously rents produced in the viscous lava as 

 it was forced onwards. The long axes of the air-bubbles are found 

 on the whole to be nearly horizontal, or pointing at moderate angles 

 upwards, and in directions very nearly parallel with the plane of the 

 dyke at the place where they occur. This is well observable in the 

 dykes Nos. 1, 5, 6, 9, 11, 17, 22, and 27. In Nos. 11, 17, and 27 

 the rents or cavities above spoken of are also to be found. 



From this great preponderant horizontality or slight slope up- 

 wards and coincidence with the planes of the dykes of the long axes 

 of these air-bubbles, I consider the conclusion is fully warranted that 

 these dykes have been filled by injection, not from below, in a 

 nearly vertical and upright direction, but horizontally, as from 

 some source or sources of liquid lava exercising statical pressure 

 upon the containing walls, as in the case of volcanic craters filled 

 more or less with liquid lava. It does not seem possible to assign 

 any physical reasons why the two surfaces of a dyke thus injected 

 should present any characteristic differences, in the material 

 of the dyke differing near one or other of its surfaces from 

 the interior of its mass, but not so differing at the other sur- 

 face, nor any reason why the two opposite surfaces of the same 

 dyke should present any obvious differences, both being derived 

 alike from the impress given to the viscous and cooling mass by the 

 opposite surfaces of the cleft or fissure into which that was injected. 

 That the parts adjacent to both surfaces of the dyke should 

 present certain differences in texture may be expected ; for if the 

 walls of the fissure be at a considerably lower temperature than the 

 viscous mass forced into it, the surfaces of the latter will become, 

 when cold, more or less vitreous, while the interior of the mass 

 remains stony or subcrystalline, this being in accordance with all 

 the well-observed facts as to the rapid or gradual cooling of molten 

 stony masses ; but that one surface of a dyke should present this 

 vitreous character and the other present none, is not easily ex- 

 plained; and equally difficult is it to explain why one surface 

 should have its irregularities mainly concave to the general surface 

 and contain imbedded pebbles picked up and partially involved, 

 and being of the same sort as those pebbles contained in the matrix 



