PRODUCTION OF VOLCANIC DYKES. 491 



for all such material is more or less compressible, the resistance to 

 compression depending upon the size, form, closenes, and mutual 

 friction of the particles composing it, whether small or great. A 

 circular embankment composed of such compressible but incoherent 

 material, when exposed to liquid pressure from within, suffers a 

 radial compression or approximation of its particles, which is greatest 

 at the interior surface and becomes less and less as we approach the 

 exterior. But the radial or normal pressures which produce this 

 compression or packing up of the particles, also tend by resolution 

 of force, as has been already explained, to separate or draw off the 

 particles one from another in the circumferential direction ; and this 

 drawing asunder of particles, like the compression radially, will bo 

 greatest the nearer we are to the interior surface. In other words 

 a circular embankment such as we have imagined that of a perfectly 

 regular volcanic cone composed of compressible though perfectly 

 incoherent or loose material, must under sufficient liquid pressuro 

 from within begin to give way by the formation of fissures (unless, 

 indeed, the interior material were viscous or plastic for a consider- 

 able thickness, the exterior still being compressible, in which case 

 the interior circumference of the cono would enlarge by mere change 

 of dimensions without any fissures being formed) commencing at the 

 interior surface and passing outwards into the mass in planes 

 whose directions would bo perfectly radial and vertical if the 

 material were perfectly uniform or homogeneous ; and if the thick- 

 ness of the embankment be sufficient in relation to the pressure 

 and the time of its continuance, the fissures may never reach the 

 outer surface. 



The hydrostatic pressure of the liquid originates the fissure into 

 which the pressing liquid at the same time enters and fills it as fast 

 and as far as it is formed. The fissure is in fact the inceptive of 

 bursting ; and if the liquid pressuro and its continuance be sufficient 

 the fissure might be prolonged throughout the entire thickness. As 

 in nature, however, the thickness is generally enormous in propor- 

 tion to the cavity of the crater, fissures commenced from within are 

 seldom found to reach the exterior surface of a volcanic cone, but 

 generally stop short within its mass, partly by reason of the cooling 

 and solidifying of the molten matter forced into the fissure. 



A fissure thus produced and at the same time filled with molten 

 matter will always be widest where nearest the crater ; and were the 

 material of the cone which it divides perfectly uniform, the general 

 form of the dyke produced would tend towards that of a wedge 

 whose outer edge would be bounded by a curve, the nature of which 

 would depend upon the vertical extent of the fissure, and the liquid 

 pressure at increasing depths, and, in nature, of course upon many 

 other conditions. 



The material composing the cone of a volcano, however, is far 

 from uniform. Besides the material of various sorts and various 

 degrees of coherence, usually small, there are intercalated beds of 

 lava of various ages, thicknesses, and qualities, all sloping more or 

 less outwards and at different depths dividing like shelves the mass 



