PRODUCTION OF VOLCANIC DYKES. 489 



volcanic axis radially, or indeed why they should show any deter- 

 minate directions at all in reference to that axis. It seems, there- 

 fore, that while all authors have tacitly or expressly supposed the 

 radial arrngement of volcanic dykes, none have made the attempt 

 to explain the mechanism by which the fissures occupied by these 

 dykes have been produced, or appear to have discerned that the pro- 

 duction of a fissure and its filling with molten matter had been 

 simultaneous and due to the same cause, viz. the hydrostatic pres- 

 sure of the liquid lava more or less filling the crater. A right 

 understanding of the physical conditions of this mechanism leads to 

 some not unimportant conclusions. 



A volcanic cone may be considered simply as an embankment 

 surrounding a central cavity or crater, approaching usually in plan 

 more or less to a circular form, an irregular cone whose base is the 

 brim of the crater. The embankment being formed, as Lyell 

 rightly insisted, merely by the continued projection of materials 

 from the crater, some poured forth as liquid lava, but by far the 

 larger proportion projected as dust, lapilli, and stones, must be 

 regarded as formed by loose material having little or no coherence. 

 It thereforo resists the tendency to being burst open by hydro- 

 static pressure of lava filling up the crater, merely by the weight or 

 mass of incoherent matter forming the embankment, the general 

 vertical section of which anywhere round the axis is rudely trian- 

 gular, the slope of the interior side being comparatively steep and 

 constantly changing, while that of the exterior at the upper part of 

 the cone depends in a great degree upon the angle of repose of the 

 loose material composing it, while further down, by the degrading 

 action of rain and other erosive causes, the angle constantly de- 

 creases to the base, whore the mountain rises from the plain. 



A straight ombankment, like one of those constructed of earth and 

 clay across a natural valley basin for the purpose of impounding water, 

 and having usually a triangular section, very much like that which 

 we have just assigned, the inner or water-slope being usually in the 

 ratio of 3 horizontal to 1 vertical, while the outer slope is generally 

 rather steeper (exactly the reverse of the case of the volcanic 

 embankment, in which the inner slope is small and may even be 

 evanescent, while the outer slope varies from perhaps 25° near the 

 top through an average of about 18° for the whole slope, becoming 

 evanescent at the bottom or plane), resists the hydrostatic pressure 

 of the water which it impounds simply by its mass, which is too 

 great to permit its being caused to slide upon its base by the hori- 

 zontal component of the fluid pressure ; and so long as the mass of 

 earth and clay is kept free from the percolation or leakage of water 

 through it, its resistance is almost wholly independent of cohesion in 

 the material of the bank, and wholly so as regards cohesion in the 

 direction of its length. The sides of a volcanic mountain are not 

 straight, but form an embankment approximately circular in plan 

 about the cavity of the crater, which it surrounds ; and hence 

 another condition of its resistance to fluid pressure within the crater 

 comes into play. The radial and horizontal pressure of the fluid 



