AFFECTING THE PILING-UP OF VOLCANIC CONES. 743 



same as that of the exterior, being always much steeper, and often 

 even vertical, and no longer dependent, as is the slope of the exterior, 

 mainly upon the angle of repose of the material. 



For our present purpose, however, it is unnecessary to enter upon 

 the more complex conditions which affect the building-up of the 

 interior sides or craters of volcanic cones. Enough has been said to 

 show that, as the deposition of the matter of the cone proceeds, the 

 pressure of the huge mass upon the base on which it rests con- 

 tinually increases with the increase of deposited matter ; and if we 

 assume that the material of the cone is discontinuous and free from 

 intercohesion of its parts — an assumption quite allowable considering 

 the very small cohesion observable in all known volcanic materials, 

 regarded in large masses, such as those of entire cones, — then the pres- 

 sure upon the base per unit of surface will be approximately as the 

 vertical depth of material above that unit of base; and as the depth of 

 deposited material is greatest at and about the summit of the volcanic 

 cone, so the maximum pressure upon the unit of base will be situated in 

 the line of a closed curve, approximating to a circle, not far distant from 

 the summit of the interior slope, or, in other words, beneath the closed 

 curve drawn through the junction of the anticlinals of the inner and 

 outer slopes. But the area of the crater of all terrestrial volcanos is so 

 small in proportion to the whole base of the mountain, that it will 

 be sufficiently accurate for our purpose to consider the pressure 

 upon the unit of surface of base to increase constantly from the 

 outer toe of the slope of the exterior of the mountain to the centre, 

 and to be everywhere proportionate to the depth of material resting 

 on the unit of base. If the ejected material, be deposited upon a 

 plane of coherent material, such as, in the exceptional case referred 

 to, is the table of granite underlying the puys of Auvergne, then 

 the arrangement of the material, deposited as just described, must 

 remain unaltered, excepting so far as future violent efforts of volcanic 

 force, or the external changes in form producible by rain-torrents, 

 &c. , may affect it. If the base of the mountain consists of less-resistant 

 strata, as in the case of the softer and more pliable stratified forma- 

 tions, and in the instance of Etna, it may yield more or less to the 

 constantly accumulating weight of matter deposited upon it, and be 

 depressed more or less below the position which it occupied prior to 

 the deposition of the cone. It follows, therefore, that, could we lay 

 bare such a base, we should find that the originally flat or level sur- 

 face had been depressed so as to form a saucer-shaped or, roughly, an 

 inverted conoidal concavity, the depression being greatest at or 

 near the central parts of the base, and becoming evanescent 

 around the margin or toe of the outer slopes of the mountain. 

 It will follow from this, that as the earliest deposited strata of the 

 cone, as has been already shown, sloped outwards at a very small 

 angle, so, during the progress of very slow descent of the mass fol- 

 lowing down into the concavity made by its own weight, the descent 

 being greatest nearest the centre, these lowermost strata of the 

 cone, could we lay them bare, would be found either to slope out- 

 ward at a less angle than they did at first, or, the amount of depres- 



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