PRODUCTION OF VOLCANIC DYKES. 481 



being constantly ejected abont the upper parts of the cone, and with 

 a weight in reference to surface increasing with proximity to the 

 crater, the tendency would be to increase the angular slope of every 

 volcano as we approach nearer the top. But as the great mass of 

 ejected material is fragmentary and incoherent, and can therefore 

 only repose permanently within a certain slope (or angle of repose), 

 so the tendency to increase the angle of the cone by the deposit of 

 new material is more or less met by irregular creeping downwards, 

 and sometimes by slipping of parts of the mass downwards as in 

 landslips, the discontinuous materials, when thus forced downwards, 

 often carrying along with them and dislocating or distorting such 

 connected beds of lava or dykes as may be involved in the mass. An 

 inspection of the general surface of the escarpment of Somma for its 

 entire length and height at once shows the observer how small, pro- 

 portionally, is the mass of lava-beds or dykes included in the enor- 

 mous surrounding mass of volcanic conglomerate, lapilli, and sand, 

 and appears to the writer fully to sustain the estimate made by him 

 in his paper "On the Nature and Origin" of Yolcanic Heat and 

 Energy " (Phil. Trans. 1873), that the incoherent material, heated 

 but not fused, ejected during volcanic eruptions, exceeds by twenty- 

 fold the mass of the material which is poured forth in fusion. 



The relatively attenuated ribs or plates of consolidated lava, present 

 extremely small resistance to the pressures produced by the enor- 

 mous weight of the discontinuous material around them, and which, 

 by reason of its discontinuity, acts somewhat as if it were an imper- 

 fect liquid or a mere heap of dry sand, in transmitting the internal 

 movements which are continually taking place within it under the 

 joint influence of slope and gravitation. 



Although the rounding, more or less complete, of the fragments 

 which constitute beds of volcanic pebbles, large or small, and of the 

 constituents of conglomerates compactsd together after deposition, is, 

 as has been stated by several authors, produced by mutual attrition 

 prior to their ejection, there can be little doubt that further com- 

 minution and rounding is produced by these powerful though little- 

 noticed movements of descent after their deposit in the mass of vol- 

 canic mountains. The same phenomena, though upon a minuter 

 scale, are to be seen in the rounding which is known to take place 

 to considerable depths in the macadamized stratum of roads which 

 have been long traversed. In these the originally angular fragments 

 of broken road-metal are found rounded when the mass is broken up 

 and exposed to view, to far greater depths than the impacts or pres- 

 sures upon the surface would seem to have been transmitted, intes- 

 tine movements, extending deep into the mass though not noticeable 

 at the surface, being sufficient to rub together and round the once 

 angular fragments to an extent less as their position is deeper from 

 the surface. 



It is obvious, from what has been stated as to the movements of 

 dykes at periods subsequent to their production, that the present 

 positions in which they may be found do not necessarily infer any 

 thing as to the position, whether in orientation or in dip, in which 



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