744 R. MALLET ON AN HITHERTO UNNOTICED CIRCUMSTANCE 



sion being sufficient, to have changed their direction of slope 

 altogether, and now to slope synclinally towards the centre of the 

 mountain, following in fact, more or less, the shape of the concavity 

 of depression of the base ; and this change of inclination will become 

 less and less as we ascend higher in the mountain mass. Further- 

 more, if the material of the base be still less resistant, so as to 

 present, when viewed on the great scale, a certain semblance of 

 plasticity, then the concave depression of the strata beneath the 

 base may give rise around the foot of the mountain to uprise of 

 the strata, and their swelling into protuberances of greater or less 

 height ; so that the final form of repose of the base, after the cone 

 shall have arrived at a certain magnitude, would consist of a central 

 saucer-shaped depression surrounded by a low convex ring of 

 irregular protuberances, rising somewhat above the level of the 

 original plane, and at their outward boundary falling into or coin- 

 ciding Avith that plane. Nearly the same succession of phenomena 

 will take place, whether the cone be deposited as we have so far 

 assumed, upon a base more or less yielding of non-volcanic forma- 

 tions, or whether, as is so often the case in old and great areas of 

 volcanic action, the newly formed cone shall have been deposited 

 upon the spread-out ruins of former volcanos or upon the volcanic 

 strata constituting the flanks of preexisting cones. Thus the 

 enormous masses of Monte Vulture penetrate directly through and 

 are based upon the Apennine limestones and other recent strata ; 

 but Monte JNuovo and numbers of other cones of the Phlegrsean 

 Fields have pierced through and rest upon volcanic formations, to a 

 large extent the ruins of previous cones. 



It follows, from what has been stated, that we should be prepared 

 to find the lower portions of many volcanic cones sloping not out- 

 wardly at all, but inwardly and towards the centre of the mountain ; 

 and that the beds around the base have, by the movement resulting 

 from the depression of the base of the mountain, had their position 

 altered so that for a considerable distance around, though not beneath 

 the base, they also slope inward and downward synclinally, coin- 

 ciding more or less with the form of depression of matter below the 

 base. Those who are unacquainted with the phenomena of dis- 

 placement, and the extent of movement that can take place in the 

 material of our earth's surface, induced by inequality of loading, 

 may perhaps at first regard with some degree of incredulity the 

 reality of such movements due to natural causes as have been 

 already described ; but facts are well known to civil engineers and 

 architects, which sufficiently prove that we do not exaggerate the 

 effects of like forces operating upon the gigantic scale of nature. 

 Railway embankments for example, when they exceed a very mode- 

 rate altitude, force themselves by their own weight into the ground 

 upon which they are placed, so that the previously flat surface be- 

 comes depressed into a hollow or long shallow trough, in breadth 

 equal to the base of the embankment ; and very frequently the 

 surface of the ground at one or both sides of the embankment is 

 forced up into a long swelling mound, more or less nearly equal in 



