1XX PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



pact lavas and the tephrines were already formed, had been spread 

 in uniform sheets over the surface, and cooled before the scoriaceous 

 matter had been forced upwards and exposed to view. The pressure 

 from below upwards, exerted by this matter on the upper sheet of 

 lava, first fissured it in a star-like form, over an extent of ground 

 proportional to the force, the radii of the sector-like spaces proceed- 

 ing from the point of application of the force ; the scoriaceous matter 

 then forced its way through by lifting up that portion of the sectors 

 which offered the least resistance, escaping by the opening formed, 

 and completing the cone, of which the elevated sector formed only a 

 part. The blocks scattered over the ground at the foot of the sector 

 were detached and projected at the time of the elevation. The mode 

 of formation of elevation-craters, M. Dumont derives from the pre- 

 ceding explanation: when, for example, the sum of the areas of 

 sectors raised constituted but a small portion of the circumference, 

 the result was a conical mountain only ; but when the larger portion 

 or the whole of the sectors had been uplifted, the scoriaceous matter 

 could no longer fill up the cavity formed, and a true crater of eleva- 

 tion was then the result. As an example, M. Dumont quotes the 

 crater north of Mayen, near Ettringen, where the beds of tephrine 

 which are horizontal in the quarries of Mayen are observed to be 

 tilted up, whilst the scoriaceous lavas are seen to underlie them and 

 to fill up several vertical fissures which correspond to the radii of 

 the sectors. M. Dumont adds that there can be no doubt from a 

 consideration of these facts, as to the mode of formation of this kind 

 of crater. Of craters properly called craters of eruption, M. Dumont 

 states that the only well-characterized one he has observed is that 

 near Gerolstein, which is situated on the summit of a calcareous hill, 

 and has a well-marked excavation, containing scoriaceous lavas. It 

 would appear, then, according to this explanation, that when the flexi- 

 bility of the strata is sufficient to yield to the pressure without being 

 broken off, either a cone or a crater of elevation will be formed ; but 

 when inflexible, the mass of the rock is torn away, and a true crater 

 formed, round which the scoriaceous matter would be then arranged. 

 The lake-craters, though apparently proceeding in the first instance 

 from the uplifting and rupture of a portion of the rocky crust of the 

 earth, do not generally exhibit any appearance of true volcanic rocks, 

 the place of the scoriaceous lavas being here supplied by agglo- 

 merates, the paste of which is dried mud, and the imbedded nodules 

 fragments of the fissured schists and grits of which the borders of 

 the crater consist. The volcanic bombs found in connexion with 

 these craters, M. Dumont considered as having been projected through 

 the mass of mud at the time of the eruption, the granular structure 

 of the bombs being analogous to that which a vitreous substance 

 strongly heated, and then suddenly cooled, would assume. This is 

 an interesting view of a very obscure subject, and, though in a certain 

 degree speculative, may be fairly considered as one of the most sy- 

 stematic explanations of the formation of craters which has, up to the 

 present time, been attempted*. 

 * Whilst this address has been passing through the press, the subject has been 



