Scrope'^ MhMolf an: iKe^'Md^) of' Central Frimce. ^^ 



" There is every reason to concltide the trachytes of the Mont Dor and 

 Cantal, as well as the phonolite of the Megen, to have been propelled from 

 a volcanic orifice in a state of liquefaction, and to have followed the incli- 

 nation of the ground they occupied, flowing in a manner differing only 

 from that of basaltic lavas in proportion to their different consistence and 

 very inferior fluidity, and the accidental circumstances which may have 

 occurred to modify their dispositions. 



*^ It is evident that, under similar circumstances of the surrounding levels 

 and of propulsive force, the tendency of a mass of lava to quit the neigh- 

 bourhood of the orifice from which it is emitted, will be in exact propor- 

 tion to its fluidity ; and when the fluidity is at its maximum, it will accu- 

 mulate immediately around the orifice : one layer of the half-congealed and 

 inert substance spreading over that which preceded it, till the whole as- 

 sumes the form of a dome or bell-shaped hillock, perforated in the centre 

 by this chimney or vent through which fresh matter may continue to be ex- 

 pelled, but which will at the end remain closed by that last sent up. Now 

 the variety of trachyte which composes the Puy de Dome and the neigh- 

 bouring Domitic Puys, consisting almost wholly of felspar, and therefore 

 possessing the lowest possible specific gravity, and at the same time a very 

 rude and coarse grain and highly porous structure, is precisely that species 

 of lava which we should expect a priori to have possessed the minimum 

 of fluidity when protruded into the air; and we therefore understand per- 

 fectly why, instead of flowing in thin and continuous sheets or streams to 

 a distance from its vent, like the basaltic lavas produced about the same 

 time and from the same fissure, it has accumulated in dome and bell-shaped 

 hillocks on the point where it was emitted. That this was the mode of 

 production of these masses of trachyte, that they were thrown up on the 

 spots they now occupy is proved by their rising in every instance either 

 from the middle or the side of a regular crater and cone of scoriae. 



" If it could be imagined possible for the volcano of the Mont Dor to have 

 sent forth a vast current of trachyte in this direction, of which these hills 

 have been supposed the remaining segments, in spite of the fact that the 

 great elevation of the granite ridge upon which they rest above the sur- 

 rounding country, renders it the last of all directions which such a current 

 could have taken, and in spite of the improbability that a rocky bed, of 

 which the Puy de Dome, a mass rising 1600 feet above its base, is merely 

 a detached remnant, should have left no traces of its existence in the inter- 

 val between that mountain and the Mont Dor, a distance of 788 miles : 

 yet a still stronger objection to this hypothesis remains behind, viz. the im- 

 probability that the position of each of these remains should severally 

 coincide exactly with that of the vent of a separate recent eruption ; that 

 the only points on which the remnants of this supposed bed are to be found, 

 should be precisely those on which, from the disturbance occasioned by the 

 volcanic explosions, there would be good reason to suppose it might have 

 been destroyed and carried off. 



" The theory of Von Buch evidently approaches more nearly to this ex- 



