J^iO Dr Daubeny (ni the Diluvial Theory^ 



poses a sort of tenacity or adhesiveness to belong to trachytic 

 lava, which, together with its inferior specific gravity, would, in 

 his opinion, exempt it from the laws which regulate the flowing 

 of igneous products in general, and cause it to accumulate 

 round the point from which it issued in concentric layers (of 

 which by-the-by the rock never exhibits any traces) until it 

 reached, at length, the height and dimensions which these cones 

 are now found to possess, of which the Puy de Dome, the lofti- 

 est mountain in the department, may give us a suitable idea. 



Unfortunately for this hypothesis, observation has furnished 

 us with no difference between the manner in which trachytic and 

 basaltic lavas flow when they issue from the earth. The current 

 of Mount Olibano near Naples, which is composed of trachyte, 

 has descended the external slope of the Solfatara, just in the same 

 way as the most modem lavas from Vesuvius; and where, as some- 

 times happens, trachyte and basalt occur together, no difference 

 has as yet been discovered in the thickness or extent of their beds, 

 such as should imply any greater tendency in the one than in 

 the other species of rock to accumulate round a central point, 

 instead of obeying the laws of gravity. 



It would he wonderful, indeed, if any such difference did 



theory is, that no part of the rock in question contains any alkali, except the 

 crystals of glassy felspar occasionally present ; now, this may be reckoned 

 as a natural eflfect of the continued action of water upon a substance of a 

 felspathic nature, as we find at present exemplified in the case of the gra- 

 nites of Cornwall, and of other localities. Thus, he attributes to the modem 

 volcanoes of Auvergne two sorts, or rather degrees, of action : — when their 

 energy was sufficient to break through the trachytic tuff", craters would be 

 formed, having their walls composed of the ejected materials, and this is the 

 more usual case both here and elsewhere ; but when the force was insufficient 

 for this effect, it might simply upheave a portion of the tuff" round^a circum- 

 scribed area, and produce certain alterations in its internal structure, of yhich 

 we have examples in the Puy de Dome, and other mountains of similar Com- 

 position. 



I had myself proposed a similar hypothesis, when I alluded in my work on 

 Volcanos to the curvature of the beds exhibited on the coast of the Island of 

 Procida, near Naples. (See page 1 79.) 



Whatever objections may be raised against that part of Mons. Lecoq*s 

 theory which respects the material from which the domite was elaborated, I 

 think it will be admitted, that the facts he has brought forward are perfectly 

 irreconcilable with the notion, that it was derived from a mass of lava which 

 congealed round the orifice that ejected it. 



