1850.] MURCHISON— EARLIER VOLCANIC ROCKS OF ITALY. 305 



that the trachytic rocks of the Mont Dore could have been formed 

 under the atmosphere, or under the same conditions as the surround- 

 ing and lower craters. The trachytes and even the domites of that 

 region were, I believe, the earliest-formed rocks along that band of erup- 

 tive agency, and were in existence before the outburst of true terrestrial 

 volcanicity. These trachytes may have been formed before the region 

 was entirely drained of the great lakes, which we know prevailed in 

 it. The trachytes of the Rhine, the Siebengebirge, of Hungary, of 

 the Ponza Isles, so well described by Mr. Scrope, as well as those of 

 the Papal States, and lastly those of Ischia, whether we judge from 

 their mineral character, the part they have played, or their physical 

 outlines, all convey assurance to my mind that they were accumu- 

 lated under water. I look upon them therefore as rocks interme- 

 diate between those termed plutonic and volcanic ; which were 

 accumulated under pressure and at greater depths, whether under 

 water or heaps of other volcanic materials, and were sometimes 

 subsequently heaved up to their present positions. In supposing 

 this manner of formation, even the symmetrically central cone 

 of the Puy de Chopine, so well delineated by Mr. Scrope, may be ac- 

 counted for. There, as in Rocca Monfina, the central cone of tra- 

 chyte may have been the result of the last grand eruption, and from 

 its structure and dimensions may have plugged up the whole of the 

 original crater. In the Auvergne case we have only to believe, even 

 if the lands were then desiccated, that the liquefaction of the trachyte 

 took place at some depth, and that v/hen it was raised by the last 

 grand effort towards eruption, the overlying dejections, finding a vent 

 in small craters, were thrown oif as a mantle around it. 



But to return to Italy. In the northern part of the Papal States 

 the order of superposition would indeed lead us fairly to infer with 

 Pareto, that the trachyte was the first-born of these submarine vol- 

 canic products, and was succeeded by the scoriaceous dejections which 

 there overlie it ; but if at Rocca Monfina this conical mass should 

 extend downwards, as there is every reason to believe it does, from 

 the discovery of another protrusion of the same rock, so as to occupy 

 a wide base, as expressed by the dotted line (fig. 5), it would in 

 such cases occupy all the crateriform depression, and we could not 

 then picture even to our imagination the source of the supposed sub- 

 sequent scoriaceous dejections, particularly those of the lofty Corti- 

 nella. In other words, the vent would have been plugged up by the 

 emission of the trachyte. 



Granting, then, that the uprising has been the last operation, and 

 that the trachyte has been so protruded as to have raised up still 

 more, and to have increased the inclination of, the surrounding dejec- 

 tions, it must have been at that time in a solid state. I do not doubt 

 that the circumambient loose materials had been ejected from a great 

 crater previously to the uprising of the trachyte from the same cavity. 

 They may in the first instance have sloped away at angles of 8° or 1 0"^, 

 and have been afterwards in certain parts raised to 20° and 30°, as 

 we now see them. 



z 2 



