30b PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



In reference to the absence of transverse fractures so pointedly 

 argued upon by Mr. Horner*, I have not sufficiently examined the 

 tract to decide affirmatively that the surrounding brim has no 

 break in it which might have been increased, if not produced, by 

 the uprising of a central mass of trachyte ; particularly if we are to 

 suppose, that that mass, being of a conical shape, widened so 

 much downwards, as I have supposed, as to plug up all the area of 

 a former grand submarine crater. For the most part, certainly, 

 the external furrows on the surface of the dejections which slope 

 down into the surrounding country are only what might be sup- 

 posed to result from thousands of years of atmospheric agency ; but 

 the chief cavity in the south-western side above Sessa, by which the 

 principal water-course escapes, is certainly so deep and precipitous 

 that it might be supposed to be a rent resulting from strain or fracture. 

 In deciding cases of this nature, however, every geologist knows that 

 the clear data which present themselves in limestone, sandstone, and 

 schists are often wanting in volcanic rocks. But however this may be, 

 neither the gorge alluded to, nor any other aperture, affords traces of 

 a subaerial coulee of lava. The eastern side is alone marked by a 

 broad general depression of the brim, which so dies away that the 

 crater may be said to open out to that side by a gradual and undu- 

 lating slope, leading down into the valley of Teano. But here again 

 we in vain look for anything like a lava-stream, or even the remains 

 of one which issued from a true terrestrial volcano ; whilst in the 

 erosion of the hillocks, as in the characters of the surrounding tuff, 

 we recognise only subaqueous action and denudation. My impression 

 therefore is, that the external tufaceous dejections of Rocca Monfina 

 were vomited from a great submarine crater, and were in the first in- 

 stance probably arranged with a certain amount of qua-qua-versal dip, 

 some of them acquiring an almost subaerial aspect, by having been 

 projected into the atmosphere before they fell back again into the 

 sea, and that afterwards the activity of this great subaqueous vent 

 was completely stopped up before the terrestrial conditions were com- 

 pleted, by the upheaval of a solid mass of trachyte which either had 

 been formed, or was kept in a state of pasty fusion at some depth. 

 I suggest that this trachyte may have proceeded from the same 

 volcanic centre, which had either been previously in activity, or was 

 continued in activity, at a greater depth, and under either the pres- 

 sure of water or superincumbent dejections. In a word, Rocca Mon- 

 fina differs only from the crateriform dejections around Viterbo, in 

 having trachyte placed in the centre of a crater, whilst in these Papal 

 examples that rock is usually a great lateral appendage or back-bone, 

 the formation of which was manifestly either of anterior date, or of 

 subsequent protrusion from a deeper-seated centre. 



In adopting this view of the modus operandi by which such an 



apparent crater of elevation was formed, we cannot indeed expect to 



find the chasm so generally present in atmospheric volcanos, with a 



lava-current issuing from it. We might, however, look out for a 



* See Anniversary Discourse, Quart. Journ. Geol. Soc. vol. iii. p. Ixxxiv. 



