1850.] MURCHISON — EARLIER VOLCANIC ROCKS OF ITALY. 301 



Daubeny'*' called the special attention of Britisli geologists, and 

 whose relations were critically discussed by Mr. Horner in his dis- 

 course as President of the Geological Society in 1847. 



This lofty tract, the interior of which was, as Daubeny has shown, 

 the stronghold of the warlike Aurunci long before Rome had acquired 

 its great power, hes between Sessa on the west and Teano on the 

 east, and derives its present name from a high rock of solid trachyte 

 (Monte della Croce) which rises in the centre of a grand upland 

 crateriform depression, and is about 3300 feet above the sea. This 

 high tract is shut out from the circumjacent low countries or bays in 

 the apennine limestone by a more or less circular ring of eminences 

 of different altitudes, the highest of which, called Monte Cortinella, 

 is nearly as lofty as the central mountain ; whilst the others are of 

 much less altitude, being on an average not more than 300 or 400 feet 

 above the upland depression. They constitute, on the whole, a brim 

 or margin surrounding the circular depression, from the centre of 

 which rises the above-mentioned trachytic mountain. These emi- 

 nences of the margin differ, however, essentially in their composition 

 from that of the central mountain. They consist of tuff, scoriaceous 

 rocks, and peperino with pumiceous or trass4ike dejections, whose 

 outward slopes extend on all sides into the surrounding low country, 

 as if radiating from a common centre. In circular arrangement, and 

 even in having a central boss, Rocca Monfina is analogous to Hanni- 

 bal's Camp in the Latian Hills ; but when we compare the two tracts, 

 essential distinctions arise. Thus, the central boss at Hannibal's 

 Camp is of the same composition as the immediately surrounding de- 

 jections of Monte Cavi, &c. ; whilst at Rocca Monfina the centre and 

 flanks are of such very different structure, that the one must have 

 been formed separately from the other. 



Pilla had remarked that the culminating-point of the central mass 

 of trachyte, or the R,occa, is precisely equidistant from all parts of 

 the escarpment of the surrounding dejections f; but he omits to state, 

 and I do not find it noticed in other authors, that portions of the 

 same trachyte protrude in lower bosses between the main mass and the 

 environing brim. This feature, however, I particularly observed to 

 the south of the hamlet of Casa Fredda, where a mount of trachyte 

 occurs upwards of half a mile distant from the slopes of the chief 

 mass of that rock, and consequently near to the surrounding belt of 

 volcanic hills. 



The diameter of the crateriform cavity is about two and a half miles, 



die Natur und den Zusammenhang der Vulcanischen Bildungen, 1841 ; and PillOf 

 Annales des Mines, 1841, and Saggio Comparativo, 1845, 



* See Dr. Daubeny's Memoir, Transact, of the Ashmolean Soc. Oxford, 1846 ; 

 and also printed in the New Edin. Phil. Journal, vol. xh. p. 213. 1 visited Rocca 

 Monfina from St. Agata and Sessa at the end of February 1848, and passed along 

 its eastern flanks in the following month on my return from Naples to Rome. 



t I presume that PiUa could only mean, that the escarpment of the limited por- 

 tion of the brim called the Cortinella described an arc exactly equidistant in its 

 parts from the summit of the Monte della Croce ; but this is very different from 

 the interpretation usually accepted. 



