282 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



journeys to and from that city by the Maremma, Siena, and Viterbo 

 on the west, and by Perugia on the east, together with excursions, 

 including one to Naples, in the spring, the abundant " earlier vol- 

 canic " products of that region were necessarily brought under my 

 notice. For, although my chief object in visiting the peninsula was 

 to ascertain the order of succession from its secondary to its tertiary 

 strata, the very existence of those formations, particularly of the 

 latter, was so intimately connected with contemporaneous or subsequent 

 eruptions of volcanic matter, that it was impossible for any geologist 

 not to be alive to the importance of observations which tended to 

 throw light on the conditions under which such igneous operations 

 took place. I had moreover a strong additional motive to enter into 

 this branch of our subject, from habitual intercourse with my friend, 

 the accomplished mineralogist. Count L. Medici Spada*, and his zoo- 

 logical coadjutor Professor Ponzi. 



Although all the materials prepared by these authors will, I trust, 

 at no distant day be publishedf , still as they will even then appear 

 in a foreign language, and will imbody local phsenomena not re- 

 quisite for the clear comprehension of the subject, I venture to offer 

 the present sketch, which, although in part derived from the labours 

 of my above-mentioned friends, contains a modification of their 

 views concerning the Latian Hills, which I consider to be of some 

 importance J. 



Not entering fully into the details of the literature of this subject, 

 I may briefly observe, that, among the writers who have treated of the 

 subsoil of Rome, Brocchi was the first modern who gave it a specific 

 geological character. Whatever errors his work, entitled ' Suolo di 

 Roma,' may have contained, have been corrected to a great extent 

 by Leopold von Buch, Lyell, Hoffman, Pareto, and other authors. 

 Still, looking at the only geological map of Italy yet published, the 

 value of the distinction which is advocated in this memoir will at 

 once be seen. For in that very useful map, the author, M. Collegno, 

 has not attempted to separate the oldest trachytes and basalts, as- 

 sociated with the subapennine strata and obviously of subaqueous 

 origin, from modern volcanos ; so that the Euganean Hills and 



* At that time Monsignore Medici Spada. 



t Monsignore Medici Spada was preparing for my use a descriptive memoir in 

 Italian, wliich it was my intention to have had translated into English, when the 

 political agitation to which Rome became subject suppressed every spark of scien- 

 tific energy. Still, before I left that city, I was fortunate enough to induce Pro- 

 fessor Ponzi to lay aside his military uniform and accompany me over the Latian 

 volcanos, of which he prepared an instructive map, of which the diagram, fig. 3, 

 p. 285, is a reduction, accompanied by sections. 



i Since these pages were written. Professor Ponzi has pubhshed a memoir, en- 

 titled " Osservazioni Geologiche lungo laValle Latina," Roma, 1849, in which he 

 shows the existence of volcanic rocks, of the same age as those of Hannibal's 

 Camp, at Tichiena and Posi, near Frosinone. I cannot agree with my friend, when 

 he places the Macigno between the hippurite limestone and the nummulite rocks, 

 nor do I admit that there is any geological distinction between the cretaceous lime- 

 stone of the Volscian Hills and that of the Sabine Hills. (See my former Section, 

 Quart. Jour. Geol. Soc. vol. v. p. 281, fig. 35.) 



