1850.] MURCHISON — EARLIER VOLCANIC ROCKS OF ITALY. 289 



but are simply the subapennine clays with yellow sands and sand- 

 stone, we see how the Etruscans, building their city on the former, 

 constructed their tombs in the dry and easily worked strata of the 

 latter, the "panchina" of the Italians. Tufaceous dejections and 

 erupted rocks containing leucite similar to those already described, 

 occupy large portions of the Papal Maremma, They are also parti- 

 cularly well developed in that undulating low country which con- 

 stitutes the north-western part of the Campagna and extends from 

 Monterosi by Nepi to Civita Castellana. The cliffs and deep denu- 

 dations of the last-mentioned place are very striking, and the right bank 

 of the Tiber near Borghetto is instructive in exhibiting the pebble 

 beds of the upper subapennine period surmounted by leucitic lava, 

 thus unquestionably proving, that this igneous rock was poured out 

 under the waters, whilst still lower down in the valley there are re- 

 aggregated heaps of a later aqueous drift, in which leucitic rocks and 

 the debris of pre-existing pebble beds are all mixed up together ; a 

 condition of things in all respects analogous to that which I have 

 described as occurring at Ponte Molle near Rome*. 



Again, in passing by Eropete and Castelluccio to Otricoli, pebble 

 beds composed of Apennine limestone are seen distinctly to alternate 

 with volcanic tuff. Associated with these subaqueous volcanic de- 

 jections of the Papal States, are travertines which have evidently been 

 formed at different periods. Thus, near Siena, the accompaniments 

 of the volcanic phsenomenon are almost confined to a copious evolution 

 of travertine, loaded with coarse angular blocks of younger secondary 

 limestone, occasionally two and three feet in diameter, which are honey- 

 combed throughout and associated with tuff, marl, &c. These masses 

 immediately overlie the subapennine marls and sand, and are covered 

 by lacustrine limestone with Lymnece and Planorbes ; and this again 

 by sandy loam with land remains and coarse alluvia. 



Travertine past and present. — The sequence of strata in the 

 southern parts of Tuscany just mentioned, shows very clearly, that 

 the purely marine condition of the strata under which the sub- 

 apennine sea-shells were accumulated, gave place gradually to other 

 subaqueous conditions, in which, after many " quasi " volcanic erup- 

 tions, accompanied or followed by the formation of much traver- 

 tine, those conditions were at length terminated by a more purely 

 terrestrial state. Evidences in the Campagna di Roma still more 

 completely sustain this view. Spada and Ponzi have, it is true, found 

 fragments of an older travertine in the tuff of this region f, but they 

 admit, that the chief masses of travertine (principally formed under 

 water) are of a date posterior to the earlier volcanic rocks I have 

 been describing. In my former communication | I exhibited a dia- 

 gram showing how the lacustrine travertine of Ponte Molle belongs 



* See Quart. Journ. Geol. Soc. vol. v. p. 296. 



t Savi describes a travertine in the Pitigliana in Tuscany, v^^hich is covered by- 

 volcanic tuff and peperino. Such cases are, however, exceptional, and Pilla admits 

 that the great masses of ancient travertine are of the younger pliocene age. — Saggio 

 Comparativo dei terreni, &c. p. 25. 



X See Quart. Journ. Geol. Soc. vol. v. p. 296, fig. 40. 



y2 



