1850.] MURCHISON EARLIER VOLCANIC ROCKS OF ITALY. 281 



of a boring moUusk would lead to the supposition that the first change 

 after the interval of repose was one of a slight and tranquil elevation, 

 by which the recently-formed deposit was raised either near to or above 

 the surface of the sea, according to the irregularities of the sea- 

 bottom . On the emerged portions the Lithodomus might have lived, 

 and there left traces of its existence, as exhibited in the borings in 

 the calcareous masses of this age at Hedgerley, and in the large and 

 thick oyster- shells of the same date at Maryland Point. The places 

 at which these remains occur, appear to me too detached and isolated 

 to favour the supposition of a continued and extended line of coast. 



The slight elevation here alluded to would also, by bringing fresh 

 currents into action, account for the fact before-mentioned of the ar- 

 gillaceous beds of the London clay sometimes passing gradually down- 

 wards into this arenaceous and conglomerate bed, and at other times 

 reposing abruptly on a mere layer of pebbles, the latter places having 

 been more exposed to the denuding action of these currents whereby 

 the lighter portions of the deposit might have been removed. This 

 movement I consider to have been but part of a very gradual and com- 

 paratively imperceptible oscillation, which, after producing a slight 

 elevation, led to a long-continued subsidence productive of further and 

 important changes in the distribution of land and water of that period. 

 By these mutations new rivers would of necessity be formed, or the old 

 ones would take a new course according as the watershed of the 

 country was shifted, and thus probably originated that powerful flu- 

 viatile action, which swept down into this Eocene sea the vast argil- 

 laceous sediment, with its rich stores of land plants and marine 

 animals, forming the London clay. 



I conclude therefore that the basement bed of the London clay 

 constitutes a well-marked horizon, dividing the London clay formation 

 by a change, both in the palseontological conditions and in the ancient 

 physical geography of the district, from the older Eocene deposits 

 which intervene between the chalk and the London clay, its mineral 

 mass being composed of the debris of the former, while its animal 

 life belongs to the period of the latter. 



I hope to treat of the remaining division of this series at a future 

 period. 



February 6, 1850. 



W. Pengelly, Esq., and Lieut. -Col. J. A. Lloyd, were elected 

 Fellows. 



The following communication was read : — 



On the Earlier Volcanic Rocks of the Papal States, and the 

 adjacent parts of Italy. By Sir Roderick Impey Murchison, 

 G.C. St. S., V.P.G.S. &c. &c. 



Introduction. — During a Avinter's residence in Rome, including 



