296 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 13^ 



mistress of the world. The oldest beds visible (and these only in 

 deep denudations to the north and west of St. Peter's and the 

 Vatican, and at the foot of the Monte Mario*) are the blue shelly 

 marls or clay, largely excavated for brick-earth. These are followed 

 by sandstone, occasionally calcareous (" panchina" of Tuscany), 

 by yellow sands, and finally by pebble-beds, the materials of which 

 have nearly all been derived from the Apennine limestone. This 

 is the subapennine shelly group of Brocchi, and in reference to its 

 fossils I may state, that although many forms are common to the 

 lower and upper beds, which are so strikingly distinguished from 

 each other in lithological aspect, there are certain species, such as the 

 Cleodora lanceolata and C. Vaticani, which pertain to the inferior 

 blue marls only, and are never found in the overlying yellow sands. 

 It may also be mentioned, that here, as at Siena, the greatest number 

 of shells are found in the beds of junction of the two divisions. 



The order in which the blue marls (the oldest stratum of the di- 

 strict) are overlaid by yellow sandstones and pebble-beds, and the 

 manner in which the latter were first associated with and next covered 

 over by volcanic materials and then elevated into land, and what 

 changes the surface subsequently underwent, I would attempt to 

 explain in this general woodcut, fig. 40. On the flanks of Monte 



Villa 

 Borghese 



North end of 

 Monte Mario. 

 3 



6 5 1 



Modern 6. Alluvium of ancient valley of the Tiber. 



Post-pliocene. . . 5. Regenerated pebble beds, gravel and travertine. 



j 4. Volcanic tutf (massive). 

 Subapennine or ) 3. Pebble beds, sands and volcanic tuff. 

 Pliocene.... ^ 2. Yellow sands, panchina, &c. 



Ll. Shelly blue marls (brick earth). 



Mario, good evidences exist of the superposition to the blue marls (1) 

 of the yellow calciferous sandstones and sands (2) . In mounting to 

 the overlying pebbly beds (3) we see the first commencement of sub- 

 marine volcanic action in dejections of finely laminated peperino and 

 tuff, which are dovetailed into the uppermost of the subapennine 

 strata. Then follow those tuffs, peperinos and other volcanic rocks of 

 the Campagna, which were so extensively spread out under a foraier 

 sea, and of which the hills of Rome and the Villa Borghese afford 

 examples and varieties (4) . These are, in fact, the submarine accu- 

 mulations which terminated the subapennine period. 



After such masses had been raised into land, and when the valley 

 of the Tiber became in the first instance either a lake or a broad 

 river, detrital accumulations were, it would appear, formed out of the 

 materials both of the pliocene strata ( I & 2) and also of all the sub- 

 aqueous volcanic dejections (3 & 4) which had overspread them. 

 The materials of the ancient gravel above Ponte INIolle decide this 



* It would appear that a good many species of shells have been detected at 

 Villa Madama, the Vatican and other localities since Brocchi wrote. It is not my 

 province to enter into these details. 



