Ch. XII.] 
SUBAPENNINE MARLS. 
159 
the thickness of an inch. In some of the hills near that city 
the marl attains, according to Signor Guidotti, a thickness of 
nearly 2000 feet, and is charged throughout with shells, many 
of which are such as inhabit a deep sea. They often occur in 
layers in such a manner as to indicate their slow and gradual 
accumulation. They are not flattened but are filled with marl. 
Beds of lignite are sometimes interstratified, as at Medesano, 
four leagues from Parma \ subordinate beds of gypsum also 
occur in many places, as at Vigolano and Bargone, in the ter- 
ritory of Parma, where they are interstratified with shelly marl 
and sand. At Lezignano, in the Monte Cerio, the sulphate 
of lime is found in lenticular crystals, in which unaltered shells 
are sometimes included. Signor Guidotti, who showed me 
specimens of this gypsum, remarked, that the sulphuric acid 
must have been fully saturated with lime when the shells were 
enveloped, so that it could not act upon the shell. According 
to Brocchi, the marl sometimes passes from a soft and pulveru- 
lent substance into a compact limestone *, but it is rarely found 
in this solid form. It is also occasionally interstratified with 
sandstone. 
The marl constitutes very frequently the surface of the 
country, having no covering of sand. It is sometimes seen 
reposing immediately on the Apennine limestone ; more rarely 
gravel intervenes, as in the hills of San Quirico f. Volca- 
nic rocks are here and there superimposed, as at Radicofani, 
in Tuscany, where a hill composed of marl, with some few 
shells interspersed, is capped by basalt. Several of the vol- 
canic tuffs in the same place are so interstratified with the marls 
as to show that the eruptions took place in the sea during the 
older Pliocene period. At Acquapendente, Viterbo, and other 
places, hills of the same formation are capped with trachytic 
lava, and with tuffs which appear evidently to have been sub- 
aqueous. 
Yellow Sand. — The other member of the Subapennine 
group, the yellow sand and conglomerate, constitutes, in most 
* Conch. Foss. Subap., torn. i. p. 82. f Ibid., p. 78. 
