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 tlje 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. FOBS, Subap., torn, i, p. 82. f Ibid., p. 78. 



