Actions of Volcanoes. 269 
Brocchi under a common term, consists of two beds, and it is 
highly essential to distinguish these where they are regular; 
because, as they are in some places much confused, they have 
been sometimes described in a careless manner, as if this was 
apart of their natural character. It will soon be seen that this 
confusion is the result of posterior, and sometimes of recent, 
causes. Where they have been described in this loose and 
general way, they have been said to consist of marl, sand, and 
gravel, together with sandstone and occasional breccias, contain- 
ing further various marine and terrestrial remains. In a general 
sense, the beds may be considered horizontal, or rather as 
placed at low angles ; and they are, consequently, unconform- 
able, under the usual variations, to the inclined calcareous 
strata of the Apennines on which they lie. 
The marl bed, which is the lowest, is, in some places, of an 
argillaceous nature ; in others, argillo-calcareous ; besides which 
it often contains mica. As it is sometimes wanting, the upper 
bed, which consists of sand and gravel principally, occasionally 
rests immediately on the solid and fundamental limestone. 
This lowest stratum is the repository of different mineral sub- 
stances, such as the sulphats of lime, strontian, and barytes,, 
flint, quartz “crystals, pyrites, bog-iron ore, sulphur, and bitu- 
men. Salt springs also rise out of it, and it occasionally gives 
vent to hot water and sulphuretted hydrogen; phenomena 
arising probably from the vicinity of volcanoes or volcanic 
materials, 
To describe the upper bed more particularly, it consists of 
siliceous or siliceo-calcareous sand and gravel, often containing 
mica and yellow ochre, while in some places, as at San Marino 
and Volterra, it becomes a solid sandstone. It does not 
every where cover the marl bed, being occasionally deficient. 
This deposit, it may be added, is sometimes accompanied by 
the partial breccias just noticed, consisting of fragments of the 
older rocks, and occasionally containing shells. 
If we take both these beds together, as Brocchi has some- 
times done, from not seeing the value of the distinction, the, 
organic remains contained in them exhibit great confusion of 
