ON THE ELEVATED SUBMARINE ALLUVIA. 301 



been described in a careless manner, as if this was a 

 part of their natural character. Thus they have been 

 said to consist of marl, sand, and gravel, together 

 with sandstone and occasional breccias, containing 

 various marine and terrestrial remains. In a general 

 sense, the beds may be considered horizontal; or rather, 

 as placed at low angles ; being, therefore, unconform- 

 able, under the usual variations, to the inclined calca- 

 reous strata of the Apennine on which they lie. 



The marl bed, which is the lowest, is, in some 

 places, of an argillaceous nature, in others, argillo- 

 calcareous ; often also containing mica. As it is 

 sometimes wanting, the upper bed, which consists 

 principally of sand and gravel, occasionally rests im- 

 mediately on the solid and fundamental limestone. 

 The lowest stratum is the repository of different 

 mineral substances, such as the sulphates of lime, 

 strontian, and barytes, and of flint, quartz crystal, py- 

 rites, bog iron ore, sulphur, and bitumen. Salt springs 

 also rise out of it, and it occasionally gives vent to 

 hot water and sulphuretted hydrogen ; from the vici- 

 nity, probably, of volcanic materials- The upper bed 

 consists of siliceo-calcareous or siliceous 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 everywhere 

 cover the marl bed, being occasionally deficient. This 

 deposit, it may be added, is sometimes accompanied by 

 the partial breccias just noticed, consisting of frag- 

 ments of the older rocks, occasionally containing 

 shells. 



If we take both these beds together, as Brocchi 

 has sometimes done from not seeing the value of the 

 distinction, the organic remains contained in them 

 exhibit great confusion of origin. They comprise 



