326 THE ROMAN CAMPAGNA 



selves. Alternations in the character of the successive 

 beds of tuff may be regarded as evidence of variations 

 in the energy and distribution of the active orifices. It 

 may be added that the tuff supplied the Romans with 

 various admirable building materials. In the days of 

 the Kings and of the Republic, its more compact kinds 

 were quarried in large quadrangular blocks for the con- 

 struction of massive walls, while in later times some of 

 its more incoherent varieties were discovered to be 

 capable of forming the most durable concrete, which 

 in the hands of Roman architects was employed with 

 a boldness and skill that have never since been 

 equalled. 



That the materials of the tuff were assorted under 

 water is suggested by their stratified structure. This 

 inference is strengthened by the intercalation among 

 them of sheets of sand, gravel, clay and marl. The 

 layers of gravel are especially important, for their 

 component pebbles of limestone and other non-volcanic 

 stones are unmistakably fragments of Mesozoic rocks, 

 which have been rolled along by running water from 

 their original resting places among the Apennines so as 

 to acquire smoothed and rounded forms. But though 

 the tuff was accumulated under water, it presents a 

 strong contrast to the clays and sands below it by its 

 generally unfossiliferous character. Leaves, branches 

 and stems of ilex, oak and other land-vegetation have 

 been obtained from it at various places, sometimes as 

 mere hollow moulds or in carbonised forms, but occa- 

 sionally with the internal structure still preserved. 

 Less frequently it has yielded the bones, antlers or 

 tusks of terrestrial quadrupeds. But both the plants 



