VOLCANIC TUFF 325 



volcanoes to whose operations the scenery of Central 

 and Southern Italy is so largely indebted. 



The solid substances ejected by these volcanoes in 

 the earlier stages of their history consisted mainly of 

 fragmentary material dust, sand, stones, scoriae, and the 

 other discrete forms in which molten lava is blown out of 

 volcanic vents by the explosion of its absorbed vapours 

 and gases. Sometimes pieces of limestone or other 

 rock, which were torn away from the older formations 

 underneath, are found dispersed through the volcanic 

 detritus. This fragmentary material, now more or 

 less compacted into the form of Tuff, extends through- 

 out the length and breadth of the volcanic tract and 

 must thus cover some thousands of square miles. In 

 the Campagna, which lies upon it and derives thence its 

 distinctive features, it reaches a thickness of 300 feet or 

 more, while inland it overlaps the Pliocene deposits 

 in detached outliers which run far up the Apennine 

 valleys, reaching heights of 1,200 feet and upward. 

 Compared with the Pliocene strata that lie below it } 

 the tuff presents some characteristic differences which 

 at once arrest attention. It lacks the rapid alternation 

 and variety of parallel layers so marked in the Astian 

 sands. Yet it can generally be seen to possess a 

 stratified arrangement. Here and there, indeed, this 

 structure gives place to a tumultuous accumulation of 

 coarse detritus, huddled together as it fell, large and 

 small lumps of lava being confusedly mingled in the 

 general matrix and forming the rock known to geologists 

 as Agglomerate. Such coarser intercalations probably 

 indicate proximity to centres of eruption, and in some 

 cases may even mark the position of the vents them- 



