326 
THE TERTIARY VOLCANOES 
BOOK VIII 
rocks. It is difficult to suppose that none of these latest dykes communi- 
cated with the surface, and gave rise to cones with the outpouring of 
basalt and the ejection of dust and stones. But of such later outflows 
of basic material over the surface of the plateaux no undoubted trace 
has yet been recognised. 
