Lava is much used for paving-stones in the neighborhood of volcanic districts, where pozzolana (for cement) and pumice stone are also important.
The mineral wealth of the Cyclades has hitherto been much neglected; iron ore is exported from Seriphos, manganese and sulphur from Melos, and volcanic cement (pozzolana) from Santorin.
Six centuries elapsed before the accidental discovery of a sepulchral chamber by some labourers digging for pozzolana earth (May 31, 1578) revealed to the amazed inhabitants of Rome " the existence," to quote a contemporary record, " of other cities concealed beneath their own suburbs."
According to modern physicians it is impossible to live many days in the caves of pozzolana in which many of the catacombs are excavated."
1 The pozzolana is the material required for building purposes, for admixture with mortar; and the sandpits are naturally excavated in the stratum which supplies it.