Pozzuolana, a reddish, porous volcanic mineral found near Pozzuoli, between Rome and Naples, and in other countries in the neighborhood of volcanoes. The catacombs of Rome were excavated in a large deposit of this argillaceous sand, and it has been found in Sardinia, in the south of France, and in Rhenish Prussia near Andernach, where it is called trass. It is mainly a dehydrized silicate of alumina with other earths and alkalies, formed by the pouring of basaltic lava floods over argillaceous beds, or by similar natural processes. According to Berthier, the composition of Italian pozzuolana and the German trass is as follows:

CONSTITUENTS.

Pozzuolana.

Trass.

Silica...

44.5

57.0

Alumina.....

150

12.0

Lime....

8.8

2.6

Magnesia....

4.7

1.0

Oxide of iron....

12.0

5.0

Potash....

1.4

7.0

Soda....

4.0

1.0

Water and loss...

9.6

14.4

Total....

100.0

100.0

Pozzuolana and trass are used for the preparation of hydraulic cements in the countries where the deposits are found, and are also exported. Their adaptability to the making of cements arises from the fact that the silica is in a condition to be easily acted on by calcic hydrate with which it is mixed, and also from its containing alkalies and oxide of iron, all of which unite in forming a mass which hardens in water. (See Cement).