The granular tufa is useless for either purpose, containing too much earth to be employed in making mortar, and being far too soft to be used as stone for building.
The Mithraeum hewn in the tufa quarries of the Capitoline Hill at Rome, still in existence during the Renaissance, is an example.
Generally speaking, they are only of local importance, their cheapness depending largely on the nearness and abundance of some suitable volcanic deposit of the trass or tufa class.
Outflows of lava and tufa cover the mountain sides and fill up the valleys.
Calcareous tufa or travertine occurs in the valley of Matlock and elsewhere, the southern part of the county, from Matlock southward by Heage, Belper and Duffield to Derby.
Cinder cones and tufa cones abound, but one of the most distinguishing features of the Hawaiian volcanoes is the great number of craters of the engulfment type, i.e.
No homogeneous series of buildings - we find various styles of construction (quasi-reticulate, opus reticulatum of tufa with stone quoins, of the time of Augustus, opus reticulatum with brick quoins or with mingled stone and brick quoins, a little later); and three styles of wall decoration fall within its limits.
Grey tufa period; ashlar masonry of tufa, coated with fine white stucco; rubble work of lava.
By abrupt tufa cliffs; but a massive wall found by excavation on the S.W.
Catacomb of benches cut out of the tufa rock.