or CALCAREOUS TUPA, is the most impure, the most irregular, and the most porous, of all the varieties of limestone. It occurs in beds generally in the vicinity of lakes and rivers; also encrusting rocks, and enveloping animal and vegetable remains in the proximity of calcareous springs. Immense deposits of calc-tuff have taken place at Terni, and on the banks of the river Anio near Tivoli; where some very curious impressions, such as that of a cart-wheel, trunks of trees, &c., are to be met with. The celebrated Grecian temples of Paestum are formed of this stone, and no doubt owe their existence at the present period to the circumstance of its becoming harder the longer it is exposed to the air; for, as the quarries whence it has been procured are in the immediate vicinity, and the stone previous to being exposed is so much softer, modern Vandals have found it easier to go directly to the quarry for what they wanted, than attack the long weather-beaten and now indurated Doric pillars of the temples. From its property of hardening so much on exposure to the atmosphere and to water, this rock makes a very useful building stone in the formation of bridges. Over the Danube at Ulm a very handsome bridge has been constructed of this stone, which, when brought from the neighbouring mountains, is cut into the required dimensions with the assistance merely of the axe and the saw.