DESCRIPTIONS OF ROCKS. A2t 
The presence of an oxidizable ingredient is a common 
source of destruction. Pyrite occurs in grains or crystals 
in almost all kinds of rocks; and it generally oxidizes 
easily whenever water and air get access to it. Only the 
firmest crystals resist change, and these not always. A rock 
containing even a little pyrite can seldom be trusted for 
architectural purposes. If a limestone contain a few per 
cent., or even one, of iron or manganese replacing part of 
the calcium, it has a souree of destruction within it. The 
iron and manganese are sure, after a while, to oxidize ; the 
iron will give rusty stains, and the manganese turn it black, 
and both will work destruction. Achemical trial is needed 
to ascertain the fact as fo the purity or not of the rock. 
The presence of iron carbonate (siderite or spathic iron) is 
the occasion, wherever it exists, of rapid decomposition as 
far down as moisture and air can reach. This has been one 
source of the changes producing the great beds of limonite 
(like those of Western Massachusetts, Salisbury, Connecti- 
cut, and other places), in which the rocks are sometimes de- 
composed to a depth exceeding one hundred feet. 
It is a fact to be remembered that a rock which has stood 
the weather for centuries in its native exposure is a safe 
material for man’s structures ; and one that is crumbling is 
worth little or nothing. 
Durability depends much on the climate. In Peru, even 
sun-burnt bricks will last for centuries. 
The resistance to crushing im rocks is ascertained by sub- 
jecting cubes of a given size to pressure. In recent experi- 
ments by P. Michelot,* Minister of Public Works in France 
(whose trials numbered over 10,000), the most compact 
limestones, weighing 2,700 kilograms per cubic meter, were 
crushed by a weight of 900 kilograms per square centimetre. 
“ompact odlitic limestone of Bourgogne and some other 
Hrench localities, weighing 2,600 to 2,700 kilograms, bore 
700 to 900 kilograms before crushing. Statuary and decora- 
tive marbles bore 500 to 700 kilograms. 
Of granitic rocks from Brittany, the Cotentin, the Vosges, 
and the Central Plateau of France, weighing 2,600 to 2,800 
kilograms, the best, which admitted of polishing, bore 1,000 
to 1,500 kilograms; while the coarser granites of Brest and 

* Exposition Universelle de 1873 4 Vienne, p. 401-432; and Annales des Ponts et 
Chaussées, 1863, 1868, 1870. 
