DESCRIPTIONS OF ROCKS. 421 



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 ciystals 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 source 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. A chemical trial is needed 

 to ascertain the fact as to 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 in 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. 

 Compact oolitic limestone of Bourgogne and some other 

 French 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; Avhile the coarser granites of Brest and 



* Exposition Universelle de 1873 a Vienne, p. 401^32 ; and Annales des Ponts et 

 Chaussees, 1863, 1868, 1870. 



