764 Transactions of the American Institute. 



a fisli, they are roe stone or oolites ; when the stone is compact, the 

 stratification is evident, and the stones split readily, they are flags. 

 To all these varieties local names are attaclied in dift'erent districts. 

 The limestones in common nse in England for architectural purposes 

 are of several distinct kinds, the Portland stones, Bath stones and 

 many others belonging to the oolites, semi-marbles of very compact 

 grain, derived chiefly from rocks of the carboniferons system, the 

 mixed carbonates of lime and magnesia almost entirely from the 

 Permian rocks on the borders of Derbyshire, Nottinghampshire and 

 Yorkshire, as magnesian limestone, and some hard varieties of chalk. 

 Elsewhere, especially in the south of Enrope, the cretaceous and 

 tertiary limestones are developed into admirable freestones of exqui- 

 site color, very compact and almost non-absorbent. In Italy 

 especially, are found the limestones properly called calcareous 

 tufa (not to be confounded with volcanic tufa), derived from 

 the exposure of waters containing a large quantity of carbonate of 

 lime. On evaporation of the water the carbonate of lime is deposited 

 in a form more or less compact according to the material on which it 

 is thrown down, and the slowness of the deposit. Some very useful 

 building materials are thus obtained, but they are limited to those 

 localities where very large quantities of mineral waters run over the 

 surface. In limestone caverns, and in veins in limestone rocks, the 

 irregular drooping pendants or stalactites, the floor called stalagnite, 

 and the lining walls of the veins are formed in the same way, but the 

 floor is very compact and may be used as marble. Oriental alabaster 

 is of this kind, and it is very abundant in Egypt. The magnesian 

 limestone, generality more crystalline than the oolites are, in propor- 

 tion to their crystalline state, valuable building materials. They are 

 heavy and hard but work well. Tlie great objection to their use arises 

 from the extreme irreguhirity of their composition and texture in the 

 quarry. Sandstones are more simple in their composition than lime- 

 stones, as they vary only in the grain, or magnitude of particles of 

 which thev are composed, in the nature of the cementing medium, 

 and in the extent to which they contain foreign impurities. There 

 are no crystalline sandstones adapted for aroliitectural purposes, and 

 the metamorphosed sandstones are too hard to be treated as freestones. 

 The color of sandstones is due chiefly to the oxyd of iron, which 

 renders them red or yellow, according to circumstances. The material 

 of all sandstones is silica. Clays will not now be considered in detail. 

 They can only be used for constructive purposes when manufactured 



