Buam.—On the Building Materials of Otago. 125 
perties of building stones generally, with special reference to the causes 
that lead to decay, and the means of preventing it. The principal bases of 
stones are silica, alumina, and lime. As can readily be inferred from the 
most superficial knowledge of these earths; the hardest and most durable 
stones are those in which the former predominates, many of them, such as 
granite and basalt, being indestructible. The building stones most subject 
to decay are sand and limestones. In the former, it is caused chiefly by 
the mechanical action of winds, rains, and frosts; and in the latter, by 
these and chemical agency combined. Sandstone is composed almost 
entirely of silica or quartz grains, or dust cemented together by lime, 
alumina, magnesia, or iron, and sometimes by a combination of two or 
more of these minerals. As the particles of quartz are, like the stones 
already mentioned, practically indestructible, the durability of sandstone 
depends entirely on the cementing material. When this is nothing but 
alumina or clayey matter, the stone is of an inferior quality, that base being 
deficient in adhesive properties, and generally soluble in water. The stone 
is therefore peculiarly susceptible to the action of the weather. The pre- 
sence of an undue preponderance of clayey matter in sandstone may be 
detected by washing small pieces in water. If a large muddy residium is 
given, the specimen should be rejected as perishable. Craig Leith sand- 
stone, the best in Great Britain, contains— 
Silica ... ae caw ae nee 98.3 
Carbonate of lime ae ca ty a 
Tron alumina ... ia de an 0.6 
100.0 
Caversham stone, on the other hand, contains— 
Silica... cas er re ‘Ke 24.4 
Carbonate of lime and magnesia__... 53.0 
Alumina ee or at ise 17.6 
Soluble clay... is ns ne 1.5 
Oxide of iron ... ie oe a 1.4 
Water and loss oe ot As 2.1 
100.0 
The reddish sandstones generally contain iron in considerable quantities ; 
when the iron is naturally in a low state of oxidation, the stone has a ten- 
dency to decay on exposure. Change from wet to dry seems to prevent 
rather than assist the cementing process. But when the iron is highly 
oxydized, and the whole a perfectly homogeneous and compact mass, the 
