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ECONOMIC GEOLOGY 
The strength of this * argillaceous sandstone" had been 
over-estimated four-fold, despite the warning of the French 
geologists, Bertrand and O. Zurcher, who had found in pre- 
paring thin sections that water reduced it to mud. 
VARIETIES OF BuiLpiNg Stones—The most durable of 
building stones is granite, which in the stone trade includes 
all coarse-grained igneous rocks such as syenite, diorite, and 
gabbro, and sometimes even granular marble. Granite, as 
defined in geology, consists of quartz, white mica, and ortho- 
clase felspar, and owes its durability to their chemical 
stability. Its absorption of water is very low and often less 
than 1 per cent. The chief trouble with granite is * spalling 
or development of cracks parallel to the surface owing to the 
three constituents expanding unequally on change of tem- 
perature. 
The basic plutonic rocks decay by weathering more readily 
than granite, and being heavier are more expensive to handle. 
The extra weight is an advantage for some uses, as in break- 
waters. 
Limestones are favourite building stones owing to their 
lightness, beauty, and ease of working. They are as a rule 
unsuitable for use in cities, as they decay in an acid atmo- 
sphere. The carbonate of lime on the surface may be con- 
verted into a fur of sulphate, which is easily removed and 
leaves another layer liable to the same change. Dolomite, 
being less soluble than carbonate of lime, may be more 
durable; but the Houses of Parliament at Westminster 
show that poor dolomite decays rapidly. The stone re- 
commended for that building was used at the same date 
for the Geological Survey Museum, three-quarters of a mile 
distant, and it has lasted exceptionally well. In the Houses 
of Parliament, owing to the difficulty of getting an adequate 
supply, dolomite of very inferior quality was accepted. A 
subsequent Royal Commission reported that much of it 
ought not to have been used for building under anv conditions 
whatever. 
Sandstone is largely used in cities being less expensive 
than granite and more durable than limestone. The dura- 
bility of sandstone depends mainly upon its cement. The 
best sandstones consist of quartz grains in a siliceous cement, 
Iron oxide cements, which colour stones red or brown, are 
generally stable.