BUILDING STONES AND ROAD METALS 179 
and observing the rate at which coloured water rises into it 
and also by determining the amount absorbed by weighing 
a test piece before and after complete immersion. A stone 
with attractive qualities may be unsuitable owing to high 
porosity where rain is flung against the buildings by high 
winds. 
Microscopic Examination—Panama “ Breaks "—These 
tests should be supplemented by microscopic examination 
of thin sections which reveals qualities that cannot be inferred 
from chemical analysis or crushing strength. The method 
quickly determines the chemical composition of coarse- 
grained rock ; it shows whether the felspar is potash-felspar, 
which is generally more durable than the soda- and lime- 
felspars. A bulk chemical analysis shows the proportions 
of the constituents; the microscope shows how they occur. 
For example, the Reigate sandstone, which was used for 
parts of Westminster Abbey, perishes in a city atmosphere, 
although it contains 80 per cent. of silica, for its cement is 
its 10 to 15 per cent. of carbonate of lime; whereas the 
Chilmark Stone with 79 per cent. of carbonate of lime better 
resists weathering because its cement of silica protects the 
calcareous particles. 
The experience at the Panama Canal 1 shows the value of 
microscopic examination of stone, and that crushing strength 
is not always a reliable guide. The International Board of 
Consulting Engineers for the Panama Canal in 1906 expected, 
from crushing tests on bore samples, that the banks would 
stand at a slope of three vertical to two horizontal, and would 
be stable in a cutting 245 feet deep. Yet the bottom of that 
excavation began to upheave when the depth was only 
65 feet. These upheavals, or “ breaks” continued in 1912 
when the slope had been reduced to one vertical to 3% hori- 
zontal. The weight of the banks forced the underlying 
mudstone, after water gained access to it, to upflow into 
the excavation. The upheavals were sometimes fast; a 
heavy steam shovel was uplifted g feet during an afternoon, 
and some machinery raised 11 feet in 10 minutes; in some 
cases the uplift recurred seven times before all the mobile 
layer had been squeezed out. 
Y Reports of the Isthmian Canal Commission, 1899-1911 Vaughan 
Cornish, Edinb. Review, Jan. 1913, pp. 21-42, and Geog. Journ., xli, 
1913, pp. 239-43.