340 SI GLDIUES INOUE (SIG OMAIN TICS, 
CHEMICAL 
The chemical analysis is the only exact method of determin- 
ing the composition of a rock in terms of the elements that 
compose it. It is also the best method of determining the rela- 
tive proportions of the mineral constituents. The presence of 
deleterious constituents, such as ferrous iron, bitumen, etc., and 
the proportion that they bear to the total mass of the rock may 
also be determined in this way. 
MICROSCOPICAL 
Much may be learned of the mineralogical composition and 
physical characteristics of most rocks by a careful examination 
of the hand specimen, especially with the aid of a magnifying 
glass. Many rocks, however, are so fine grained that the min- 
eralogical composition and texture can only be accurately deter- 
mined by an examination of thin sections under a compound 
microscope. 
It is thought that the microscopical examination is of much 
greater practical importance and less expensive than the chem- 
ical analysis. By use of the microscope and thin sections both 
the mineralogical composition and texture of a rock can be 
determined with a high degree of accuracy. The relative 
abundance of the different minerals and even the chemical 
composition can be approximately estimated. Minerals that 
are easily decomposed and liable to cause discoloration can be 
identified, and the presence of cracks, strains, and gas bubbles 
can be detected. A single caution should be observed in this 
connection. Cracks and strains are thought to be frequently 
due to stresses resulting from cutting and grinding the thin sec- 
tion, on account of which care should be exercised in drawing 
conclusions therefrom. The size and abundance of the pore 
spaces can be estimated from the texture, closeness, and manner 
of contact of the grains. All the characteristics of a rock which 
contribute to its strength, hardness, elasticity, capacity to resist 
alternating and extreme temperatures, and immunity from the 
