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THE STEUCTUEE OF EOCK MASSES 
(FOLIATION AND STEIATION). 
By DAVID FOEBES, F.E.S. 
[PLATE LXXIII.] 
I N a previous communication on this subject* the three 
structures, stratification, joints, and cleavage, were described ; 
it now remains to take into consideration the two other systems 
of parallel structure of common occurrence in the metamorphic 
and eruptive rocks, viz. foliation and striation. 
Foliation . The terms foliated rocks and foliation appear to 
have been introduced into geological nomenclature some 
twenty-five or more years ago, by Darwin, and since then they 
have met with very general acceptation by geologists both 
abroad as well as at home ; by the term foliation is signified, 
such parallel structure as makes its appearance in rock masses 
owing to the arrangement of certain crystallised minerals in 
more or less parallel lines, along which their crystals lie on 
their flat sides or lengthways, i.e. having their longer axes in 
the direction of, and not against, the grain of the rock. 
All foliated rocks come under the definition of metamorphic 
rocks, that is, rocks which subsequent to their consolidation 
have undergone a change in the molecular arrangement of 
their original component mineral particles, which change in 
many, if not most instances, has been at the same time ac- 
companied by a re-arrangement of their chemical elements 
also. 
The subject of foliation is one of the most intricate problems 
in geology, and opinions are much divided as to the significa- 
tion and origin of these lines of parallel structure, so that, in 
the present communication, the views propounded must not 
be regarded as representing any universally accepted doctrine, 
* Popular Science Eeview for April 1870. 
VOL. X. — NO. XL. R 
