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through along the planes of stratification by the two granite 
dykes shown in the section. 
Many years ago, however, attention was directed by Keilhau 
and others to the fact that the planes of foliation did not 
always coincide with those bounding the larger beds or masses 
of rock, seen to he distinct from one another by their differing 
in mineral composition, and that they were sometimes even at 
high angles to them. The subsequent observations of Darwin 
and Sharpe proved, in very different parts of the world, that, 
so far from being identical with stratification, the lines of 
foliation over large areas were also those of cleavage ; and Mr. 
Sharpe, who regarded foliation as the final result of the same 
cause which induced cleavage in rocks, generalised, from ob- 
servations in Scotland and the Alps, that the lines of these 
two structures, taken together, were parts of great arches 
many miles across — an hypothesis, however, which has not 
received subsequent corroboration. In 1854 the author an- 
nounced, as the results of experimental as well as field 
observations, that the foliation in rocks appeared to be a struc- 
ture induced subsequent to the consolidation of the rock, 
following the direction of the planes of least resistance in the 
rock, whether such planes were those of original sedimentary 
stratification, subsequent cleavage, or (in eruptive rocks) the 
strise of fusion ; and this view explains why the lines of foli- 
ation and cleavage so often coincide, since, if a rock had once 
undergone cleavage and subsequently became foliated, the 
foliations would naturally follow the planes of cleavage in pre- 
ference to those of stratification, since the former would be 
those of least resistance. 
It is therefore of the utmost importance that geologists, 
when observing in the field, should — especially in districts con- 
sisting of metamorphic schists and gneiss — continually bear in 
mind that the planes of foliation may not necessarily be in 
any way connected with those of sedimentary deposition, and 
that, in such districts, the only means of arriving at any 
sound conclusion as to what the probable original bedding had 
been, is by carefully studying the difference in mineral cha- 
racter of the various rock masses superposed one on another. 
What the cause of foliation may be, is a problem as yet but 
little investigated or understood. Heat (not necessarily intense) 
appears certainly to have played an important part, since 
foliated rocks are rarely or ever met with unassociated or at 
any great distance from rocks of eruptive, i.e. of igneous origin. 
Experiments made by the author between 1849 and 1853 
showed, when blocks of massive or amorphous soapstone were 
exposed for some months to a temperature not exceeding red- 
ness (infinitely below what would be sufficient to fuse or even 
