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1855.. FORBES FOLIATION OF ROCKS. 179
sideration, as being in some cases the agent in causing as well as
modifying foliated structure.
Whatever may he the cause of the arrangement of foliation in cer-
tain directions, I believe that most geologists agree in supposing that
fohation itself is due to the action of heat, which has reduced the
rock in situ to a fluid, or at least semifluid state, to which some add
the action of gases and pressure.
On this point I would wish to make some remarks ; and I may
premise by stating, that to me foliation appears to be the result of
chemical action in recombining the elements existing in amorphous
sedimentary rocks, together with a simultaneous molecular move-
ment of the products thus formed ; also, that this action is efi'ected
by heat, but has taken place at temperatures lower than even neces-
sary to change the external form of the masses, or to produce any
semifusion or even softening.
As corroborative of this, I may refer to fig. 10, which Keilhau
Fig. 10. — Section of the gneiss in the Island of Jomfruland^
Norway.
has also observed and made mention of*. It represents an appear-
ance in the gneiss on the island of Jomfruland.
Here we have a vein of hornblendic character running across the
gneiss, and disturbed by the fault AB, which throws it downwards.
On examining the lines of foliation, it will now be found that those
lines which are most distinct are not at all affected by the fault, and
continue throughout with the greatest regularity ; so that it can
hardly be doubted, that the foliation of the gneiss took place later
than the formation and subsequent dislocation of the hornblende vein ;
and consequently we cannot suppose that the mass could have been
in a fused or softened state at the time of fohation without the
obliteration of these appearances. Again referrmg back to fig. 8,
p. 177, it will be seen that the fault b, the origin of which apparently
was previous to or simultaneous with the granite-vein, has altered the
position of the foliation on the one side and bent it upwards. The
faults c, D, and e do not effect this at all.
If, however, the rock had been in a semifused state when foliation
took place, or when the granite-vein was injected, we should have

• Norske Magazin for Naturvidenskab, vol. iii. p. 175.