Alfred Harker—Physics of Metamorphism. 19 
converted into heat or chemical energy or both, and so contribute 
to the chemical part of the metamorphism. 
There is another case which must be to a certain extent realized 
by some rocks, and which we may picture as that of a mass consist- 
ing of grains individually rigid, but capable of sliding upon one 
another. This is the “granular medium” investigated by Professor 
O. Reynolds.1_ Here a pure shear is impossible, since any change 
of shape in the whole, effected by rearrangement among the grains, 
involves a change of total bulk. If such a mass has already been 
“packed” into a state of minimum volume (not necessarily the 
least possible volume), any deformation will increase its bulk, and 
so will be resisted by the pressure. Such a condition could be per- 
manently relieved only by chemical action, beginning perhaps by 
the solution of the grains at their points of contact. It appears too 
that the intercalation among more yielding materials of a granular 
rock, which becomes in places ‘“‘ packed” to a minimum volume, will 
often determine internal disruption and faulting of the mass. 
The relation between foliation and the banded or gneissic structure 
is a question which need only be referred to briefly. When the 
latter is produced by the same prime agent as the former, as for 
example in the manner described by Mr. Teall,’ the two structures 
will necessarily be parallel to one another. But when foliation is 
set up by dynamical agency in rock-masses possessing a previous 
banded structure, whether from stratification, or earlier metamorphism, 
or any other cause, the former will not of necessity be parallel to 
the latter, although it will tend to become so with increasing 
deformation. Numerous examples of foliation oblique to gneissic 
structure have been described and figured.* For the sake of clear- 
ness, it does not seem advisable to name this peculiarity ‘double 
foliation.” If we use the term foliation strictly with reference to 
the intimate structure of the rock, and not to alternations of different 
petrological types, a rock-mass cannot present two foliations in the 
same place: the second will destroy the first as a direction of true 
schistosity. The same is true of slaty cleavage, notwithstanding 
various statements to the contrary. I have examined numerous 
examples of the local phenomenon styled “double cleavage” in the 
Ardennes, and in all cases resolved the second set of structural 
planes into a false (or ausweichungs-)cleavage, {consisting in a 
succession of minute folds or faults of the kind figured by Heim, 
Reusch, and others. The “cross-cleavage” of Wales and the Lake 
District appears to be merely a fine jointing. 
In conclusion, it may be well to remark that the analysis of 
internal forces into uniform pressure and shearing stresses is quite 
general. Such ideas as torsion or wrench on a screw do not import 
1 Phil. Mag. ser. 5, vol. xx. p. 469, 1880. 
2 Grou. Mac. 1887, p. 484. 
3 Q.J.G.S. vol. xliv. p. 898; 1888: Reusch, Krystallinischen Schiefer, pp. 74 
and 115; 1883. 
4 Mecanismus der Gebirgsbildung, plate xv. figs. 8 and 11; 1878: Krystallin- 
ischen Schiefer, p. 51; 1883: Bémmeloen og Karméen, p. 196; 1888: Report Brit. 
Assoc. for 1885, pp. 838 and 840. 
