ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 845 
which are clearly referable, in whole or in part, to the action of mechanical 
forces. With the views of those geologists who ascribe the formation of 
certain schists to unique physical conditions prevailing in Pre-Cambrian 
times, we are in no wise concerned. Neither is it within the province of 
this paper to discuss superinduced foliation on pre-existing structural 
planes in a rock. We will only remark that such foliation in a stratified 
rock may apparently be produced upon any kind of structural planes 
previously existent in the rock, whether of lamination, bedding, current- 
bedding, cleavage, or jointing; and that the foliation will follow which- 
ever of these sets of planes is the most pronounced, according to Forbes’s 
law of ‘least resistance.’ Further, such superinduced foliation may 
apparently be formed upon structural planes in a crystalline rock. 
Referring to the central portion of the Alps, Professor Bonney! says: 
*The gneissic mass has been crushed, cleaved, and on the cleavage planes 
films of a hydro-mica have been developed. We cannot fail to be struck, 
when once our eyes have been opened to it, by the frequency of a slabby 
structure in the more central parts of the Alpine ranges, the surfaces of 
these slabs being coated with minute scales or films of mica.’ These he 
regards as ‘records of a rude cleavage which has been impressed upon the 
more central and less flexible portions of the Alps during the great earth- 
movements which they have undergone since they were first meta- 
morphosed.’ 
YX. Physical and Chemical Changes dependent on Pressure. 
A simple pressure acting at any point of a mass in a definite direction 
is mathematically equivalent, as may readily be proved,? to (i.) a uniform 
pressure in all directions, combined with (ii.) certain shearing stresses. 
The former tends to produce a compression of volume without change of 
shape, the latter a deformation without alteration of volume. It is then 
to the latter that the mechanical deformation or fluxion of rock-masses 
must be attributed ; while the former, which is of the nature of a hydro- 
static pressure, may prove to have been at least an important factor, if 
not indeed the prime agent, in the mineralogical and chemical changes 
observable in the same rocks. We shall therefore, to fix ideas, disregard 
for the present the shearing stress and the consequent movements of the 
rock, and consider only that ‘ quaquaversal’ or uniform pressure with 
which hydrostatical theories render us familiar. 
The importance of pressure as an element in the conditions of mole- 
cular and chemical changes has from an early date been more or less 
clearly recognised by geologists as well as physicists. It figures, at least 
as an accessory, in all modern theories of ‘ regional metamorphism,’ and 
high pressures play an important part in the experiments of those who 
have worked at the artificial production of minerals and the imitation of 
the structures of crystalline schists. 
Pressure, however, does not appear to be always ranked in its proper 
place as a condition, codrdinate with temperature, determining the action 
of all physical and chemical forces ; and its importance as such does not 
seem to be thoroughly realised in the discussion of all geological questions. 
Indeed we may say with Professor Lehmann? that ‘ we stand at present 
1 Proc. Roy. Inst., vol. xi. p. 64 (1885). 
* ? Thomson and Tait, Natwral Philosophy, vol.i. pt. ii. § 682, new ed. (1883). 
3 Op. cit., p. 244. 
