844 REPORT—1885. 
due solely to the action of molecular forces, which would probably be 
intensified by the extreme pressure, is one of the questions which must be 
left for further elucidation.! But as bearing upon this question, and as 
showing that it isa speculation by no means foreign to the subject of slaty 
cleavage proper, we may notice the interesting case described and figured 
by Professor Bonney,” of a ‘ structure imitative of foliation produced by 
pressure on a stratified rock without important mineral change.’ Here, in 
a cliff section at Torcross, South Devon, are alternate layers of fine dark 
mud and silt, the former predominating, which are much folded, and con- 
sequently inclined at various angles to the cleavage which traverses the 
whole. Where the cleavage and bedding coincide, the layers, though 
much compressed, are distinct ; but where the angle between the direc- 
tion of pressure and the normal to the surface of the bed has been a large 
one, the stripe is obscured or obliterated, and a new structure produced. 
The broader gritty bands have their edges ‘ frayed out,’ or send out comb- 
like processes into the finer bands in the direction of the cleavage ; the 
narrower bands are entirely obliterated and replaced by a new structure 
of parallel lenticular streaks or elongated ‘eyes’ extending in the direc- 
tion of the cleavage. 
Connected with the tendency of a heterogeneous mass in flowing 
motion to separate into layers or patches is its liability to unsteady or 
sinuous motion. The very tortuous fluxion structure exhibited by many 
rocks of volcanic origin must be ascribed to this circumstance. As Mr. 
Poulett Scrope* remarked with reference to the ribboned and banded 
trachytes, &c., the contortions are owing to ‘the various degrees of 
mobility of the different layers, those of coarser grain . . . retarding the 
motion of the proximate layers which possessed a greater liquidity.’ In 
fact, the resistance to motion within a moving mass of the kind con- 
sidered would be more of the nature of surface friction than of viscosity, 
and so would be a condition unfavourable to steady motion. A similar 
explanation may apply to those cases in which contorted foliation is 
attendant upon the fluxional motion of rock masses.? Such contortions 
occur, it should be noticed, both along the dip and along the strike of the 
foliation. 
The constant association of intense mechanical deformation and 
crushing of rocks with molecular and atomic changes of a certain 
character naturally leads to the inquiry, what is the nature of their con- 
nection, and to what extent can the two phenomena be supposed to have 
acommon origin? In fact, in what measure the intense lateral pressure 
which we assume to have been the agent by which a fluxional structure 
was impressed upon solid rock masses, may be held to account for con- 
comitant changes of a mineralogical and chemical kind. Some materials 
for such an inquiry will find their place in the following section. 
But to prevent misconception, I must point out at this stage, that in 
the present paper, foliation, except in so far as it is related to cleavage, 
has no proper place. Our subject embraces only those types of structure 
» Cf. Lehmann on the ‘ Augengranulites ’ of Saxony, op. cit., S. 202 et seq. 
2 Quart. Journ. Geol. Sve., vol. x1. pp. 18 et seq. 
8 Trans. Geol. Soc., 2nd ser., vol. ii. p. 195 (1826). 
* Osborne Reynolds, ‘An Experimental Investigation of the Circumstances which 
Determine whether the Motion of Water shall be Direct or Sinuous, &c.,’ Proc. Roy. 
Soc., vol. xxxv. p. 84 (1883). 
° Cf. Fisher, Physics of the Earth’s Crust, p. 124 (1881), London. 
