ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 843 
flowing. In describing the phenomena in the neighbourhood of the great 
‘thrust-planes’ of Eriboll, the officers of the Geological Survey’ re- 
peatedly make use of the expression ‘ fluxion-structure ’ as one specially 
applicable to the result of a shearing motion in the rocks. According to 
the character of the rocks affected and the intensity of the forces to 
which they have been subjected, the deformation of the mass appears to 
have proceeded partly by internal fracture and faulting of the rocks, and 
partly by their gradual and continuous distortion in the manner of plastic 
bodies. In fact, continuous and disruptive movements seem to have 
taken place concurrently, in such a way that the planes of shearing with 
the consequent ‘ grain ’ or fluxion-structure are not always distinguished 
from planes of abrupt slipping with their associated slickensides. A 
characteristic of these intense mechanical changes is that they are fre- 
quently accompanied, and their results complicated, by the paragenesis, 
recrystallisation or destruction of some of the constituent minerals of the 
rocks and the production of new ones. In this way foliation and gneissic 
structure may be produced as the result, partly of mechanical, partly of 
mineralogical and chemical agencies. Apart from these last, however, 
the plastic movement or flow, which in a crystalline rock gives rise to 
‘foliation, is precisely analogous to that which in a clastic rock produces 
slaty cleavage; and in fact where rocks of the required lithological types 
are found associated together and affected by the same movements, the 
foliation of the one passes into the cleavage of the other. As, moreover, 
the development of new minerals takes place on the cleavage-planes of the 
latter as well as on the foliation-planes of the former, it is impossible to 
draw any line of demarcation between the two structures. As instances 
of foliation associated with, and in part at least produced by, mechani- 
cally caused movements, we may cite the hornblende-schists of the Bergen 
peninsula and the Saxon Erzgebirge, and the granulites, and probably in 
part the gneisses of the latter region. An instance on a small scale has 
been described by Mr. Teall in a dyke at Scourie, Sutherland, where the 
conversion of dolerite into hornblende-schist is clearly exhibited. 
By regarding cleavage and some cases of foliation in the light of 
fluxional structures, we gain a clue to some of the difficulties connected 
with those phenomena. The segregation of the several constituents into 
lenticular patches is a peculiarity of some types of foliation, and, indeed, 
in some gneisses, presents a very characteristic appearance. Now it isa 
matter of common observation that a ‘stream of liquid moving in any 
general direction tends to divide itself, more or less, into veins and 
threads composed of particles of different degrees of coarseness, and, con- 
sequently, of different degrees of mobility, and moving at different rates.’ 
The significance of this was pointed out by Mr. Poulett Scrope? in con- 
nection with his theory which made the foliation of gneiss and mica-schist 
an original structure produced during the protrusion of an igneous mass 
in a partially molten state, and the same principle would probably have at 
least a limited application to the flow of solid rock masses which gives rise 
to the foliation or fluxion structure in the Eastern gneiss of Sutherland 
and other rocks of like character. Whether the segregation in these cases 
is to be referred in any measure to the cause described, or whether it is 
1 Geikie, Peach, and Horne, Nature, November 13, 1884. Cf. Lapworth, Geol. 
‘Mag., March 1885, p. 97. 
wey vol. i, p. 363 (1858). See also Quart. Journ. Geol. Soc., vol. xii. p. 345 
