842 REPORT— 1 885. 
microscopic, consequent upon a flowing motion of a mass of any material. 
The comparison between slaty cleavage and the various types of fluxion- 
structure met with in different rocks has been made by Mr. Poulett 
Scrope and others.!_ The movement which the parts of a rock experience 
under the action of the forces which produce slaty cleavage may evidently 
be regarded in the light of a flowing motion along the cleavage-planes in 
the direction of their dip, and so is analogous to the similar flowing of 
other plastic materials, such, for instance, as a viscous lava.2 Thus the 
rearrangement of the ultimate fragments of a slate-rock in the general 
direction of the ‘ grain’ or cleavage-dip corresponds to the arrangement 
of the microlites in a rhyolitic or andesitic lava parallel to the lines of 
flow: the distortion of fossils, &c., in the slate is analogous to the draw- 
ing out of vesicular cavities in the lava into amygdaloidal or ellipsoidal 
forms: the more resisting fossils, around which the cleavage is slightly 
curved, are paralleled by the porphyritic crystals in the lava, round 
which the lines of flow sweep in curves: and, finally, both the fossils in 
a cleaved rock and the porphyritic crystals in a lava are occasionally 
found to be shattered and the fragments drawn apart in the direction of 
flow. Fluxion structure, in its various phases, is moreover common, 
though local, in plutonic, as well as volcanic rocks; and where a rock is 
largely made up of parallel tabular or linear crystals of such minerals as 
felspar, &c., it is not infrequently found to possess in consequence a kind 
of rude macro-cleavage, either lamellar or fibrous, in the direction so 
determined. 
This ‘ parallel-structure,’ to be clearly distinguished from foliation on 
the one hand and from platy jointing on the other, was long ago re- 
marked by various geologists ;3 the ‘grain’ in certain Cornish granites, 
described by Professor Sedgwick‘ is a structure of this character. In 
rocks which have a large proportion of isotropic ground-mass, the fluxion- 
structure does not produce any marked tendency to split parallel to it. 
Hitherto we have spoken of the flowing of igneous rocks only when in 
a partially molten condition ; but the comparison may be carried much 
further. Since the experimental researches of M. Tresca® the flowing of 
solid bodies is no longer a contradiction in terms, and of recent years the 
plasticity of rock-masses has been the subject of much discussion. In 
greatly disturbed regions even the hardest rocks are found to have 
suffered a mechanical deformation, which is in part of the nature of 
 Scrope, ‘On Lamination and Cleavage Occasioned by the Mutual Friction of the 
Particles of Rocks while in Irregular Motion, Quart. Journ. Geol. Soc., vol. xv. p. 84 
(1859). Geologist, vol. i. p. 361 (1858). Heim, op. cit., Bd. ii. S. 56. 
* Cf. Miller, ‘ Fluxion-structure in Till,’ Geol. Mag., 1884, p. 472. 
’ Von Buch, Scrope, &c. For the earlier notices of both parallel structure and 
gneissic structure in igneous rocks, see Naumann, ‘ Ueber die wahrscheinlich eruptive 
Natur mancher Gneisse und Gneiss-Granite,’ Neu. Jahrb. fiir Min., Sc., 1847, 8. 297 ; 
trans. in Quart. Journ. Geol, Soc., vol. iv. pt. ii. p. 1 (1848). 
* Trans. Geol. Soc., 2nd ser., vol. iii. p. 483 (1835). Introduction to Synopsis of 
British Pala@ozoie Rocks, p. xxiv. (1855). 
° Comptes Rendus, 1864, p. 754; 1865, p. 1228; 1867, p. 809. Mém. Savants 
Etrangers, vol. xviii. p. 733; vol. xx. p. 75. Sur Vécoulement des solides, 1872, Paris. 
‘Flow of Solids,’ Proc. Inst. Mech. Eng., 1878, p. 301. 
® Heim, Ueber den Mechanismus der Gebirgsbildung, 1878, Basel; Zeitsch. Deutsch. 
Geol. Geselisch., BA, xxxii. 8. 262 (1880). Stapff, Ueber die Plasticitit der Gesteine 
beim Zusammenpressen, &¢., ibid., 1879, 8, 292, 792; 1881, Bd. i. S. 185. Pfaff, Der 
Mechanismus der Gebirgsbildung, 1879, Heidelberg. Lehmann, Zntstehwng der 
Althrystallinischen Schiefergesteine, §c., 1884, Bonn. 
