840 REPORT—--1885. 
be expected, the hade of the faults, measured from the axial plane, be- 
comes less as we approach that plane, and the throw of the faults dimin- 
ishes also. These faults are on an average about =4,-inch apart; a 
cleavage-structure on a like scale, and showing altogether very similar 
appearances, has been figured by Prof. Heim.' Owing to subsequent 
actions, the Amlwch rock shows no marked tendency to split in the 
direction determined by this microscopic faulting, but on examining one 
of the contorted surfaces of ‘ foliation,’ the traces of the minute faults are 
visible as a very fine striation running parallel to the strike of the con- 
tortions. 
Dr. Sorby,? in his Presidential Address to the Geological Society in 
1880, described and figured an interesting case of cleavage due to close 
parallel planes of discontinuity in slate-rock at Liskeard, the planes being 
apparently about ,1,-inch apart. This instance is instructive, because in 
the less disturbed parts of the same rock, where the cleavage is very im- 
perfect, the earliest stage of the structure is seen in the shape of minute 
Fic. 14. 
contortions of the lamine of stratification. He also pointed out that in 
some other cases, such as the pencil-rock of Shap, the effect of lateral 
pressure has apparently been to break up the laminz of deposition in an 
irregular manner, so that the laminated structure is almost obliterated, 
and the microscopic flakes of mica, which constitute nearly the whole 
mass of the rock mentioned, are practically inclined about equally in all 
directions. This removed a difficulty which had previously been felt 
with regard to his original theory of slaty cleavage, viz., the postulate 
that the rocks which now form roofing slate had, prior to their becoming 
cleaved by lateral compression, &c., a structure in which the flat consti- 
tuent particles were not arranged in lamine of deposition, as is commonly 
the case in uncleaved shales, &c., but distributed at random in all posi- 
tions. Dr. Sorby? had formerly endeavoured to account for this assumed 
original structure of slate-rock by the supposition that the microscopic 
flakes of mica in it were of secondary origin; but we now see that the 
required arrangement might be brought about by the earlier effects of 
1 Opycit., at. xyv., fic. 8: 2 Quart. Journ. Geol. Soc., vol. xxxvi. Proc. p. 73- 
8 Brit. Assoc. Rep., 1857, Trans. Sect. p. 92. 
