ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 837 
the term ‘ cleavage’ ceases to be properly applicable to them. The same 
_ laws of formation seem to govern the small and the great. Professor 
Heim in his figure! illustrating the passage of an overfold into an over- 
fault by the obliteration of the middle limb gives the scale as ‘ 22° to 
topoo Of the natural size.’ Some of the macro-structures analogous to 
this type of cleavage may therefore be found worthy of note. 
First, however, it may be remarked that that flattening and elongation 
of the individual particles of the rock, which constitutes Mikroclivage, has 
its counterpart in the similar distortion of fossils, nodules, and pebbles; 
and where pebbles or other fragments of visible dimensions make up the 
bulk of a rock which has yielded uniformly under pressure, we have, in 
the flattened pebbles é&c., a representation on a large scale of the micro- 
structure of slate as pictured by Mr. Sharpe. A cleaved conglomerate 
of this kind was long ago noticed by Professor Ramsay ? at Llyn Padarn. 
It consists of ‘ slaty pebbles in a slaty matrix.’ A similar rock, in which 
both pebbles and matrix are apparently compacted volcanic ash, occurred 
to me in the Boulder-clay at Nantlle. In this specimen the closely-packed 
pebbles are all strongly distorted in the same directions into approximately 
ellipsoidal forms. They are all very nearly of the same shape, the ratios 
of the axes of the ellipsoid being about 1°6 : 1:0 : 0-23, which figures are 
very nearly the same as those for the green spots in the slates of the 
_ district. Numerous examples of distorted pebbles in conglomerates occur 
in the eastern states of America ;* the remarkable squeezed conglomerates 
of the Bergen peninsula have been well described by M. H. H. Reusch.! 
In such cases the pebbles are either so closely packed as to be in contact 
with one another or, as in the case of the Welsh conglomerates, the 
pebbles and matrix are about equally hard, and so yielded equally to 
pressure. 
Another type of what we may call macro-cleavage corresponds to the 
structure produced by the parallel flakes of mica in the Llanberis slates ; 
_ the difference being that in this case the flakes are less minute, and have 
had an original arrangement parallel to the lamine of stratification. The 
laminez are thrown into a series of small contortions in such a manner 
that the flakes of mica lie chiefly along certain definite parallel planes 
oblique to the general direction of stratification, which thus become 
planes of easy fracture or ‘cleavage.’ This kind of structure forms a 
connecting link with Ausweichungsclivage, to which type, indeed, it is 
referred by M. Reusch, who gives a good example from the black, mica- 
ceous clay-slates of the Bergen district. In his figure the pseudo-cleav- 
age-planes appear to be about }-inch apart, and this interval is determined 
by the scale of the small contortions. 
As an example of true Ausweichungsclivage arising from minute and 
regular contortions, I may instance a black slate-rock from the pass of 
Drws-y-Coed, near Snowdon. In this rock the contortion has taken 
' Op. cit. Atlas, Taf. xv., fig. 14. 
* Mem. Geol. Surv. Gr. Brit., vol. iii., ‘Geology of North Wales,’ p. 179, 2nd ed, 
(1881). 
; * Hitchcock, Crosby, Wadsworth, &c. For references, see Proc. Bost. Nat. Hist. 
Soc., vol. xx. pp. 308 et seq. (1879). 
* Silurfossiler og Pressede Konglomerater i Bergenskifrene, 1882; German trans., 
Die fossiiienfiihrenden hrystallinischen Schiefer, gc. 8. 22, 51, figs. 11, 12, 33, 37 
(1883), Leipzig. 
5 Op. cit., 8. 51, fig. 32. 
