ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 839 
examples, associated also with curved and irregular contortions, are met 
with in what Professor T. McK. Hughes! calls the ‘ gnarling’ of the per- 
plexing beds about Amlwch in the North of Anglesey. Some of these are 
figured in the Survey Memoir on North Wales.’ In this case, if, as Pro- 
fessor Ramsay supposes, the planes such as AA’ in fig. 14 indicate the 
original stratification of the rocks, and the zigzag planes represent an obso- 
lete cleavage, the shearing has taken place along the bedding, and has 
affected certain beds to the exclusion of others. Professor Liversidge * 
has described and figured a slate-rock in which the undoubted cleavage- 
planes are thrown into well-defined acute zigzags by subsequent con- 
tortion. 
To return to Ausweichungsclivage, it is evident that planes like AA’ 
and BB’ in fig. 14, passing through the angles of the zigzag contortions, 
must be planes of structural weakness in the rock, even though no actual 
disruption of the lamine of stratification may have occurred at the places 
where they are sharply bent. M. W. 
C. Brégger‘ states that such planes of Fie. 18. 
weakness, which he terms Knickungse- 3 
bene, are common in the friable, finely A 
laminated beds of his Silurian Etage 2. pire tay 
in the Christiania district. Ihave no- ~~ es 
ticed a dark shale at Porthwen onthe ~ \/ 
north coast of Anglesey, in which the tte Low ok 
lamine of stratification, marked by ae es Rte! 
graptolites, are thrown into sharply de- Was a aed ee 
fined zigzag contortions, the longer and we > 
shorter limbs of which are about 2 inches ais he eg 
and }-inch respectively, in length. A pe cen fae 
similar instance is figured by M. Reusch® (ak 
from Débeln in Saxony. These rocks A’ B’ ( , 
are readily fractured along the Knic- oi 
kungsebene. 
Lastly, there is the variety of Ausweichungsclivage, in which the 
‘cleavage-planes’ are actual surfaces of discontinuity in the rock—in fact, 
minute faults. Prof. Heim regards the faulting in this case as a further 
stage of unsymmetrical contortion of the laminw of bedding, so that the 
dislocations are of the kind which he names /fold-faults (Faltenverwerfun- 
gen), as distinguished from the ordinary fissure-jaults (Spaltenverwerfun- 
gen). Examples on a microscopic scale are not uncommon: the ‘gnarled 
beds’ of Amlwch ® afford a beautiful instance (fig. 14). As seen in the 
figure, the faults are related to the visible contortions of the rock, being 
roughly parallel to the axial planes of the zigzags or contortions (the 
apex of one of which is shown in the figure), but curving away upon 
reaching a felspathic layer, through which they do not pass. It is indeed 
evident that the faults could not be perpendicular to the direction of the 
lateral pressure which produced them. In accordance with what might 
1 Quart. Journ. Geol. Soc., vol. XXXVi. p. 237 (1880). 
2 Mem. Geol. Surv., vol. iii. pp. 237-240, figs. 91-96, 3rd ed. (1881). 
3 Paper read before Roy. Soc. New South Wales, December 6, 1876. 
4 Die silurischen Etagen 2 und 3 im Kristianiagebiet, Sc., 8. 216 (1882), Chris- 
‘tiania. 
5 Op. cit. 8. 52. 
6 [Some slides of the Amlwch rocks show very clearly the passage of an ‘ over- 
fold’ into a ‘ fold-fault,’ as described by Heim. ] 
