14. Prof. Hughes—Lower Cambrian, Bethesda, N. Wales. 
contorted, so that fossils are often found along an even line on the | 
edges of the cleavage planes. : 
Cleavage may be superinduced upon a rock already affected by 
fault and fold structure, especially in a rock of a fine and homo- 
geneous character, and this is what seems to have taken place in the 
case of the Penrhyn Slates. 
Doubts as to the nature of cleavage may often be traced to the 
want of a clear distinction between these two superinduced struc- 
tures—the molecular rearrangement due to cleavage and the kneading 
of the mass with fault and fold schism. 
Why pressure should in one case crumple up the beds and in 
another make them yield as a plastic body, is a separate question 
probably capable of receiving different correct answers in different 
cases. What we are aiming at now is the establishment of the fact 
that there is this two-fold action, and resulting different structure, 
and the application of it to the explanation of some stratigraphical 
difficulties. 
In these lower Penrhyn Slates the direction of displacement of 
well-marked beds is not even generally coincident with cleavage. 
We see here evidence that there is not only a thickening of homo- 
geneous masses by molecular movement approximately at right 
angles to the direction of pressure as indicated by the cleavage, 
but also a folding and crumpling of the beds, so that one layer is 
repeated three or four times in the same vertical section. 
Thus we get a suggestion of the reason why fossils may be found 
only here and there in a rock-mass which appears to be throughout 
of a similar character and to have been subjected to similar con- 
ditions. The cleavage often leaves the fossils on planes along which 
the rock will not split, at the same time distorting them beyond the 
possibility of recognition in small fragments. But when the rock 
has been crumpled and folded previous to or coincidently with the 
cleavage, some specimens here and there will lie on faces which have 
suffered less deformation or coincide with easy divisional planes. 
The great rock-masses near Bethesda roll away in great undu- 
lations, having a general dip to the south and east; but subordinate 
masses are puckered, behaving under this enormous pressure as if 
plastic, the greater surface extension being compensated by dimi- 
nished thickness and vice versd. . 
Ramsay! pointed out that the vertical extent of some of these beds 
was probably greatly in excess of their original thickness. 
This kind of thing is, of course, most likely to occur in such 
cases as that which we are considering, where the newer beds 
abut against an axis of older rocks which have been already 
squeezed and shrunk beyond the possibility of much further com- 
pression, just as in a crowd the man who is jammed against the 
wall or post gets most hurt. 
We further notice that along the old Archzan axes two things 
happen which account for a very rapid disappearance of beds 
which are seen to be of great thickness in a closely adjoining area. 
1 Mem, Geol, Survey, vol. iti, pp. 190-196. 
