12. Prof. Hughes—Lower Cambrian, Bethesda, N. Wales. 
I. Below these come the slates of the Tramway cutting, to be 
more fully described in illustration of the chief point of this com- 
munication, viz. the nature of the deformations which have taken 
place in the Penrhyn slates, and their bearing upon the manner of 
occurrence of fossils in different parts of the series. . 
M. To the north of these the St. Anns Grit crops out. It was 
touched in breaking ground for the new brickworks near St. Anns, 
and was quarried for use in some of the buildings connected with 
them. It contains numerous and conspicuous grains of hyaline 
quartz, as well as of pink quartz and jasper, in this resembling the 
Cambrian grits of the Bangor-Carnarvon area. 
Such is the succession of rocks seen south of Bethesda. There is 
no proof here of a repetition by complete inversion of any large 
portion—though the rocks have been much crushed up in detail. 
One of the various difficult questions raised by the examination of 
this district is, What were the conditions which favoured the pre- 
servation of fossils only here and there in this great thickness of 
rock? The first and most obvious reason which suggested itself 
was that the great unyielding mass of the Bronllwyd Grit, imme- 
diately below which the fossils occurred, saved the underlying bed 
from the severity of the crush, and that the finer rock therefore 
split along bedding planes parallel to the base of the grit. An 
examination of the section, however, soon showed that this explana- 
tion would not hold: for the fine green rock is strongly cleaved 
tight up to the base of the grit, and the grit itself has evidently 
suffered much in the crush, as shown by the squeezed-up mudpans 
and the contortions of the finer parts. Moreover, if the occurrence 
of fossils 1200 feet lower down in the quarry and far from any 
unyielding mass of grit be verified, we must seek some other reason 
for the exceptional preservation of such traces. 
When in the quarry we endeavour to seek an explanation, we 
notice first that the fossils do not occur in the planes of bedding 
as inferred from the lie of the large consecutive masses of rock of 
different lithological character. Yet the fossils can never have been 
of such rigidity as to have been thrust into approximate coincidence 
with the planes of cleavage during the crush which produced it. 
Further we observe that there are no bands or lines suggestive 
of stratification running parallel to the dip inferred from the alter- 
nating beds of grits and slates. Any such bands and lines are 
irregular, interrupted, of small extent, and at all angles to the 
constant cleavage. 
Tn rocks of such a uniform character as the Penrhyn Slates the 
relative displacement of parts is never easy of detection. If, how- 
ever, we follow them to where some of the beds are picked out by 
differences of colour and texture, so that we can disentangle the 
complications, we get a clue to the processes by which the rock has 
been reduced to its present state, and an explanation of the irregular 
manner of occurrence of the fossils. In the Tramway cutting for 
instance, between the St. Ann’s Road and the Quarries, we see that 
the bands which here mark bedding are twisted, drawn out, doubled 
