chap. XI 
NORTH WALES 
163 
Iiave undergone considerable deformation from the same cause. Their thinner 
portions are so well cleaved that they have been mistaken for included bands 
of green slate (Fig. 42). But these cleaved branches may sometimes be 
traced into a thicker and more solid dyke, wliose uncrushed cores still 
preserve the original character of the rock and prove it to be eruptive. 
The rocks which si^cceed the porphyry in the Valley of Llanberis are of 
great interest, for they contain abundant proof of contemporaneous volcanic 
activity, and tliey show that, so far from there being any marked hiatus 
Fere, there is evidence of the persistence of eruptions even into the time of 
Pig. 43. — Section of well-cleavcd tull’, grit ami breccia passing np into rudely-cleaved conglomerate .and 
well-bedded cleaved tine conglomerate and grit. East side of Llyn Padarii. 
tbe Llanlieris Slates.^ Considerable misapprehension has arisen from the 
attempt to make one of the conglomerates the base of the Cambrian series, 
f^iid the real significance of the volcanic detrital strata in association with 
It was consequently missed. The conglomerate does not lie on one definite 
horizon. In truth, there are several zones of conglomerate, each with some 
difference of composition, thickness or extent." These may be well studied 
both on the south and the north side of the porphyry ridge at the lower 
^ The sections in the Vale of Llauberi.s on cither side of Llyn Padarn have been again and 
*<'gain described and fought over. Some of the papers are cited in the following pages, but it 
"'ould be inipos.sihle in this volume to find room for a full diseu.ssion of the difference.s of opinion. 
What is stated in the te.xt is the result of my own study of the rocks on the ground, eou])led 
'"’ith a careful consideration of the work of other observers. 
^ I can find no evidence of unconformahility beneath any ot tlie conglomerates. The section 
described by Professor Green, Qiijirl. Journ. Geul. Soc. vol. xli. (1885), p. 74, merely shows the 
difference between the elfects of cleavage on the fine tulfs and the more nias.sivc resisting con- 
glomerate which overlies them. This section is represented in Fig. 4.3. At first sight tlie 
conglomerate apjiears to be lying on the vertical edges of an older group of slates, but any one 
S'Cquaiuted with cleavage can trace this structure from the tuffs into the conglomerate and 
■■osuniing its course again in the finer sediments above. The wliole series of deposits in the 
section is continuous ami conformable. The section on the slate railway has deceived Mr. 
Blake a.s well as Professor Green (QunrL Journ. Oeol. Soc. vol. xlix. (189-‘i), p. 445). The correct 
Hiterpretation is given by Professor Bonney and Miss Raisin {o}). dt. vol. 1. p. 592). 
