'64 a. MAW ON AN UNCONFORMABLE BREAK 01' 



43. 0)1 an Unconformable Break at the Base of the Cambrian 

 Rocks near Llanberris. By George Maw, Esq., F.L.S., 

 F.G.S., &c. (Read April 3, 1878.) 



The short paper which I have to lay before the Society has been 

 suggested by several previous communications on the subject of 

 rocks older than, and unconformable to, the Cambrian Slates of 

 North Wales. 



In the year 1867 I brought before the British Association at the 

 Dundee meeting the accompanying section on the Llanberris and 

 Carnarvon Railway, then for the first time exposed in a new cut- 

 ting near Llanberris. The section was subsequently published in 

 the ' Geological Magazine' for March 1868, to which I must refer 

 the reader. 



Up to this time no suspicion existed of the occurrence of an un- 

 conformable series of ancient rocks in North Wales older than the 

 Cambrians. 



Previous to this the researches of Dr. Hicks in Pembrokeshire 

 established in that district the fact of the occurrence of great series 

 of beds unconformable to the overlying Cambrians ; but during the 

 succeeding seven or eight years no fresh observations were made 

 in the Llanberris district. 



At the reading of Dr. Hicks's paper on December 2, 1874, I 

 pointed out an apparent analogy between the Pembrokeshire series 

 and that at Llanberris ; and this, I believe, induced Professor 

 Hughes and Dr. Hicks to visit Llanberris, and their observations on 

 that district were recently communicated to the Society (Q. J. G. S. 

 vol. xxxiv. pp. 137 & 147). 



Rather singularly, they disagree with me in the conclusion I 

 arrived at, that there was a visible break in the series of rocks ex- 

 posed in the Llanberris railway-cutting, but admit that such a 

 break does occur at Moel Tryfane, where the unaltered Cambrian 

 slates rest unconformably on the adjacent altered rock. 



It is unquestionable that both series are identical, and include 

 the same succession of rocks ; and if the unconformity exists at 

 Moel Tryfane it must also appear on the banks of Llyn Padarn, 

 where a fairly continuous section is exposed in the cuttings. 



The substance of what I recorded in 1867 was to the effect that 

 the Cambrians of Llanberris do not pass into the great metamorphic 

 mass of porphyritic rocks crossing the north-west end of Llyn 

 Padarn (marked " Felspar-Porphyry " in the Survey map), but that 

 they rest unaltered and unconformably on a more ancient series of 

 distinctly stratified rocks which graduate into the metamorphic 

 mass, having near Betwys Gannon a distinct igneous nucleus. 



I had the good fortune, in 1867, to see the section when freshly 

 exposed, which enabled me to note all the details from careful and 

 exact measurement. Ten years weathering has partly obscured 



