TRANSACTIONS OF SECTION C. 669 



out that these gabhros differ from those of the Tertiary volcanic district of the 

 ■west of Scotland in being largely composed of saussurite and hornblende, and in 

 frequently exhibiting a foliated structure. He described certain types of foliated 

 gabbro under the ievmsflaser-c/ahbro, angen-cjahhi o, and gabbro-schist, and discussed 

 the origin of the foliation. He concluded that it (the foliation) was the result of 

 the deformation of the solid rock-masses by the intense mechanical forces that have 

 acted upon the district, and he attributed the replacement of felspar and diallage 

 by saussurite and hornblende in a great measure to the same agency. 



6. Introduction to the Monian System of BocJcs. 

 By Professor J. F. Blake, M.A., F.G.S. 



It has already been recognised by various authors that there are in several parts 

 of Wales a group of rocks which are older than the Cambrian system. Certain of 

 them have received names such as Dimetian, Arvonian, Pebidian, but as a whole 

 they have only been recognised as Pre-Cambrian or Archaean. They are best deve- 

 loped in the island of Anglesey, and a study of them in that region shows that 

 they form a well-defined system, of quite equal importance to the Cambrian or 

 Ordovician, and presenting very well marked and peculiar characters. It is pro- 

 posed, therefore, to create for them a new ' system,' the ' Monian,' after the place of 

 their development, which shall take its place below the Cambrian as the oldest 

 series of stratified rocks, without being a full member of the great group known as 

 Archaean. The system is divisible into two parts ; the Lower is remarkable as pre- 

 senting a transition from true crystalline gneisses into earthy slates, the Upper for 

 the development of vast masses of volcanic debris, with infusions of quartz and cal- 

 cite, and containing also as intrusive masses rocks of peculiar character which may 

 be called ' Dimetite.' The rocks of St. David's belong exclusively to the Upper 

 Monian. 



7. On the Igneous Rocks of Llyn Fadarn, Yr Eifl, and Boduan. 

 By Professor J. F. Blake, M.A., F.G.S. 



These masses have been claimed as Pre-Cambrian, but the evidence obtainable 

 on the north side of Llyn Padarn does not warrant this conclusion. Cambrian 

 rocks are found to the west ; no remarkable conglomerate lies between them and 

 the quartz-felsite mass, but on the eastern side is one with enormous blocks of 

 the felsite and of other, probably Cambrian, rocks. This is conterminous with the 

 igneous mass rising into crags at Clegyr and Moel Tryfan. The felsite of Moel 

 Gronw is a distinct and later outburst. The mass north of Llyn Padarn consists in 

 ascending order of felsite, brecciated felsite, agglomerate, felsite of different charac- 

 ter, agglomerate and ash, indicating an outburst to a level above the earlier de- 

 posited rocks, and therefore immediately exposed to denuding influences. It is 

 believed, therefore, that this mass may be best interpreted as a submarine eruption 

 in the middle of Cambrian times, i.e., younger than the Cambrian rocks to the 

 west, and older than the conglomerate which is locally derived from it, and also 

 older than those slates which lie to the east. There is no other conglomerate com- 

 parable with this, and the outburst of Moel Gronw has no conglomerate on its 

 western side. 



The igneous rocks of Yr Eifl and Boduan come up in irregular masses and 

 clearly overlie the Ordovician slates, after the manner of intrusive igneous rocks. 

 They show a rather remarkable system of horizontal (columnar ?) jointing, which 

 gives them a very bedded aspect as seen from a distance, disappearing on closer 

 examination and only affecting some part of the massif. The Boduan mass may be 

 possibly contemporaneous, as the overlying slates are somewhat conglomeratic, but 

 there is not the slightest evidence of either of these being either Cambrian or 

 Pre-Oambrian in age. The case is different with the ' Rhos Hirwain syenite.' 



