442 Geological Society : — 



great masses of quartz-felsite, foliated at the edges, are intruded. 

 Here also are found the quartz knobs and sporadic limestones as -well 

 as diabase-flows. At Mynydd ystum is found an isolated patch of 

 grey gneiss. 



The area between Bangor and Caernarvon has lately been shown 

 to contain some felsite-flows, and also granites, apparently intrusive 

 into ashes, which may belong to the volcanic facies. 



At Hoivth, near Dublin, the rocks have all the characters of the 

 South-Stack series, to which they may be correlated ; and these are 

 followed upwards by the well-known Bray-Head rocks, which differ 

 from them in character, but whose fossils are not of Cambrian 

 species. 



The succession thus shown in the various districts consists of the 

 following in ascending order. The grey gneiss, becoming more 

 quartzose, micaceous, or chloritic in parts, and so representing the 

 quartzite and the chloritic schists of other districts ; changing through 

 chloritoid schists into two facies, viz., (1) the slaty, represented 

 best in the northern district, and also as the South-Stack series ; 

 and (2) the volcanic facies. No further deposits are recognized in 

 the areas of the volcanic facies, but in the slaty area of Howth the 

 Bray-Head rocks succeed. 



To the whole system of rocks the name of Monian is applied, as 

 derived from Mona, or the Isle of Anglesey, and the several parts 

 are distinguished as the Holyhead group, or Lower Monian, the 

 St. David's group and the equivalent of the South-Stack Series, or 

 Middle Monian, and the Brag-Head group, or Upper Monian. 



The " Pebidian " represents the St. David's group, and but for its 

 termination, which indicates a system, might be used as an alterna- 

 tive. The " Dimetian and Arvonian " are intrusive granites, or 

 felsitic flows associated with the same group. 



The Monian system, though much metamorphosed in its lowest 

 parts, is not considered Archaean, but as a lower sedimentary system 

 than the Cambrian, and hence the lowest system of our ordinary 

 stratified rocks. 



March 28.— W. T. Blanford, LL.D., F.R.S., President, 

 in the Chair. 



The following communications were read : — 



1. "On some Eroded Agate Pebbles from the Soudan." By 

 Prof. Y. Ball, M.A., F.R.S., P.G.S. 



The majority of the pebbles in a collection made by Surgeon- 

 Major Greene in the Soudan, and presented by him to the Science 

 and Art Museum in Dublin, are of very similar character to the 

 agate and jasper pebbles derived from the basalts of India. It 

 may be concluded inferentially that they came originally from a 

 region in which basaltic rocks occur to a considerable extent. A 

 certain number of them are eroded in a manner unlike anything 

 noticed in India, though it is probable that similar eroded pebbles 

 will eventually be found there. 



