Reports and Proceedings — Geologicnl Society of London. 333 



to the directions of the prevailing winds of the district ; they showed 

 stages between stones eroded on one face, or on two faces, and the 

 typical ' Dreikanter ' ; tliey illustrated the polishing action of the 

 agent on hard, fine-grained material, its differentiating action on 

 coarse-grained ; they retained the original water-worn surface and 

 shape on the under side, and a growth of lichen on such small 

 portions of the upper surface as had been sheltered from the wind. 

 Applying this explanation to Mr. Darbishire's specimen, there still 

 remained an unsolved problem: Was the pebble eroded before or 

 during the deposition of the Runter stratum, or at some time 

 before, during, or subsequent to tlie transport and deposition of the 

 Drift? The mode of occurrence of the North German specimens 

 shows that they were eroded after the deposition of the beds at 

 the top of which they now lie ; and, in the absence of contrary 

 evidence, the same answer must be given provisionally to the 

 preceding questions. 



The following communications were read : — 



1. " On the Geology of Northern Anglesey." By C. A. Matley, 

 Esq., B.Sc, F.G.S. ; with an Appendix on the Microscopic Study of 

 some of the Rocks, by Professor W. W. Watts, M.A., Sec. G.S. 



The strata which occupy the northern part of Anglesey have been 

 the subject of much controversy, some geologists considering them 

 (with the exception of a few patches in the extreme north) to be 

 pre-Cambrian, while others maintain that they are of Bala age, and 

 that they are an upward continuation of the black slates that every- 

 where appear to underlie them to the south. 



The author attacks this problem from its palfeontological as well 

 as its stratigraphical side. He divides the rocks into three groups, 

 namely : (1) the 'green series,' which forms the floor of the greater 

 part of the area; (2) the 'northern complex,' occupying a tract 

 along the northern coast and including in it (3) some undoubted 

 Ordovician strata. He adduces paleeontological evidence that the 

 Ordovician rocks lying in the northern complex are mainly, if not 

 wholly, of Llandeilo age, while the black slates of Central Anglesey 

 appear to range into Llandover}'. The field-evidence shows 

 inter alia that the asserted interstratifications of the Green Series 

 and the southern Ordovician are the deceptive results of thrust- 

 planes which have driven the barren green rocks over the 

 Ordovician beds, the latter being in other places made up of 

 fragments derived both from the Green Series and from the rocks 

 of the northern complex. 



The contortion, overfolding, cleavage, dislocation, and disruption 

 which the rocks have undergone are next described. Disruption 

 is traced from its early stages into 'crush conglomerates.' Some 

 of the disrupted rocks are Ordovician, and traces of ancient dykes 

 have been found rent to pieces by the movement, which is stated 

 to be post-Ordovician and pre-Carboniferous. The detached masses 

 of limestone and the isolated ' quartz-knobs ' of the northern 

 complex are considered to be portions of strata which have suffered 



