Hartfell- Valeutian Succession around Plynlimon. 333 



The Plynlimon Stage is developed in the northern part of the 

 district, between Plynlimon and Pont Erwyd ; the Pont Erwyd 

 Stage along the two valleys of the PJieidol and the Castell, which 

 converge near Pont Erwyd village ; while the Ystwyth Stage is 

 developed on the plateau-like tract extending from the Castell 

 Valley to the Ystwyth Valley. 



The stratigraphical succession is demonstrated by a series of 

 sections and traverses across various parts of the district, and 

 lists of fossils (mainly graptolites) collected from the various zones 

 are given. The palaeontological evidence is in entire accord with 

 the stratigraphical evidence. 



The structure shows many points of interest, and is clearly 

 brought out by the mapping. Three types are dealt with — 

 (1) folding, (2) strike-faulting, and (3) normal faulting; but the 

 first is predominant. 



The rocks are folded into a primary anticlinal fold or anti- 

 clinorium, with a southerly ' pitch ' of 10° to 15°. This primaiy 

 fold is composed of a number of secondary folds of a symmetrical 

 type, with axes ranging nearly due north and south and pitching 

 as above. Occasionally, the secondary folds are complicated by 

 smaller folds or erumplings. The effect of the plication-structure 

 on the rock-outcrops, and especially on the topographical features, 

 is dealt Avith in some detail. The latter are shown to be dependent 

 in an important degree upon the pitch, and to a much smaller 

 degree upon the dip of the rocks. 



Strike-faults play a minor role in the structure, and their effects 

 are of little importance ; they have not been observed to carry 

 mineral deposits. 



The normal faults of the district are of greater interest. They 

 range in an east-north-easterly direction, and nearly always carry 

 sulphidic ores of lead, zinc, or copper. They appear to be quite 

 independent of the folding, and behave generally as if they were of 

 later date. 



A brief account is given of the district lying on the western limb 

 of the anticlinorium between Pont Erwyd, Devil's bridge, and 

 Aberystwyth. Evidence is given for assigning to the 'Aberystwyth 

 Grits ' of earlier observers a position much higher in the geological 

 sequence than has hitherto been attributed to these ' Grits.' 



The paper concludes with a tabular list of fossils, correlation - 

 tables, and a description of two species of graptolites of zonal 

 importance. 



2. ' The Geology of the Neighbourhood of Seaford (Sussex).' By 

 James Vincent Elsden, 13. 8c, F.G.S. 



This paper illustrates the application of zonal methods to field- 

 geology in a Chalk area. It deals with a portion of the South 

 Downs lying between Eastbourne and Newhaven. The inland 

 outcrops of the uppermost zones of the Chalk are mapped. In 

 tracing the boundary-lines, fossil evidence is alone relied upon, 



