'536 THE HARTEELL-VALENTIAN SUCCESSION [Nov. 1909,. 



EXPLANATION OF PLATES XXIV & XXV. 

 Plate XXIV. 



Geological map of the district around Plynlimon and Pont Erwyd, on the 

 scale of 1^ inches to the mile. 



Note.— The distinction between the colours of the Dolwen Mudstones and 

 the Devil's Bridge Group is not so clear as was intended. 



Plate XXV. 



Plan of the Eheidol Gorge between Pont Erwyd and Parson's Bridge, illus- 

 trating the relationships of the zonal subdivisions and the geological 

 structure, on the scale of 12 inches to the mile. 



Discussion. 



Dr. J. E. Marr congratulated the Author on successfully working- 

 out a district which, as he knew from a visit made with the late 

 Walter Keeping many years ago, was one of extreme complexity. 

 The Author had mentioned the similarity of the jointing in his 

 convolutus beds to that of the same beds in the Lake District. 

 This led the speaker to discuss the frequent importance of litho- 

 logical characters for purposes of correlation, — not always the most 

 prominent characters being of importance for this purpose, but often 

 obscure characters the very origin of which was not yet understood. 

 In this connexion he referred to the ' pattern ' described by the- 

 Author, and asked for information concerning a certain band in the 

 l&ptoiheca zone which he understood was on the horizon of a ' green 

 streak ' in the same zone of the Lake District (known there as the 

 argenteus zone). This streak had now been traced in the North 

 of England from near Broughton-in-Eurness, to Cautley near 

 Sedbergh, a distance in a direct line of 31 miles. 



Dr. H. Lapwoeth congratulated the Author on the completion of 

 a most interesting and valuable piece of work. He had visited the 

 district with the Author, who had shown him his main sections- 

 and demonstrated the general stratigraphy. He had been greatly 

 impressed by the remarkable lithological similarity between the 

 rocks of this region and those of Ehayader. In the middle and 

 upper portions of the Pont Erwyd Group it was possible almost. 

 to correlate each band with its equivalent 20 miles to the east. 

 There were, however, one or two striking discordances, which 

 threw light on the geological history of Mid-Wales. In the- 

 Plynlimon district the Author had found no well-marked litho- 

 logical change at the base of the Lower Llandoverj 7 , such as existed 

 at Ehayader ; and here, as at Tarannon, there was a continuous, 

 passage upwards from the convolutus beds into the sedgwicki and 

 maximus zones, in contrast with the pronounced unconformity at 

 Ehayader. 



Farther north Mr. Fearnsides, at Criccieth, and the speaker, at 

 Aberdovey, had found the Valentian rocks to be of the Lake- 



