262 l>ROF. A. H. COX AXD ME. A. K. WELLS ON THE [vol. lxxvi, 



(i) The Niobe Beds. — As already remarked, the dark lamin- 

 ated shales of the Dolgelley Beds pass quite gradually into the 

 paler and harder Tremadoc Slates. These lowest heds of the- 

 Tremadoc Series do not show any obvious lamination, and they 

 are much less pyritous than the Dolgelley Beds, so that, although 

 frequently assuming a red staining on weathering, they do not 

 become covered with the very thick coat of rust that is so often 

 seen in the Dolgelley Beds. In the course of about 200 feet the 

 beds become harder and more flaggy, and the red staining becomes 

 less pronounced. The uppermost flaggy beds are Yerj fine-grained,, 

 and at first sight recall beds that have been baked by intrusions ; 

 they possess sometimes almost the appearance of a chert or an 

 adinole, and weather completely white. They give rise to a 

 marked escarpment on both sides of the Gwynant Valley : the one 

 on the east side — Graig Llwyd — attaining a height of 1011 feet 

 (PI. XVI). At this locality Niobe liomplirayi and various ex- 

 amples of Lingulella lepis Salter were obtained from the highest 

 beds of the flaggy group. The characteristic fossil, N. homphrayi, 

 was obtained by Belt from corresponding beds on the northern 

 front of Mynydd-y-Gader. 



(ii) The Lower Dictyonema Band. — Immediately succeed- 

 ing the hard flagg} r strata come the softer beds of the Lower 

 Dictyonema Band. They consist of dark-blue shales passing up 

 into harder blue-grey mudstones, which weather to a light grey. 

 The soft beds determine the position of a hollow which is often 

 very deep, as (for example) east of Garth Angharad. The fossils, 

 are usually to be found in the lower part of a small craggy scarp 

 which overlooks this hollow, exactly as described by Prof. Fearn- 

 sides in the case of the corresponding beds of the Tremacloc- 

 district. 1 On the western bank of the Gwynant the small scarp is 

 crowned by a wall, and numerous examples of Dictyonema sociale- 

 were obtained from the beds underneath the wall at various points 

 between the river and the 350-foot contour. Higher up the hill- 

 side the Dictyonema Band is largely covered by screes from the 

 overlying harder slates. The small scarp is also developed east of 

 the river, and here again fossils were obtained from the lowest beds 

 of the scai'p at the point where it crosses the road. Farther east 

 fossils were discovered at various points in the hollow immediately 

 south of Graig Llwyd, just above (stratigraphically) the beds 

 yielding Lingulella lepis; but at this last locality the Dictyonema- 

 Beds are badly cleaved, and hence good specimens are difficult to 

 obtain. The close association of Lingulella lepis and Dictyonema 

 sociale is well-known elsewhere. Still farther east Dictyonema 

 was found near Bron-y-gader by Belt, 2 and on the northern flank 

 of Mynydd-y-Gader by Mr. Lake & Prof. Reynolds. 3 



1 Q. J. G. S. vol. lxvi (1910) p. 157. 



2 Geol. Mag. 1867, p. 542. 



3 Q. J. G. S. vol. lii (1896) p. 514. 



