Belt— On the " Lingula Flags:' 541 



ing influence of the Ehobell-fawr igneous rocks, cross the Wnion near 

 Glyn Maiden, and then by Gwern-y-barcud and Tyn-y-craig, range 

 to Coed-y-garth, and into the estuary of the Mawddach. To the 

 south of Dolgelly, at Pandy and Bryn-rhug, they are brought in by a 

 branch of the great Bala fault, and are there much disturbed and 

 altered by intrusive igneous rocks. 



Very durable building stones and some good rough flags are 

 obtained from these strata. In the neighbourhood of the intrusive 

 diabases the lower beds are often largely impregnated with iron and 

 copper p3'rites, and have been mined for the latter with some 

 success at Glasdir. The numerous quartz veins intersecting the 

 same beds in the neighbourhood of Dol-y-frwynog all contain a 

 little gold, but have nowhere on this horizon been worked with 

 profit. 



Dolgelly Gkoup. 



Lower Dolgelly Beds (No. 7 in Section), — The next beds in 

 ascending order are hard, blue slates, characterized by containing, 

 in great abundance, a small species of Orihis, and Paraholina (Olenus) 

 spinulosa, Wahl, P. sjpinulosa was first found in the Dolgelly dis- 

 trict by Mr, Williamson, in loose boulders, in the valley of the 

 Mawddach, below Ehiw-felyn. In consequence of this discovery we 

 searched the rocks in the neighbourhood together, and soon found, 

 not only the above fossils in situ, but, above the strata contain- 

 ing them, the Upper Dolgelly beds crowded in some parts with 

 Trilobites of various genera. Shortly after, I found the lower beds 

 with Orthis and P. sjpinulosa at Gwern-y-barcud, and more lately, 

 have detected them on Mynydd Gader, at both places lying con- 

 formably upon the Upper Festiniog beds. I have also recognised 

 them on the eastern flank of Moel Cors-y-Garnedd ; but there, 

 through the inversion of the strata, they lie below instead of above 

 the Upper Festiniog beds. Since their discovery, the lower beds 

 have been well searched for fossils ; but the only species found 

 in addition to the two mentioned above, have been some speci- 

 mens of a Lingulella, and a single fragment of a species of Ag- 

 nostus. In a loose stone, which probably came from these beds, 

 Mr. Hicks, of St. David's, found a species of Protospongia. It adds 

 to the probability that the specimen came from the Lower Dolgelly 

 beds — that I have foimd two species of the same genus in Lower 

 Tremadoc beds — so that it must have existed from the Menevian 

 epoch, where it first appears, up to the time of the deposition of the 

 Ti-emadoc strata. 



I think that the Lower Dolgelly beds are about three hundred feet 

 thick ; but they are so much jointed and faulted that I have nowhere 

 been able to get a trustworthy measurement of them, and my estimate 

 of their thickness is little more than a guess. 



Upper Dolgelly Beds (No. 8 in Section). — To these beds I have 

 already alluded, when mentioning the discovery of the last. They 

 are soft, black slates, much jointed and often intensely cleaved. 

 They generally contain numerous fine grains of pisolitic iron. Near 



