7 6 



PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



tributed much to our knowledge of the fauna and geology of the 

 Lingula-flag series. He was then engaged in investigating the 

 Dolgelly and other districts, especially with reference to the divi- 

 sions of the Lingula-flags called the Maentwrog, Ffestiniog, Dolgelly, 

 and Tremadoc groups, lying between the Menevian and Tremadoc 

 beds, and occupying extensive areas near Dolgelly and in the 

 Mawddach valley, also at Maentwrog near Ffestiniog &c. 



In 1847 Prof. Sedgwick, on palaeontological evidence, removed 

 the previously named Tremadoc series from the "Lingula-nags," 

 and designated the latter the Ffestiniog group. Salter divided the 

 Lingula-nags, in 1866, into Lower, Middle, and L T pper. Again, 

 Salter and Hicks, in the same year *, included the Lingula-nags of 

 South Wales only in the Ffestiniog group, describing them as being 

 characterized by " hard siliceous sandstone with grey flaky slate " con- 

 taining f 6 Lingulella Davisii." In character the North- Wales group 

 of rocks differs altogether from those of South Wales, the arenaceous 

 flags and shales with L. Davisii forming only a subordinate part of 

 a series of fine-grained dark blue and black slates containing many 

 genera of Trilobites. Mr. Belt restricted the Ffestiniog beds to the 

 flags containing Lingulella Davisii and the Phyllopod Crustacean 

 Hymenocaris vermicauda, as originally applied by Sedgwick, and 

 proposed to name the slates and flag3 below them the Maentwrog 

 group, characterized by peculiar Oleni. These Maentwrog beds are 

 exhibited to great perfection at Maentwrog, S.W. of Ffestiniog. 



The blue and black slates occurring above the Ffestiniog series 

 Belt named the Dolgelly group, with reference to the circumstance 

 that it is only in this area that both members have been found. 

 This upper member of the Lingula-nags is characterized by peculiar 

 forms of Oleni, comprising the subgenera Splicer ophtlialmus, Para- 

 bolina, &c. These three groups — the Maentwrog, Ffestiniog, and 

 Dolgelly — are palseontologically and lithologically distinct ; none of 

 the Trilobita passes from one group to another, peculiar genera occur- 

 ring in each. Each group also has well-defined lithological charac- 

 ters. The Maentwrog is readily distinguished by its dark-blue 

 jointed ferruginous slates ; the Ffestiniog by hard micaceous flags 

 with abundant Lingulella Davisii, Hymenocaris, and Conocoryphe ; 

 and the uppermost or Dolgelly, by species of Agnostus, Olenus and 

 its subgenera, which are abundant in the soft black slate, which 

 shows a black streak when scratched. Mr. Belt divided the Maent- 

 wrog slates into a lower and upper series : the lower are ripple- 

 marked, and have many Annelide-tracks. They are conformable to 

 the blue-black Menevian slates which mantle round the Merioneth- 

 shire anticlinal. This lower group is about 700 feet thick. Olenus 

 gibbosus, Wahl., and Agnostus pisiformis, Linn. (A. princeps, Salt.), 

 var. obesus, Belt, occur in the slates. The range between the Eden 

 and the Mawddach, and near Dolmelynllyn, in the Mawddach, are 

 the chief localities for fossils. The upper Maentwrog series is 

 nearly 2000 feet thick. Agnostus princeps is abundant in the flaky 

 * Brit. Assoc. Reports, 1866. 



