ANNIVERSARY ADDRESS OE THE PRESIDENT. 



77 



beds near the summit above Dolgoed ; in thin slaty shales it occurs 

 in thousands, associated with Olenus truncatvs, Ang. ; the higher 

 beds contain 0. cataractes, Salt., which replaces A. princeps. This 

 group is completely developed in the Maentwrog district, fine sec- 

 tions occurring in the Waterfall valley and in the valley running 

 from Taffarn-helig to Caen-y-coed. Olenus cataractes abounds here. 



Lower Ffestiniog Beds. 



Thick micaceous grey flags, 2000 feet thick, conformably overlie 

 the upper Maentwrog beds ; the lowest series are bluish-grey slates, 

 and contain Lingulella Davisii, M'Coy, with Annelide-tracks ; and 

 the highest part of the Lower Ffestiniog slates yields Hymenocaris 

 vermicauda, only known in this division of the Lingula-flags. 



Upper Feestiniog. 



This group succeeds the Lower series, and is not more than 50 feet 

 thick, but distinct in fossil contents. The characteristic forms here 

 are Olenus micrurus and Better ophon cambrensis. This thinly deve- 

 loped division occurs at Gwern-y-barend, in the Mawddach near 

 Craig-y-dinas and on Mynydd-Gader. 



The river Mawddach cuts through the whole of these beds 

 between Ehiufelyn and Hafod-fraith, thence ranging across the east 

 end of Moel Hafodowen and by Pen-y-bryn. 



Dolgelly Group. 



This uppermost division of the Lingula-flags is physically divided 

 into two series, lower and upper, the former composed of hard 

 blue slates abundantly filled with Olenus (Parabolina) spinulosis. 

 The thickness of this series is about 300 feet. 



The upper division differs essentially from the lower. It consists 

 of soft black slates, with black streak, intensely cleaved and often 

 pisolitic (fine grains of pisolitic iron-ore). The constant character 

 of the black streak distinctively marks this group, and in the most 

 disturbed areas may be relied on as a never-failing criterion of the 

 upper series. To Mr. Belt is due the determination of this character 

 over a large area — a fact of inestimable value in the identification 

 of strata so disturbed and intricately faulted as those round Dolgelly, 

 Bhobell-fawr, and on Mynydd-Gader. The black shales of Malvern, 

 overlying the Hollybush Sandstones, and containing Olenus bisulcatus, 

 Phill., O. scarabceoides, Wahl., O. pauper, O. humilis, Conocoryphe 

 mcdvernensis, Agnostus princeps, A. M'-Coyi, &c, belong here; they 

 underlie the Dictyonema-sh&les, and are exposed in the valley of the 

 White-leaved Oak. The upper Dolgelly beds are by far the most 

 fossiliferous ; and nearly every species in any of the areas is confined 



