538 Belt— On the " Lingula Flags" 



I shall now describe the strata lying between the Menevian and 

 Tremadoc beds in ascending order. 



Maentwrog Gkoup. 



Lower Maentwrog Beds (No. 3 in Section). — The blue black slates 

 of the Menevian group, which everywhere around the Merioneth- 

 shire anti-clinal overlie conforsnably the Harlech grits, are followed 

 by a series of sandy and slaty beds. The lowest of these are grey 

 and yellowish grey, fine-grained, pyritic flags, with hard felspathic 

 bands and rusty partings. Alternations of more arenaceous and 

 gritty beds are not unfrequent, and beds of yellow grey shale also 

 occur. Some of the beds are ripple-marked and traversed by worm- 

 tracks, but neither Trilobites nor shells have been found. 



These arenaceous and shaly flags are about 400 feet thick, and are 

 succeeded by bluish grey, blue, and blue-black, jointed, fossiliferous 

 slates, alternating with bands of slightly arenaceous, grey and yellow 

 grey flags. The slates contain (Menus gibhosus, Wahl ; Agnostus 

 nodosus, Belt ; and A. pisiformis, Lin., var. ohesus, Belt. Fragments 

 of these fossils were first found by Mr. Williamson, near Cefn- 

 deuddwr in 1864 ; but it was not until Mr. Barlow found them in 

 great abundance near Dolmelynllyn in 1866 that I was able to 

 determine their specific distinctness, and to separate the beds con- 

 taining them from those above, with which they had been until then 

 confounded. 



The fossiliferous beds, and also the underlying arenaceous and 

 shaly flags, may be well studied on the range between the Eden and 

 the Mawddach, a little above the junction of these rivers, but the 

 best locality for the fossils is that discovered by Mr. Barlow, near 

 Dolmelynllyn, in the Mawddach, opposite the fifth milestone on the 

 Trawsfynydd road. 



The fossiliferous beds are about 300 feet thick, making the total 

 thickness of the Lower Maentwrog beds about 700 feet. 



Upper Maentwrog Beds (No. 4 in Section).— The last beds are 

 overlaid by yellow and bluish grey fine grained flags, sometimes 

 a little arenaceous, but never so coarse as some of the gritty beds 

 lying below. They are often finely laminated and flaky, especially 

 towards their upper limit, where Agnostus pisiformis, Lin., is not 

 imcommon. The top beds are very fine grained and flaky, and of a 

 bluish or brownish grey colour. There are about 600 feet of these 

 fine grained flags. 



From the bluish grey beds there is a gradual passage upwards 

 into dark, dull blue slates, much jointed and weathering to a rusty 

 colour. There are occasional intercalations of bluish and yellowish 

 grey beds ; and where sections of the rocks are exposed in the beds 

 of streams, thin alternations of blue, yellow, and grey layers give 

 them a banded appearance. This part of the series is at least 1200 

 feet thick, and may be more. The rocks are so much faulted that it 

 is impossible to obtain more than an approximation towards correct 

 measurements. _ _ 



Throughout the blue beds Agnostus pisiformis, Lin. is found, but 



