﻿Tol. 66.~] TEEMADOC SLATES OF SOUTH-EAST CAEXAEVOXSHIBE. 157 



with the nets of this graptolite. Somewhat less resistant in its 

 weathering than the beds above and below, the Dictyonema Band 

 generally crops out along a slight hollow, and the fossils must be 

 sought along the under side of the outstanding rock-rib which 

 comes next above the prominent Obolus Bed described above. The 

 Dictyonema rock itself is quite distinctive. It is a fine-grained 

 rock of a darkish blue-grey, and with faint pale bandings, which 

 ■come to maxima three or four times in each inch. It breaks 

 not along its bedding, but along joints, into pieces shaped like 

 irregular paper-knives, with very sharp edges, each three or four 

 times as long as broad. As it weathers the surfaces first turn 

 green, and then to an iridescent gold which is characteristic ; 

 later, they are covered with a foxy-brown rust-film, which they 

 retain until the whole rock is bleached to a pale but bright creamy 

 paste. 



Dictyonema sociale was discovered and first described by Salter 1 

 from this district, but his specimens were not of the best, and the 

 species and its variations require, and are worthy of, a modern 

 revision. As generally collected, the specimens are in the form of 

 casts, filled with iron oxide ; but, although such specimens show the 

 form of the net and the association of individuals, they do not retain 

 the details of original cell-structure, and from a zoological point of 

 view are uninteresting. Fragments of more satisfactory specimens 

 rare occasionally to be obtained from fresh and un weathered rock, but 

 the slate will break along the bedding only by accident, and the 

 pieces of the graptolite seen are small. In such rock the Dictyonema 

 is found replaced by the unstable form of pyrite (marcasite) and 

 is preserved in full relief. 



The outcrop of the Dictyonema Band is indicated with sufficient 

 care upon the map (PI. XVII), and I need only add some notes 

 upon localities at which one can be sure of finding the rock in a 

 condition suitable for the collecting of fossils. 



At the eastern outcrop, by the sea, below Bron-y-garth, the 

 richest fossil bed is mostly covered by scree. At Lletty and along 

 the little cliff which bounds the paddock between Bryn-parc and the 

 wood of Parc-y-Borth, there is a fine exposure, which is maintained 

 through the northern corner of the wood and through the plantation 

 beyond the Morfa Bychan road along the cart-road to Llanerch 

 Farm. The fault there carries the outcrop to the 400-foot contour 

 south of the eastern summit of Moel-y-gest, and there fine exposures 

 •occur. Hence rising, it attains 600 feet at the wall south of the 

 western summit, whence descending it skirts Coed-y-cefn, and 

 crosses the road from Wern to Bron-y-foel, just below the 300- 

 foot contour, with open moorland exposures all the way. Lower 

 down, at the 200-foot contour on the same road, is the original 

 locality of Salter's Dictyonema (called from the keeper's house 

 <Cefn-cyfanedd), and there, at the turn of the road, is a wall every 



1 Mem. Geol. Saw. vol. iii (1866) Pal. Appendix, p. 331. 



