﻿Vol. 66.'] TREMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 163 



begins at the cart-track 150 yards S. 40° W. from the first mile- 

 stone on the Tremadoc-Carnarvon road. 



Thickness in feet^ 



Thinly bedded dark-blue slate, bladed irregularly along the bed- 

 ding, weathering to iridescent tints and a rich red-brown. 

 Contains abundant tails of Ogygia and numerous pitted heads 

 of Angelina 20' 



Very thinly-bedded shivering slate-rock, with thin white- 

 weathering bands. Some of its beds are calcareous, and 

 almost consist of broken fragments of Angelina 20 



Flaggy slate, prominently ribbed in. 3-inch bands, paler and not 

 so rusty as the higher beds, and forming an outstanding rock- 

 rib. Large nodules of cone-in-cone ironstone not infrequent. 

 A flaggy bed in the midst has yielded several specimens of 

 Dicettocephalus furca 3*0 



Above these rocks, between the Ogygia Band and the great 

 Penmorfa Fault, comes some 50 feet of thinly-bedded Angelina- 

 bearing slate, but this is too much crushed by the fault to yield; 

 good specimens. 



At Garth Terrace the rock is similarly thin-bedded and silky ,~ 

 but the needle-cleavage is strong, and it is difficult to break open? 

 any bedding-plane over the area of a whole trilobite. At the lodge- 

 gate to Tu-hwn't-i'r-bwlch, and along the Criccieth road as far as the 

 Tu-hwn't-i'r-bwlch quarry, similar banded needle-rock is exposed^ 

 and fragmentary fossils can generally be had for the seeking. As we 

 go westwards along the road the needle-jointing gives place to blading, 

 and fossils become easier to get. Farther west, at Penrhyn-llwyd: 

 and north of the railway at Penamser, quite good trilobites have 

 been collected, and at the last-named locality the same succession 

 as at Penmorfa has been made out. 



On the Criccieth side of the anticline three specimens of Angelina 

 were taken from a block of hard blue slate on the road-bank, 250 

 yards south of Braich-y -saint. 



Another famous locality for Angelina is Ynys-towyn, on the east 

 side of Portmadoc Harbour. Most of the Tremadoc rock there was 

 quarried away in the making of the road-and-railway embank- 

 ment, but the remaining hill-mass shows a fine exposure of Garth 

 Hill Beds and of the overlying Ordovician grit. Along the Oakeley 

 slate-wharf the rocks are needle-slates like those of the Garth Hill 

 dip-slope ; but north of the roadway the higher beds, less intensely 

 cleaved, are pyritous leaden-grey raudstones with coarser flaggy 

 partings. These weather with an inky or ochreous-brown rusty 

 crust, which is very characteristic, and contain rows of spreading 

 sheet-like concretions of cone-in-cone ironstone at many horizons. 

 Some of the mudstone beds are quite massive, to a thickness of 

 more than a foot. The Tremadoc rocks exposed at Ynys-towyn are 

 about 90 feet thick. 



The Yynys, upon which Pen-syflog House is built, shows rock 

 like that of Ynys-towyn. The small rock in the field south-west 

 of the farm yields abundantly Ogygia and Angelina. 



