Nov. 28, 1884.] 



• KNO^A/'LEDGE • 



437 



former village, we walk across the sandhills which fringe 

 the beach to the rounded nioorlaad ridge of Penrhynddu, 

 where St. Tudwal's twin isles lie at our feet, their rocks 

 being an extension of those of the mainland. Walking 

 southwards along the precipitous cliffs, the little bay of 

 Porth-Oeiriad is soon reached. Here the Lingula Flags* 

 form magnificent cliffs S-JO ft. in height, the rocks dipping 

 to the east at an angle of 45deg. ; in the middle of the 

 bay they have been eroded down to the sea level, and the 

 hollow so formed is filled by a great mass of boulder 

 clay rising perpendicularly 80 ft. or 100 ft. above the 

 shore (Fig. 1). 



550 FEtT 



PENRHVN-DU 



LI NCU LA FLAGS 



LINCULA FLAGS 



Fig. 1. — Section in Bay of Porth-Ceiriad. Lingula Flags form 

 cliffs on the east and on the west ; the hollow between being filled 

 up by glacial deposits. 



The stratified Cambrian rocks here consist of hard mica- 

 ceous sandstones (ripple-marked and showing worm-tracks 

 on the surfaces) with dark-grey and black slabs. Similar 

 beds extend westwards as far as the broad bay called 

 " Hell's Mouth," whose low shore is marked by a little 

 cliff of boulder-clay and fringed by green fislds which, 

 smiling in the summer-snn, belie the ominous name. But 

 woe to the sailing-vessel which gets becalmed between 

 its two projecting headlands ! Strong currents set 

 towards the beacli, and the projecting ribs and timbers 

 of wrecked ships mark their fatal ertect. A little girl, who 

 acted as my guide, told me that sealed bottles which had 

 been thrown overboard from ships in peril were not uncom- 

 monly found on the shore of Hell's Mouth, and I saw in a 

 cottage a number of West Indian seeds picked up there. 

 In the absence of fossils, the hard grits and shales— 1,200 

 feet in thickness — which we have just described, were long 

 considered to belong to the Ilarlcch Series, which lies at or 

 near the base of the Cambrian formation. But, in 

 1876, Professor Ramsay found specimens of the trilobites 

 Agnostus and Olenus, with the little brachiopod shell 

 Linguhlla lepis in roadside quarries near Porth-Ceiriad ; and 

 as these fossils are peculiar to the Lingula Flags, this fortu- 

 nate discovery at once settled the question of the true age 

 1)1 the strata. 



Passing now to the north-east from Abersoch, through 

 Pwllheli, it is a distance of fifteen miles before we reach 

 the Lingula Flags, in the east of our district, where they 

 mantle i-ound the hill called Moel-y-gest, whose profile is a 

 gigantic Wellington silhouette. Here the whitish flaggy 

 sandstones form successive step-like terraces at the western 

 foot of the hill, whence they run down to the coast at 

 Ogof ddu, about a mile east of Criccieth. Their thickness 

 here is above 2,000 ft , forming a dome-shaped mass, of 

 which the northern half only is visible. The sandy beds 

 are surmounted Vjy black slates, and there are black slates 

 underneath them also, in which the caves at Ygraig-ddu — 

 the Black Rick — have been hollowed out. Fossils are 

 numerous, but not easy to find or to extract ; the best 

 localities are Ogof-ddu, Penmorfa Church, Carreg-wen, and 

 Borth (south-west of Portmadoc). 



"' This name is derived from the abundance of the brachiopod 

 shell LiiujnJa (now LinrjvIclUi) Davisii, discovered by Mr. Davis in 

 1845. 



The Tremadoc Slates lie above the Lingula Flags, 

 between Portmadoc and Criccieth, but they decrease in 

 thickness from 3,000 ft. near the former, to 200 ft. near the 

 latter, town, and they probably die out altogether further 

 west, for there is no trace of them above the Liugula Flags 

 south of Abersoch. They derive their name from the pretty 

 little town of Tremadoc — which, in its turn, was christened 

 after its founder, a Mr. Jladocks, who flourished as a 

 notable land-reclaimer in this region, at the close of the last 

 century. 



It is not possible for me to leave the Cambrian beds of 

 Carnarvonshire without acknowledging the great services 

 rendered in the study of these rocks by Messrs. David 

 Homfray and Ash, of Portmadoc, to the former of whom 

 I am personally indebted for guidance and information. 

 The late Mr. Salter did some of his best work down here, 

 about 18G0; but the main relations of the different beds 

 _had been ascertained by Sedgwick some ten years' pre- 

 viously. 



Lower Silurian Formation. — The junction of the great 

 Cambrian and Silurian formation is marked by a bed o'; 

 grit, well seen at Garth Hill, near Portmadoc, but only 

 10 feet thick where it runs out to sea at Ogof-ddu, east of 

 Criccieth. This grit forms the base of the Arem(j U'rotip, 

 which consists mainly of iron-stained black slates, con- 

 taining beds of pisolitic iron ore and a large collection of 

 fossils — the yraptoliles being especially numerous and note- 

 worthy. Arenig Rocks also occur in the western tract, 

 between Abersoch and Llanengan. Here, they dip north- 

 wards, and are se])arated from the Lingula Flags beneath 

 by a line of fault, whose direction, east and west, can be 

 clearly traced by the line of chimneys marking the numerous 

 lead-mines ; for the " fault " has been filled up with (among 

 other things) much galena (sulphide of lead) deposited by 

 the heated water which traversed the line of weakness, 

 resulting from the severance of the rocks. The rubbish 

 heaps and heaps of picked ore surrounding the mouths of 

 the shafts of the lead-mines form a fine collecting-ground 

 for the mineralogist. The pisolitic iron-ore, by which the 

 Arenig strata can be best recognised in this western portion 

 of Carnarvonshire, is only known to occur at one other 

 place in Lleyn, viz., at Trwyn-y tal, on the coast, about a 

 mile north-west of Yr Eifi. 



If it be asked why the line of demarcation between two 

 great geological series is drawn at the base of the Arenig Beds, 

 the answer is to be found in the great change which there 

 takes place in the fossils — the remains by which we judge 

 of the life of the period. No fewer than fifty-five new 

 genera of animals make their first appearance in the 

 Arenig Beds, including the remarkable fossils called 

 graptolites, which Prof. Lapworth has .shown to be so 

 useful as marking by distinct species distinct beds of rock, 

 and so enabling us to identify the order of succession among 

 strata which may be hundreds of miles apart. 



The Arenig Beds are also interesting, because they tell 

 us of the volcanic action on a grand scale which took place 

 daring the time of their deposition. In the Arenig 

 Mountains, the Arans, Cader Idris, and elsewhere, we find 

 the Arenig slates interbedded with enormotts masses of 

 old lava and beds of volcanic ashes. In the district we 

 are now describing, a bed of igneous rock of this age forms 

 the top of Moel-y-gest, and constitutes the grand precipices 

 north of that hill between Tremadoc and Brynkir (Fig. 3). 

 On the Survey Map it is styled greenstone, a name which 

 now means nothing save that some igneous rock is meant. 

 Modern petrologists would call the Moel-y-gest rock an 

 ancient basalt. 



(To le continued.) 



