﻿Vol. 66.'] TREMADOC SLATES OF SOUTH-EAST CARNARVONSHIRE. 149 



Thickness in jeet* 

 Thin and thick, alternating grey and blue, flaggy shales, much 

 cleaved and puckered, with ringers 4 to 6 feet apart. Notable 

 ringers at 20 and 60 feet below the sill, and a 3 to 6-inch 



grit-band at 40 feet 80 



Sill along which the main cave has been eroded 3 to 6- 



Flaggy slates with ringers : 4-inch ringers at the base 10 



Splintery shales with thin ringers : 4-inch ringers at the base . 20 

 Flaggy beds with strong double ringer : 9 inches to 1 foot 



thick at the base 30 



Rusty slaty flags, with strong ringers, composite : 1-foot ringer 



at the base 50 



Intrusive sill. 



Rusty slates, with about three ill-defined ringers to each yard. 50 



Massive, fine-grained, flaggy band 1 



Thinly-bedded silky flags or slates, without ringers 50 



say 300 



Ill-preserved specimens of Olenus cataractes from 1 to 2 inches'- 

 long are found in the splintery slates, some 25 feet below the sill 

 of the main cave ; small Agnosti and an indeterminate Conocoryphe- 

 are not infrequent in the silky flags and rusty slates at the base of 

 the series. 



This section is much complicated by small anticlinal folds and by 

 faulting, the effect of which is not easy to make out. It is, how- 

 ever, washed clean by the tide and fretted or polished by the wind- 

 driven sand, and so affords a most instructive exposure. Joints and 

 decayed sills and dykes of an ancient dolerite have given scope for 

 differential erosion by the waves, and the beautiful caves so produced 

 have long been famous. In these caves the colour-banding of the 

 slates and flags which form their walls adds largely to the fascination. 

 Greens, greys, and purples are the colours which appear ; and one 

 can often count some two or three large, and perhaps twenty to- 

 thirty small, colour-changes on each inch of rock. 



The cliffs below Cefn on the east of the bay were probably at 

 one time equally well exposed, but now the sand-hills partly covering 

 and partly protecting the cliffs from the direct action of the sea 

 have spoiled the section, and one can do little more than note the 

 rarity of ringers among the slaty flags exposed. The contorted 

 Olenus-be&r'mg flags of Ffynnon-Ochr-cefn appear to be identical 

 with the fossiliferous splintery slates which form the low detached 

 stack on the other side. 



Rocks referable to the top of the Maentwrog Series appear again 

 along the old sea-cliffs to the east of Morfa Bychan, from Gareg- 

 wen to Pechan Point, and in the outstanding rocks of Careg-cnwc 

 and Gareg-goch. In all these places they are thinly bedded and 

 well striped, but not very rusty. They contain trilobite fragments 

 and small Lingula?, and are associated with greenstone intrusions. 



The land topography of the Maentwrog Series is uniform and 

 uninteresting. The ringer-beds of the coast-section seem to conceal! 

 themselves, while the igneous sills which weather into sea-caves 

 stand up in slight relief. The soil is everywhere scanty, but is- 

 generally fertile enough to be worthy of cultivation. 



