MONIAN SYSTEM OF ROCKS. 477 



with the purple slates which are not foliated at all ; and on the 

 south-western side there are parts which are both cleaved and 

 foliated, and others which are neither. The disturbance and con- 

 tortion are much greater on this side, because the rocks have 

 yielded to the pressure. The}?- are also cleaved in parts, because 

 they are of suitable material. The accidents to which both parts 

 have been subjected since their formation have affected them in 

 different ways and in a different order ; but this throws no light 

 on their relative age, which must be determined, if possible, by 

 stratigraphy ; and if not, by correlation with deposits whose order 

 is known. 



The South-Stack Series is divided by the erosion of the sea into 

 two distinct parts, the one from Gogarth below the Holyhead moun- 

 tain to Porth-y-corwgi, the other near Eoscolyn from Bwa-du to 

 Berth "Wen. The two halves do not correspond, and it is not clear 

 whether they are alternative or consecutive. 



In the Eoscolyn mass we find their junction with the chloritic 

 schists. The bounding fault comes out in the centre of Berth "Wen, 

 and an epidote-infusion marks the boundary of the chloritic schists 

 towards the north-east. On the other side a mass of grit is brought 

 against these schists in a low clifi' in the hollow of the bay. When, 

 now, the series is traced eastwards into the promontory, this grit is 

 seen to lie upon a more micaceous variety, and this passes down, on 

 the small peninsula, into a rock, which can be distinguished neither 

 macroscopically nor microscoiDically from the typical chloritic schist 

 of Holyhead. So unprepared was I for this passage, that I labelled 

 the rock when first found " pseudochloritic," and paid a second visit 

 to the spot to find, if possible, some fault which had been over- 

 looked. ISTo fault, however, occurs, and it is certain that the South- 

 Stack Series succeeds conformably, and must therefore be the non- 

 volcanic equivalent of the pelites of the north. 



This Eoscolyn area must contain the lower beds of the series, 

 whatever the other area may be. In it we meet for the first time 

 the phenomenon of cleavage in these rocks. As soon as we leave the 

 chloritic schists this cleavage sets in, and along the planes of it abun- 

 dant chlorite or mica is developed, so that the rock is also foliated, 

 and on the surface of the ground the bedding is entirely masked. 

 Under the weathering of the sea all looks alike, with vertical lines 

 of subdivision, and stratigraphy has to be abandoned as hopeless. 

 But in a deep cleft of the cliff, marked by a fault on the Survey Map, 

 sudden light is afforded, and we learn that in spite of the cleavage 

 and foliation we are dealing with well-bedded rocks, each with its 

 distinct character and dipping in a northerly direction, i. e. partly 

 towards the fault and partly in the direction that will bring in 

 higher beds as we proceed north-westward (see fig. 1). It is the 

 lower beds of this section that we have been passing over since 

 leaving the chloritic schists of Borth Wen and the higher beds that 

 rise into the summit of Mynydd Eoscolyn. Thence they have 

 already come down to the level of the top of the cliff, and shortly 

 they occupy the whole of it. Special attention may be drawn to 



