2(56 PROF. A. H. COX AND JIB, A. K. WELLS OS THE [vol. lxxvi, 



less felspathic, and more like a quartzite. Their texture is usually 

 medium to rather fine-grained, and true pebbles are wanting, 

 though small pellets of mudstone, possibly derived from Tremadoc 

 Slates, are fairly abundant in the actual basal bed. 



Some 10 to 15 feet up in the series argillaceous material begins 

 to make its appearance, distributed in thin shreds and streaks ; 

 these increase in amount upwards, and so give rise to a set of 

 streaky grits that are highly characteristic of the Basement Series. 

 The striping of the grits is at first irregular, owing to the lenticular 

 arrangement of the alternations of arenaceous and argillaceous 

 material, the latter being distributed in thin films that curve round 

 the sandy lenticles in a manner suggestive of ripple-marking and 

 false bedding. The sandy material is sometimes white, sometimes 

 grey, or almost blue, according to the amount of chloritic matter 

 present. 



Still higher up the argillaceous partings increase in number, and 

 at the same time the alternations of sandy and muddy material 

 become much more regular and more closely spaced ; this gives 

 rise to a group of flags with very regular striping. The sandy 

 material is now almost invariably pale-coloured, and is also rather 

 liner-grained than in the irregularly- streaked grits below. Among 

 these regularly-banded flags is intercalated a massive band of white 

 fine-grained grit which has almost the character of a tiue quartzite. 

 It is overlain by a small thickness of flags similar to those below. 



The group is terminated upwards by aboiit 6 feet of slates. The 

 slates are themselves somewhat flaggy, owing to their micaceous 

 and slightly arenaceous character. They are dark bluish-grey, and 

 ■contain a fair amount of pyrites. Being so thin the slate-band is 

 not often exposed, but it is well seen at the point where the stream 

 issuing from Llynau Crogenen crosses the outcrop of the Basement 

 Series. The junction of the slates with the massive rhyolitic rocks 

 above is here excellently displayed. 



The streaky grits and the banded flags are constant members of 

 the Basement Series ; but the actual basal grits, such as those seen 

 near Llyn Wylfa, are not always developed. Nor is the higher 

 massive white quartzite in variably present : for example, it seems 

 rather impersistent near Llyn Wylfa. The Basement Series has 

 a thickness of 150 to 200 feet. 



These basement beds frequently occur on the slopes of a steep 

 scarp overlooking the Tremadoc Slates, the top of the scarp being 

 occupied by the rhyolitic rocks of the Lower Acid Series with their 

 attendant diabases. For this reason outcrops are often obscured 

 by screes from above. This is the mode of occurrence for 

 the whole distance between Llyn Wylfa and the stream issuing 

 from Llynau Crogenen. Near this stream are good exposures. 

 Still farther west other good exposures occur on each side of 

 the Arthog Glen and Waterfalls, on the east side especially near 

 Pant Phylip, and on the west side near Crogenen Farm and at the 

 edge of the wood above Arthog-Hall Hotel. In this neighbour- 

 hood the outcrops are affected by an anticlinal fold, and they are 



