169 



-sand Bay, however, and at Eamsey Island, the characteristic Lin- 

 gulella Davisii is to be -found in great abundance. At St; David's 

 the series is made up of very even deposits from the beginning to 

 the close, and no sudden change takes place anywhere in the cha- 

 racter of the sediment. In North Wales, before the close of the 

 epoch, there is evidence that a depression of the sea-bottom rather 

 suddenly took place ; and with this depression a fine silt was thrown 

 down, in which a fauna unknown to South Wales occurs. These 

 are the deposits now known as the O^mts-shales, which are found 

 in North Wales and in the neighbourhood of Malvern. Eor the 

 same reasons we find that the Tremadoc group, which immediately 

 succeeds the Lingula-flags, is also rather different lithologically in 

 North and South Wales. The depression in the St.-David's area 

 continued to go on very gradually even in the Tremadoc epoch ; 

 hence the character of the deposit is on the whole that of shallower 

 water than that in North Wales, and the fauna varies accordingly, 

 as may be noticed in the unusual prevalence and size of the Lamelli- 

 branchiata and the Brachiopoda found in these beds at St. David's. 

 Some parts of the series, however, are very similar in character, 

 and indicate the presence of like conditions in both districts, espe- 

 cially in the earlier stages of the formation. 



The beds of the Tremadoc group can be seen on the east side of 

 the anticlinal, resting conformably on the Lingula-flags about Tre- 

 manhire, and beyond Caerfegga ; but as there are only a few expo- 

 sures of the strata in that neighbourhood, in consequence of the 

 ground being covered over by drift, the further succession will have 

 to be illustrated chiefly by reference to the sections on the north 

 side of the axis. In the north end of Bainsey Island, or rather 

 at the north-east point (fig. 3), the Tremadoc group is seen in an ex- 

 cellent coast-section with the lower beds resting quite conformably 

 on the Lingula-flags, and the change from the one group to the 

 other is very gradual. The group consists for the most part of 

 dark earthy flags and flaggy sandstones, with some iron-stained 

 slates at the top, and it is rather less than 1000 feet in thickness* 

 The beds here are exceedingly rich in fossils, and they have yielded 

 many important new forms. At Trwyn-hwrddyn, in Whitesand Bay, 

 the lower beds of the group are also seen resting conformably on the 

 Lingula-flags (fig. 4), but the upper portion is cut off by an east and 

 west fault. A little further inland from this spot, however, the 

 fault has crossed the series, and the group is again seen in its proper 

 thickness *. Near Lanveran the beds are well exposed, and the 

 characteristic fossils are found in them. Prom here they take a 

 more easterly strike ; and they may be followed for several miles in 

 that direction. 



The next succeeding groups are the Arenig and Llandeilo \ and 

 these I shall have to refer to more in detail, as they have not 



* The fault of which this is a branch, as will be seen by reference to the Map 

 and Section IV. (PI. VIII. figs. 1 & 5), has somewhat interfered with the suc- 

 cession along the north edge of the anticlinal by cutting off the whole of the 

 Menevian group and part of the Lingula-flags. ■ . -I 



