and the Paleozoic System of England, ~ 4^ 



VII. Llandeilo Flag of the Valley of the Towy, and its relations 

 to the neighbouring groups, 



I will not detain my readers by any elaborate details respecting 

 this remarkable group. It had long been known to fossil col- 

 lectors from its specimens of Asaphus Buchi and other fossils, 

 and for ages its limestone beds and flags had been quarried for 

 economical use, but the definition of its geological place was 

 first attempted by the author of the ' Silurian System.' It 

 contains many calcareous bands, as may be well seen in the 

 Silurian sections, and still better in the maps and sections of 

 the Government Survey. These limestone bands (some exhibited 

 as mere streaks of limestone, alternating with, and passing into 

 dark shale and flagstone, others as thick and solid masses) are 

 numerous. Within the group are also bands of a brown, arena- 

 ceous flagstone, and sometimes of very hard and coarse siliceous 

 grits passing into a conglomerate. So far as I know, its base is 

 not exposed in any section of the Towy, but its highest beds (as 

 I interpret the group) are on its north side composed of a dark- 

 coloured, pyritous, indurated shale, passing into a slate, which in 

 a few places {e. g. N.E. of Carmarthen, and at Pengoylan near 

 Llangadoc) has been quarried for use. Some of these slates, 

 not being affected by transverse cleavage, show the Diplograpsus 

 pristis in considerable abundance. 



The group is in the form of a saddle, but these highest bands 

 are not so well exposed on its south side as they are on the north 

 side. They are represented partially, and may perhaps be here 

 and there overlapped and concealed by the Wenlock shale, which 

 (on the south side of the Towy from Llangadoc to Llanarthney) 

 seems to form the immediate southern boundary of the group. 



Most of these peculiarities of structure are seen in the sections 

 published by the Government Survey, and in the original sec- 

 tions of the ' Silurian System;' and with no idea of blinking 

 the difficulties of the subject, but in the hope of making the 

 relations of the groups clear, I gave an ideal section {supra, fig. 6, 

 p. 314) to show the order of the deposits down the valley of the 

 Towy, through a space of nearly twenty miles below Llangadoc. 



The disturbing forces that caused the complicated derange- 

 ment of the Llandeilo group aff^ected also the beds much further 

 towards the north-east, but without bringing up the pommel of 

 the saddle; for above Llangadoc, the valley for several miles 

 must originally have been bridged over by the higher geological 

 groups, such as those above described, which ended, here and 

 there, with a kind of top dressing of the May Hill sandstone. 



The slight ideal sketch (fig. 3, p. 314) above noticed was from 

 memory ; but since my recent visit to the valley of the Towy, I 



