LLANDEILO FLAGS — (DISTINCTIONS OF). 



223 



lions to graduate downwards into the older strata of the Cambrian System, as re- 

 presented in the woodcut, p. 196, (see also PI. 34. figs. 3. and 8.). In the immediate 

 neighbourhood of Ludlow and Wenlock, on the contrary, there is a great break along 

 the base of the Caradoc sandstones, occasioned by the outburst of trap which will be 

 soon described (PL 31. figs. 3. and 4.), so that the exact place of the Llandeilo forma- 

 tion cannot be there established. Without, however, quitting Shropshire and the ad- 

 jacent tracts of Montgomeryshire, we can prove the flags to be of older date than the 

 sandstones, because the latter, as exhibited on the slopes of Caer Caradoc, contain no 

 strata lithologically resembling these flags, whilst in several situations on the north- 

 western side of the Longmynd and Stiper stones, true dark-coloured Llandeilo flags, 

 containing the characteristic trilobites, rise from beneath sandstone charged with many 

 well known Caradoc fossils (PL 32. figs. 1. and 2.). That these Llandeilo flags dip be- 

 neath the Caradoc formation, is also proved by continuing a transverse section to the 

 north-west of the Vale of Meifod (PL 32. fig. 9.) j when, after passing over many ridges 

 composed of Caradoc sandstone with fossils, we meet with dark calcareous flags charged 

 with the Asaphus Buchii, &c, emerging from beneath the sandstones, and reposing upon 

 the slaty rocks of the Berwyn mountains. (See the end of Chap. 24.) It is from these 

 combined evidences, drawn from different localities, that the Llandeilo flags are proved 

 to be of higher antiquity than the Caradoc sandstone, properly so called. Some geolo- 

 gists, seeing the vast thickness of the Caradoc sandstone, and the comparatively small 

 dimensions of the Llandeilo flags, with the occasional recurrence of sandy beds beneath 

 them, might prefer to comprehend under the former name both deposits. Taken in 

 this sense the Llandeilo could be considered only a subdivision of the Caradoc forma- 

 tion. After mature consideration, however, and the examination of numberless sec- 

 tions, I have preferred retaining the Caradoc sandstones and Llandeilo flags as separate 

 formations, terming the whole "Lower Silurian Rocks", and premising, that where cer- 

 tain calcareous bands and peculiar beds of fossils disappear, it is not practicable to 

 maintain the division of the group into distinct formations. On the contrary, however, 

 where fully expanded, as in Caermarthenshire and Pembrokeshire, these flags are often 

 of much larger dimensions than the Caradoc sandstone : besides which the flags are so 

 strongly marked by lithological characters, and the presence of distinct trilobites, that 

 there is no difficulty in distinguishing them from the overlying sandstone. 



In the vicinity of trap rocks, these flags, like the Caradoc sandstone, frequently 

 contain mineral veins, chiefly of lead. In the Shelve and Cornden district, such ores 

 are largely worked. On the flanks of the Berwyns in Montgomeryshire, and near 

 Llandrindod in Radnorshire, they are small and unproductive. The productive ores of 

 the Cornden and Shelve district will be described at length in the twenty-second 

 chapter. 



It is scarcely possible to estimate the thickness of these flags, nor is it necessary to 

 do so. In Caermarthenshire, where the overlying formations are attenuated, they are, 



2e 2 



