Chap. V.] 
LOWER AND UPPER LLANDOVERY ROCKS. 
87 
Noeth Grug. (From the Government Survey.) 
S.E. 
b 
o 
f 
e 
d 
c 
a 
a 
a. Schists, &c., representing the Caradoc formation, b. Lower Llandovery rocks 
(Llandovery Sandstone), c. Upper Llandovery rocks : equivalent of the May Hill 
Sandstone, d. Tarannon Shale, or ' pale slate ' (of which hereafter), e. Wenlock 
and other Upper Silurian strata, without subdividing limestones. /. Base of Old Red 
Sandstone. (Compare with Section, Sil. Syst. pi. 34. f. 3.) 
Wild and uninhabited, this small tract is truly remarkable in being the 
only one in England and Wales wherein the lower and upper zones of the 
Llandovery rocks have as yet been observed in a united mass, and with 
clear relations to the inferior and superior strata. In the last chapter a 
succession from the Caradoc formation into the lower member of this group 
was, indeed, indicated as occurring over large districts of the west of Car- 
marthen and Cardigan. But there the upper member of the Llandovery 
rocks seems to be wanting, and no overlying Upper Silurian strata are 
contiguous, as in this district. In the bare and rocky undulations, around 
Noeth Grug, no unconformity is observable between the underlying beds, or 
the representative of the Caradoc formation, and the Llandovery strata 
surmounting them, which are laden with Pentameri and other fossils of the 
group under review. Nor is any discordance visible either between the 
Lower and Upper Llandovery rocks, or between the latter and the overlying 
Upper Silurian strata, as well as the Old Red Sandstone. What then is 
the age and equivalent of the lower and larger portion of the hard gritty 
sandstone (b) ? If mineral aspect be considered, this rock would certainly 
be ranked as one of high antiquity ; for the softer and more argillaceous 
sandstone and building- stone of Llandovery has in the short intervening 
distance become a hard intractable grit, traversed by planes of slaty clea- 
vage, as shown in the white lines of the drawing. 
Again, if the fossils of the lower beds of these sandstones be appealed to, we 
find in them, either at this or other localities, the following unquestionable 
Lower Silurian types : — Illsenus Bowmanni, Salter ; Lichas laxatus, M'Coy ; 
Murchisonia simplex, id. ; Orthis insularis, Eichwald ; O. calligramma, PL V. 
f . 9 ; 0. Actoniae, ib. f. 11 ; Strophomena bipartita, Salter j Leptaena sericea, 
PI. V. f. 14 ; Cyclonema crebristria, M'Coy j Lituites cornu-arietis, PI. XL f. 1 ; 
Orthoceras bilineatum, Hall. (See also woodcuts in Chapter IX.) 
In addition to these forms, none of which rise higher in the series than the 
zone under consideration, the following species, which are also of Lower Silurian 
date, range upwards through all the Llandovery rocks into undoubted Wenlock 
or Upper Silurian. These are : Atrypa marginalis, PL IX. f. 2 ; Orthis elegantula, 
f. 19 ; 0. occidentalis, Hall ; Strophomena antiquata, PL XX. f. 18 ; S. pecten, 
Dalm. ; Conularia Sowerbyi (quadrisulcata, Sil. Syst.), PL XXV. f. 10 ; Bellero- 
phon dilatatus, f. 6 ; Orthoceras ibex, PL XXIX. f. 3. (See Plates VIII. to 
XI and woodcuts in Chapters VIII. & IX.) 
