Chap. V.] TAR ANN ON SHALE AND DENBIGH GRIT. 
103 
or pale slates (b), which in Wales occupy an intermediate space between the 
Llandovery rocks and the unquestionable Upper Silurian, — the overlying 
grits and shales, c, like those of the previous section, forming the base of 
the Upper Silurian. Again, in the third section, we have before us the 
Radnorshire, S. of Llanbister. (Horiz. Sect. Geol. Survey, No. 27.) 
N.W. S.E. 
be d c b 
b. Tarannon shales (pale slates), c. Denbigh grits, &c. d. Wenlock shale in its 
ordinary characters. 
true Wenlock shale, d, above all the masses exhibited in the previous 
sections. 
The Tarannon shales, occasionally of hard, slaty character, and of 
various colours (in some places so pale a grey as to have been termed 
' pale slates,' in others of purple colour), have been shown by Mr. Jukes 
and Mr. Aveline to form a geological band of great persistence, which, 
beginning, according to the last-mentioned surveyor, in small dimensions 
near Llandovery, expands in its course through Radnor and Montgomery, 
and in North Wales becomes an important subdivision. It is largely and 
clearly exhibited near New Bridge, and at Tarannon, between Llanbryn- 
maer and Llanidloes in Montgomeryshire. Fossils are rare, and those 
which occur do not absolutely determine whether the beds should be 
classed with the Upper Llandovery rocks we have been considering or 
with the Wenlock formation. In a physical and geological sense, Prof. 
Ramsay and Mr. Aveline would class them with the latter. Mr. Salter 
would rather connect them with the Upper Llandovery rocks. For my 
own part, I have only to say that they occupy the same intermediate 
place in the Silurian system, and connect the Lower with the Upper Si- 
lurian rocks. 
However this may be, the next band, c, which, from its mineral cha- 
racter, was once termed 1 Caradoc sandstone,' must clearly, through 
its fossils, be classed with the overlying shale, d, of the Wenlock forma- 
tion. The sandstone, e, is, in short, known to be a prolongation of the 
' Denbigh grits and flags,' which, from their being unconformable to the 
older slates and rocks, and containing Wenlock fossils, Professor Sedgwick 
properly considered to be the base of the Upper Silurian of North Wales*. 
Among the most abundant of these fossils there is, however, one form, 
Bhynchonella decemplicata, which is also characteristic of the Llandovery 
rocks. But with it are many common Upper Silurian fossils, such as 
Phacops Downingise, P. caudatus, Spirifer trapezoidalis, Cucullella ovata, 
* Quart. Journ. Geol. Soe. vol. i. pp. 19, 21. 
