76 
SILUEIA. 
[Chap. IV. 
Igneous rocks associated with the Lower Silurian. — To make the reader 
acquainted with the complete aspect and nature of the British Lower Si- 
lurian rocks, it is essential to explain how volcanic action affected the 
bottom of the sea in certain districts during the deposition of those sedi- 
mentary strata, and how those strata (particularly of the Llandeilo forma- 
tion) were also penetrated by powerful eruptions. 
In the vicinity of some igneous rocks, the schists and calcareous flag- 
stones have also been filled with mineral veins, and have undergone various 
changes*. Thus one of the tracts in the original Silurian region where 
the Llandeilo formation is most productive in lead- ore is the rugged, hilly, 
district of Shropshire which lies around the village of Shelve and the 
Corndon Mountain, and extends westward from the Stiper Stones (see 
Section, p. 26, and Map). 
The student of natural phenomena is again specially invited to examine 
this tract, the ordinary strata of which, and their organic remains, 
have before been described, because it exhibits all the cotemporaneous 
formations in North Wales, in hills which are much more accessible than 
the steep flanks of the slaty, subcrystalline mountains of Snowdon and 
Cader Idris. 
This typical Lower Silurian district was long ago shown by me to con- 
tain two classes of submarine igneous rocks, — the one bedded and regu- 
larly interlaced with the strata, and therefore formed at the same time 
with them, the other unstratified and posterior. Each of these is divided 
into several varieties, for a full acquaintance with which the reader must 
consult the original descriptions f. 
Of the interstratified felspathic ashes ('volcanic grit' of the 'Silurian 
System'), there are very numerous examples between the west flank of the 
Stiper Stones, where they first appear in the ascending order, and the pic- 
turesque defile of Marrington Dingle near Chirbury on the west, or, in 
other words, through a space of four to five miles in breadth, and eight to 
nine miles in length (see Sil. Syst. p. 268). 
At the Bog Mine, on the western slope of the Stiper Stones, and therefore in 
the lowest member of the Llandeilo formation, the mineral veins and adits tra- 
verse a vast number of the thin-bedded felspathic grits or ashes, interstratified 
with the schists and flags charged with Trilobites and Graptolites. Not less 
than seventeen courses of these igneous rocks are marked in the Govern- 
ment Geological Map of this district (Sheet 60) as occurring in the space of 
little more than a mile, between the Bog Mine and the environs of Hyssington ! 
Some of these courses are traceable for four miles on their line of bedding. 
They crop out in parallel rocky ridges, whilst the associated schists and shales 
have been worn into alternating furrows or depressions. The broadest of these 
hard felspathic bands, as seen upon the surface, ranges from near Symmonds 
Castle on the S.S.W. by the Roundtain Hill to Stapeley, beyond which, or be- 
tween Shelve and Rorington, it is divided into five or six of the thinner courses 
* Besides ores of lead and copper, it will be also t See Sil. Syst. pp. 269, 324, and passim ; and 
shown, in a separate chapter, that rocks of this for observations on these and other igneous rocks 
ape were rendered partially auriferous in Wales, by Mr. D. Forbes, see Appendix (B). See also 
and largely so in other countries. p. 49. 
