Chap. IV.] VOLCANIC KOCKS WITH LLANDEILO FLAGS. 
79 
the strike of the beds, by the same individuals who sought for lead-ore. g. Hard, 
thick-bedded, porphyritic felstone. h. Flagstone, with Ogygia Buchii, and much iron- 
pyrites indurated by contact with the igneous rock. i. Grrey porphyritic clay-stone. 
j. Black schist, with some hard stone bands, in parts pyritous. h. Slaty porphyritic 
felstone. I. Black shale, with stone bands and concretions of argillaceous limestone. 
m. Thin band of decomposed granular felstone, weathering to a rusty colour, and 
looking like a coarse sandy oolite, n. Black pyritous schists, much contorted near 
the mouth of the gallery. 
Thus, in a thickness of only 350 feet across the beds, this detailed sec- 
tion of the order exhibited in one of the original typical Silurian tracts, 
displays felstones and porphyritic rocks, occasionally both crystalline and 
amygdaloidal, with clay-stone, granular felspar, &c. These materials 
alternate, six or seven times, with finely laminated flagstones and black 
schists, containing Graptolites and Ogygia Buchii (PL III. f. 1). In 
truth, therefore, this little section exhibits an epitome of the structure of 
vast tracts in North Wales. 
The intrusive rocks which have broken through and diversified these 
tracts of Shropshire, Montgomeryshire, and Radnorshire, now under con- 
sideration, whether large masses or mere dykes that traverse the contiguous 
strata, were most of them laid down in my map of the Silurian region, and 
have since been more accurately defined by the Government Surveyors (see 
Sheets 60 & 61). They are chiefly coarse-grained greenstones and fel- 
stones, some of the former passing into basalt. In many cases the shale 
or schist in contact with the eruptive rock has been converted into 
porcellanite, with surfaces smoother than the finest lithographic stone. 
These altered shales are, in fact, the ( Brand-Erde,' or burnt-earth, of the 
Germans * (Sil. Syst. p. 275). 
The Breidden Hills, including the picturesque Moel-y-Golfa, on the right 
bank of the Severn, near Welsh Pool, also exhibit similar illustrations, 
both of cotemporaneously bedded ashes and of eruptive or intrusive igneous 
rocks, which have broken out along the same line at different periods f. 
The annexed drawing, taken from the terrace of Powis Castle, shows 
these hills in the distance to the north-east. They mark a line of erup- 
tion that separates the Lower Silurian rocks, on the left hand, from the 
Upper Silurian of the Long Mountain on the right. Now, it has been de- 
monstrated that along this line of eruption, ranging from S.W. to N.E., 
some volcanic dejections were spread out on the sea-bottom during the 
formation of the Lower Silurian strata, and that other eruptions and dis- 
locations afterwards took place along it, heaving the strata into highly 
inclined positions, like those on which Powis Castle stands. It further 
appears that on the edges of these Silurian and volcanic rocks Carbonife- 
rous deposits were afterwards accumulated, and that again, after all these 
and other formations, including the New Red Sandstone and Lias, were 
* In England, curious actual proofs of this con- shale and sandstones into burnt-earths, or porcel- 
version occur m the South Staffordshire coal- lanites of divers colours, some of them resembling 
field, near the town of Dudley, where the long- the riband-jasper of mineralogists, 
continued combustion of subterranean coal m t See detailed descriptions, Silurian System, 
abandoned mines has converted the associated p. 287 et seq. 
