204 
THE SILURIAN VOLCANOES 
BOOK IV 
The volcanic rocks of Builth were first described in the “Silurian 
System.” Murchison clearly recognized that they included some which 
were “ evolved from volcanic apertures during the submarine accumulation 
of the Lower Silurian rocks,” and also “ imbedded volcanic masses which had 
been intruded subsequently, dismembering and altering aU the strata with 
which they came in contact.” ^ These igneous rocks ivere mapped in some 
detail by the Geological Survey, and their general relations were expressed 
in lines of horizontal section.^ They were likewise described by Eamsay in 
the Catalogue of the lloch-specimens in the Jermyn Street Museum, specimens 
of them being displayed in that collection.*' The tuffs and lavas were 
distinguished, and likewise the intrusive “greenstones.” But no attempt 
wms made towards petrographical detail. 
This interesting district has recently been studied by Mr. Henry Woods,^ 
who has grouped the igneous rocks in probable order of appearance as 
follows 1st, Andesites ; 2nd, Andesitic ash ; 3rd, Ehy elites ; 4th,Diabase- 
porphyrite ; and 5 th, Diabase. 
Some of the andesites are described as intrusive in the Llandeilo strata. 
Jhe ash in its lower part contains numerous W'ell-rounded pebbles of 
andesite, usually five or six inches in diameter, but sometimes having a 
length of two feet. It contains fossils (Orthis calligramma, Leptmna sericea, 
SerpulUes d%spar, etc.), and as it is overlain with shales containing Ogygia 
Buchii, it may be regarded as probably of Lower Llandeilo age. The 
rhyolites are feebly represented, and some of them may possibly be intrusive. 
Among them a nodular variety has been noticed, the nodules being solid 
tliroughout, varying up to two inches in diameter, and formed of micro- 
crystalline quartz and felspar, with no trace of any radial or concentric 
internal arrangement. The diabase-porphyrite, the most conspicuous rock 
of tlie district, is intrusive in the andesites and ashes, and occurs in four 
separate masses or sills. The diabases are all intrusive and of later date 
than any of the other igneous rocks, and as they traverse also the Llandeilo 
shales, they are probably considerably later than the previous eruptions. 
But as they do not enter the surrounding Llandovery and Wenloek strata 
they are regarded by Mr. Woods as of intermediate age between the time 
of the Llandeilo and that of the Upper Silurian formations. 
About nine miles in a west-south-westerly direction from the southern 
extremity of the Builth volcanic area, another mucli smaller exposure of 
Igneous rocks has been mapped by the Geological Survey at the village of 
Llanwrtyd. This tract is only about three miles long and half a *^niile 
broad. The volcanic rocks are represented as consisting of three or more 
bands of " felspathic trap ” interstratified in the Lower Silurian strata, and 
folded into an anticline along the ridge of Caer Cwm. No published line of 
» Silurian System, 1889, p. 330. The oceun-eiice of “ trappean ash ” with fossils in the Builth 
district was noticed by De la Bechc, Mem. Geol. Sun. vol. i. (1840), p. 31. 
See Sheet 56 of the oue-inch map and Sheets 5 and 6 of the Horizontal Sections. 
^ Catalogue of Rock Specimens, 3rd edit. 1862, p. 36 ct scq. 
* Quart. Joum. Geol. Soc. vol. 1. (1894), p. 566. 
