1848.] RAMSAY AND AVELINE ON THE STRUCTURE OF WALES. 295 
Above these shales the Ludlow rocks of Montgomeryshire appear 
in their usual position, capped by certain outliers of old red sandstone 
long since mapped by Sir Roderick Murchison. From beneath the 
Wenlock shales of Montgomeryshire rise the slates and associated 
contemporaneous igneous rocks of the country north of Bishop’s 
Castle. On a smaller scale these rocks present the same charac- 
teristics as the igneous rocks and slates of Merionethshire, and from 
beneath rise the old purple, green and grey sandstones of the Long- 
mynds, bearing great resemblance in lithological character to the 
sandstone series of Barmouth, and occupying the same position with 
regard to the Bishop’s Castle traps that the Barmouth sandstones do 
to the igneous series of Merioneth and Caernarvonshire. 
A mere trace of Caradoc sandstone is occasionally to be seen be- 
tween the Wenlock shales and the older members of the Bishop’s 
Castle district. The Wenlock shale on the south runs across the 
strike of this boss, resting alike unconformably on both. On the east 
of the Longmynds is a great fault, throwing down the country to the 
west, and running from the new red sandstone of Shropshire in a 
south-west direction into Radnorshire, on the east of Builth. 
On the north-east of Welshpool from beneath the Wenlock shale, 
the black slates and their associated contemporaneous traps again rise 
to the surface. On the north of Builth the same black slates, with 
beds of greenstone and volcanic ashes, again appear. The Wenlock 
shale, without the intervention of the Caradoc sandstone, laps nearly 
completely round this district, resting in the east on the lowest beds, and 
in the west on the highest beds of the igneous series. At Llanwrtyd 
and Baxter’s Bank in Radnorshire ashy traps again appear in small 
bosses from beneath black slates, and at St. David’s in Pembroke- 
shire the same kind of contemporaneous traps, also associated with 
black slates, come to the surface. At the last locality purple slates 
and sandstones rise from beneath, having exactly the same relation to 
the igneous rocks there that the Barmouth sandstones bear to the 
igneous series of Merionethshire, and the Longmynds to the parallel 
rocks north of Bishop’s Castle. 
The fossils of the slates associated with this i igneous series are so 
well known, that it is unnecessary to particularize them. We would 
however invite attention to the fact, that wherever the disturbances 
of the country bring up the deeper parts of the series, beneath the 
fossiliferous slates associated with contemporaneous traps and volcanic 
ashes, there are certain sandstones and slates, generally of a purple 
and greenish hue, and these, in spite of all the search that has been 
made in them, seem to be perfectly unfossiliferous. 
The igneous rocks that occasionally appear in the line of the preat 
Shropshire and Raduorshire fault are of different date and structure 
from those heretofore alluded to. They are always massive (green- 
stones, syenites, &c.); they invariably appear in the line of great 
dislocation, and alter by baking or semi-fusion, whatever strata they 
chance to come in contact with, of whatever age these strata may be. 
We shall now endeavour to show what have been the successive 
disturbances that affected the country, as this throws much light on 
Z 2 
