80 
SILUEIA. 
[Chap. IV. 
completed, still more recent eruptions occurred along this same line of 
fissure*, as indicated by igneous dykes which traverse those Secondary 
formations to the north of Shrewsbury. 
View op the Breidden Hills near Welsh Pool, prom Powis Castle. 
(From a Drawing by Lady Murchison, Sil. Syst. p. 302.) 
The shortest method of explaining to the reader the structure of the 
Breidden Hills is to exhibit this small diagram. It represents a mass of 
Section across the Breidden Hills. 
(Taken from a Section by Mr. Aveline of the Geological Survey.) 
N.W. S.E. 
Eodney's 
Pillar. 
b * b * b b* b d 
b. Lower Silurian slaty rocks, b*. Cotemporaneous volcanic breccia, d. Wenlock 
shale, or base of the Upper Silurian. # Eruptive rocks. 
porphyritic and amygdaloidal greenstone, which in its protrusion has 
carried up included portions of slaty rocks, and has thrown off pebble- 
beds and Upper Silurian rocks to the S.E., and Lower Silurian to the 
N.W. 
One of the igneous rocks most easily examined, on this line of habitual 
outburst in ancient periods, is the large basaltiform mass at Welsh Pool. 
* For the proofs of this see Sil. Syst. p. 299 et seq. 
