290 
SILURIA. 
[Chap. XII. 
Having alluded to the convolutions of the Pembrokeshire limestone, it is right 
to explain that the violence of the movements which produced them affected 
still more remarkably all the overlying Carboniferous rocks of that region, and 
particularly the coal. Thus the latter, which is there nearly all in the state of 
culmf or anthracite, has been for the most part shivered into small fragments, 
and is frequently accumulated in little troughs or hollows, the ' slashes' of the 
miners. Of the great lateral pressure and violent fractures to which the strata 
have been subjected, the following woodcut may convey some idea. 
Slash op Culm, in Pembrokeshire. (From Sil. Syst. p. 377.) 
b 
a. Contorted Culm strata, with stone-coal, a*, b. Fault, c. 1 Slash' of finely triturated 
culm between violently contorted strata, and probably upon a great line of fracture. 
All these convolutions and fractures were produced long after the solidification 
of the deposits under review. 
"Whilst the Carboniferous Limestone is separated, as before stated, from the 
Devonian, or Old Red, by shales (which in Pembrokeshire are in parts both 
sandy and calcareous), it is there, as in most other districts of England, sur- 
mounted by light-coloured sandstones of considerable thickness, known under 
the name of Millstone-grit. 
The English student may, in a small compass, examine the Carboniferous series, 
from its base upwards to the productive Coal inclusive, on the south-east slope 
of the Titterstone Clee Hill in Shropshire %. Ascending from a depression in 
the Old Red Sandstone near Cleobury to the eastern summit of that hill, he 
passes successively over the three divisions of Shale and Limestone, Millstone- 
grit, and productive Coal, — the whole being capped by basalt, which is seen to 
have been erupted through the entire series, and to have overflowed on the top 
of the hill. 
Section across the Cornbrook Coal-basin op the Clee Hills. 
(From Sil. Syst. p. 113, pi. 30. f. 6.) 
b * b a 
a. Upper beds of Old Red or Devonian, b. Carboniferous Shale and Limestone, c. 
Millstone-grit. d. Coal-measures. * Erupted basalt, which has risen through and 
overflowed the coal. 
The Lower Carboniferous members, or the shale, limestone, and grit (b,c), are, 
however, of small dimensions in and around the Silurian region, when compared, 
t The variations in mineral character of the an- great mineral changes, within a few miles, of the 
thracitic or culm trough of North Devon (doubt- same rocks in our own isles ! 
less a mere extension of the Pembrokeshire coal- I Any one who is desirous of understanding the 
field) have been partially adverted to in the pre- peculiarities of this coal-field may refer to the 
vious Chapter (see p. 271-4, and"section, p. 272). ' Silurian System,' p. 113, where the proofs of the 
One thin course of black limestone with Posi- existence of a vertical mass of basalt rising up 
donomyae is, in North Devon, the true repre- through the strata are explained, as well as the 
sentative of the massive, white, calcareous cliffs extraction of coal by shafts from beneath the 
of the opposite coast of Pembroke, and of the overlying table of basalt, 
diversified North British series: so much for 
