78 
SILUKIA. 
[Chap. IV. 
with the ordinary dark- coloured muddy sediment. The diagram further 
shows how a volcanic cone similar to that of Graham Island (He Julia of 
a a 
Ideal Representation of the manner in which Submarine Volcanic Dejections 
were probably formed during the early sllurian period. 
(From bottom of Map, Sil. Syst.) 
the French) in the Mediterranean, and indicated by the dotted lines, may, 
after rising into the atmosphere, have disappeared ; its scoriaB and ashes 
having mixed with the ordinary marine mud or sand. 
One of the larger masses of eruptive rock, which penetrated the sedi- 
mentary and associated volcanic deposits at a subsequent period, is seen in 
the Corndon Mountain, as expressed in the sketch at p. 49, the undulating 
ground around it being composed of strata of the Llandeilo formation and 
beds of felspathic ashes. 
Equally instructive examples of the alternations of bedded igneous matter 
with Lower Silurian strata, and also of posterior eruptions, are seen in the 
rocky tract of Eadnorshire, extending from Llandegley and Llandrindod by 
the hills of Gelli, Gilwern, and Carneddau, to Builth. This example of 
felspar-porphyries, which are there regularly stratified, is taken from one 
of my earliest illustrations. 
Alternations of Llandeilo Flags and Schists with Volcanic Grits, exposed in 
A RAVINE ON THE NORTH-WEST FACE OF GeLLI HlLL. 
(From Sil. Syst. p. 325.) 
a 
a. Coarse slaty felspar rock, both porphyritic and amygdaloidal, containing elongated 
concretions of greenearth. This rock is regularly stratified in beds three to four 
feet thick, and forms the mass of the hill, rising into the higher ground, b. Finely 
laminated greenish-grey sandy flagstone, apparently hardened near the top. c. Fine- 
grained granular felstone and courses of clay-stone, some of which are used as oven- 
stone, d. Altered flags, having a conchoidal fracture, in parts almost lydian-stone, 
with crystals of iron-pyrites, e. Grey felspar rock, the laminae of deposit marked 
by ferruginous streaks, probably due to the decomposition of some other mineral. 
/. Black shivery shale, containing a few concretions of argillaceous limestone, with 
veins of calcareous spar. One of these, which fell under my notice, was a septarium, 
two or three feet in diameter, containing many impressions of Graptolites. This 
band of black shale was foolishly excavated to some distance in search of coal (!) on 
