Chap. IV.] 
IGNEOUS ROCKS— LOWER SILURIAN. 
77 
interlaminated with the Llandeilo Flags. Among the higher members of these 
stratified tuffs, which were manifestly accumulated under former seas, some of 
the finest examples are to be seen in the picturesque defile of Marrington Dingle. 
There the lowest strata, exposed in numerous quarries, are felspar rocks of a con- 
cretionary structure, which are surmounted by coarse mottled volcanic giits. 
The latter consist of a base of lightish and greenish-grey granular felspar, mixed 
with sand and chlorite, and contain angular fragments of schists and porphyritic 
greenstones, the whole being arranged in beds from two to four feet thick, and 
dipping at an angle of about 40°, as expressed in this vignette. 
One of the Whittery Quarries in Marrington Dingle. (From Sil. Syst. p. 270.) 
In many other parts of this district, we meet with felspathic agglomerates 
and ash-beds, or volcanic grits, as well as slaty porphyries with crystals of fel- 
spar. Some of these alternate, as before said, in ridges with the schist contain- 
taining Trilobites j others constitute courses of a few inches thick only, and oc- 
casionally include fragments of Ogygia Buchii. Organic remains are also found 
in beds composed almost exclusively of volcanic materials, thus showing that 
volcanic action was rife at the sea-bottom in which these Lower Silurian strata 
were accumulated. 
Volcanic Breccia or Ash, b, alternating with beds of Schist, a, and enveloping other 
fragments of schist and slate, c. 
(From Sil. Syst. p. 271.) 
Another diagram, taken from the coloured sections on the margin of my 
old map of the Silurian region, explains theoretically the manner in which, 
by the action of submarine volcanos, such igneous dejections are supposed 
to have accumulated. The igneous or volcanic materials are represented 
by the light- coloured layers, which, issuing from beneath a, a, alternate 
