82 
SILURIA. 
[Chap. IV. 
Llandrindod, Builth, Llanwrtyd, &c. that gush out around or near this 
volcanized tract. 
In North "Wales, some of the finest examples of the stratified and co- 
temporaneous beds of felspathic ash or volcanic grit are exhibited in the 
environs of Ffestiniog. There the interval between the green and grey 
schists, of the age of the Lingula-flags of Tremadoc, and certain overlying 
black roofing-slates is occupied by syenite, satiny schists, and volcanic 
grits, the last being partly coarse conglomerates. To the west of Tan-y- 
bwlch, these beds assume the form of ashes, which are there as finely 
levigated as the materials of many schists. They are associated and alter- 
nate with hard grit, penetrated by large branching veins of white quartz. 
Occasionally this felspathic ash is separated into thick beds formed of 
slightly inclined laminae of deposit, and is traversed by numerous highly 
inclined lines of slaty cleavage, from which the associated coarse grits are 
exempt. 
An adequate acquaintance with such features can be attained only by 
traversing those mountains with the remarkable illustrations of the Geo- 
logical Survey in handf . In the area of a few square miles, the geologist 
there meets with numerous grand alternations of interstratified volcanic 
tuffs and conglomerates, mostly felspathic, but sometimes sandy and cal- 
careous, which have been pierced by eruptive igneous matter, whether 
syenite, felstone, or greenstone, here and there porphyriticj. It is, in- 
deed, sometimes most difficult to separate the cotemporaneous igneous rocks 
(' porphyries ' of Sedgwick) from the subsequently intruded masses. 
Then, again, let the student have with him the minutely worked maps 
of these districts, and, observing the faults or fractures as delineated on 
them by the great number of white lines, he may form some idea of the 
consummate skill and labour by which such results have been obtained. 
This brief sketch of the igneous operations which were the accompani- 
ments of the Lower Silurian sediments cannot, indeed, be better termi- 
nated than by exhibiting two woodcuts, reduced from the large Sections 
of the Survey, which record the labours of Ramsay and Selwyn in North 
Wales. The first of these diagrams (p. 83) represents the relations of the 
different rocks composing the mountain of Cader Idris, near Dolgelly. 
The lowest strata here indicated, a, are slaty schists, containing a 
few Lingulse ; they have been traversed by porphyry, *, and towards their 
summit contain some layers of felspathic ashes. Then follow grand masses 
t The repeated alternations of these volcanic trated in the new work of the Survey, by Eamsay, 
rocks with the schists and slates of North Wales 'The G-eology of North Wales.' 
can be best understood by reference to the J A fine example of this succession to the north 
maps and sections prepared by Eamsay and Sel- of Ffestiniog was pointed out to M. de Verneuil 
wyn. See Sheets 75 and 78 of the Geological Sur- and myself by Professor Eamsay in 1851, the real 
vey Map, and the illustrative Sections of the same merits of whose labours have recently been made 
on the six-inch scale. In the small Map attached known in an admirable Memoir of the Geological 
to this work, and which, as before explained, is Survey. It is, indeed, a perfect model of what a 
simply a reduction of that of the Government work ought to be which professes to describe the 
Surveyors, numberless details are necessarily true structure of a complicated region. The fossil 
omitted. These have now been thoroughly illus- remains are well described by Mr. Salter. 
