Chap. IV.] VOLCANIC ROCKS, BEDDED AND ERUPTIVE. 
83 
of porphyry and greenstone, *, which alternate with Lower Silurian slates, 
probably of the Llandeilo age, b, and some accompanying ash-beds ; 
Cader Idris. 
a. Upper part of the Lingula-schists, with porphyry. * Grand masses of porphyry 
and other igneous rocks, with courses of hard slaty Lower Silurian schists, b. c. Lower 
Silurian, assuming its ordinary character of shale when removed from the igneous 
rocks. 
whilst the overlying slates and schists, c, are Lower Silurian rocks, 
which resume their ordinary characters when removed from the igneous 
foci of which Cader Idris was one great centre of action. Thus we see, 
in examining this mountain, that the crystalline and slaty strata on its 
flanks, b, are succeeded by other masses, c, which, in proportion as they 
recede from the igneous rocks, partake more of the character of ' mud- 
stone.' This character prevails, indeed, wherever the strata are unaltered, 
from the base of the Silurian system to the very youngest of its beds. 
The following section (p. 84) illustrates the structure of Snowdon, and 
shows to what an extent the sediments constituting the highest mountain 
in England and Wales have been modified. Independently of the slaty 
cleavage by which large portions, and particularly its western flanks, have 
been impressed, as seen in the fine lines which cut across the beds, under 
the word ' y-Tryfan,' the student will perceive that rocks inferior to any 
which are sketched in the foregoing section of Cader Idris are here repre- 
sentedf. 
The strata, c, which constitute the lower portion of Snowdon, and repose 
upon the older slates and Lingula- flags, 6, consist of dark bluish -grey 
slaty schists, representing the inferior part of the Llandeilo formation. 
In them, however, no clear fossil evidences have been detected. They 
are traversed by masses of eruptive rock ( * ), which, whether consisting 
of porphyry and greenstone or of compact felspar (felstone) have been 
described by Sedgwick, and their relations have been completely defined 
by Ramsay. In following, however, these same beds a few miles, to the 
environs of Ffestiniog, we meet with the fossils of the lower member of 
t Professor Kamsay's observations on these place during the Lower Silurian epoch. He also 
igneous phenomena in North Wales, are elaborated infers that all those igneous rocks were formed 
with great ability and precision, in his recent pub- during the older Silurian period, commencing at 
lication in the Memoirs of the G-eological Survey, the close of the deposition of the Lingula-beds. 
in Chapters xvii. & xviii.,which are specially reeom- He establishes this important inference by point- 
mended, to the attention of geologists. He assigns ing out that the intruded masses have all been 
good grounds for the belief that, although some of subjected to subsequent forces which bent, con- 
the intruding bosses and dykes of porphyry,syenite, torted, and fractured those ancient accumulations, 
and greenstone were posterior to the sedimentary Hence both the interstratified and the intrusive 
felspathic ashes and other volcanic sheets which igneous rocks pertain to the same great Lower 
they traverse, others probably represent the Silurian epoch. (See also Quart. Journ. Geol. Soc. 
deep-seated portions of the volcanic centres vol. ix. p. 170, 1853.) 
whence ejections and flows of felspathic lava took 
G 2 
