Chap. IX] 
KELATIONS OF CAMBRIAN ROCKS. 
31 
dinate courses of grit, with rocks of igneous origin intermixed, they 
are seen to fold over and plunge to the E.S.E., so as to pass under the 
great and massive succession of schists constituting the distant heights 
of the Snowdon range. The reader will observe that faint slanting lines 
have been introduced into the drawing to represent this ascending order, 
which the geologist can observe for himself, as he examines the sides of the 
valley in passing from the unfossiliferous slaty rocks of Llanberis, up to the 
overlying strata (2 a , 2 b , 2 C , of the section below the Map), which, by their im- 
bedded organic remains, are known to be of Lower Silurian age. The 
rocks in the foreground stand in the place of the Longmynd, whilst the 
overlying or distant masses are the equivalents of the Stiper Stones, Shelve, 
and Corndon of the previous section, and even of the Caradoc Sandstone of 
Shropshire. 
A like succession, and a clearer one as regards the exposition of the 
fossils of the next overlying zone, is seen near Tremadoc on the north, and 
Barmouth on the south side of the hard Cambrian rocks of Harlech, which 
represent the upper portion of the Longmynd series. There they also pass 
under the lowest member of the Silurian rocks — the Lingula-flags — as 
explained in a diagram prepared for me by Professor Ramsay, for which 
see the next Chapter. 
The generalized coloured section at the bottom of the annexed geological 
map, also prepared by Professor Eamsay, who, with his associates, Jukes, 
Aveline, and Selwyn, established the important identification of the slaty 
rocks of North Wales with their unaltered equivalents in the Silurian Re- 
gion, will at once render the subject intelligible to the reader (see Preface). 
In the two fundamental masses, therefore, of the Longmynd, on the east, 
and of Llanberis and Harlech, on the west, are represented the oldest 
sedimentary rocks of England and Wales *. 
Rocks similar in character to those of the Longmynd, Llanberis, and Har- 
lech, and rising out, like them, from beneath all the Lower Silurian rocks, 
occur also at and near St. David's in South Wales. There again, as in North 
Wales, no fossils have been detected in them; but they are conformably over- 
lain, according to observations made by Mr. Aveline and Mr. Salter, as in 
Shropshire and Merioneth, by strata in which the earliest conspicuous groups 
of animals have been detected. These British examples are well supported 
by an analogous succession in Bohemia, where Barrande has indicated the 
presence of an enormous mass of clay-slate apparently devoid of fossils f, and 
forming the natural base of his ' Silurian basin of Bohemia.' In that region 
Sedgwick, Quarterly Journal of the Geological tion of the strata. In both, copper and lead ores 
Society of London, vol. iii. p. 138. For a correct occur, and in parts of Merionethshire gold also 
acquaintance with all the faults and fractures of appears. The auriferous conditions of the Lower 
these rocks, the geologist will, of course, consult Silurian rocks of North Wales will be subsequently 
the subsequently published maps and sections of explained. 
the Government Surveyors. t No fossil, except Eozoon Bavaricum, Giimbel, 
* It is also worthy of remark, that in the long and some Annelide markings or Fucoids, has been 
transverse section below the Map, extending in a discovered in the vast thickness of clay-slate of this 
right line over 90 miles of country, the lowest region ; whilst Eozoon Canadense is quoted from 
rocks, near each extremity (in Carnarvonshire as the underlying crystalline limestone subordinate 
in Shropshire), are accompanied by mineraliza- to the Laurentian gneiss. See Chapter XV. 
