Ciiap. IY.]                    THE CAEADOC FOEMATIOX.                                     63
 

CHAPTEE IV.
LOWEE SILUEIA^T EOCKS {continued).

THE CARADOC FORMATION.—SHELLY SANDSTONES OF CAER CARADOC.—GENERAL CHARACTER
AND ORDER IN THE TYPICAL SILURIAN TRACT OF SHROPSHIRE.—CHIEF ORGANIC REMAINS
AS DISTINGUISHED FROM THOSE OF THE LLANDEILO FORMATION.—GREAT MASSES OF THE
SLATY ROCKS OF WALES, INCLUDING THE BALA LIMESTONE, SHOWN TO BE THE EQUIVA¬
LENTS OF THE CARADOC OF SHROPSHIRE.--IGNEOUS ROCKS, COTEMPORANEOUS AND ERUP¬
TIVE, OF LOWER SILURIAN AGE.

That certain sandstones and shales with occasional calcareous or shelly
courses overhe the schistose rocks of the Llandeilo formation in Wales, has
already been indicated *. But before we pursue their clear and consecutive
relations, let us view those masses of the same age in Shropshire, which, with
their fossils, were originally described as a formation younger than the
Llandeilo flags, and as underljing all the Upper SUurian rocks. For,
this Caradoc formation and its characteristic fossils having been described
and named many years before its equivalents in AYales were brought into
comparison or their fossils examined, the account of the original type
naturally precedes any description of strata subsequently ascertained to be
of the same age.

In Shropshire, the Caradoc Sandstone, so named from the ridge on the
flanks of which it is weU exposed, is cut off, as formerly shown, from the
next deposits below it, namely the Llandeilo flags, by the intervention of
the Cambrian rocks of the Longmynd (see Map). Whilst a perfectly sym¬
metrical ascending order occurs, as already stated, on the west flank of the
Longmynd, from the Cambrian into the lowest of the Silurian rocks, and
from them into the LlandeUo formation, in vain do we look on the eastern
side of that mountain for any representative of the Stiper Stones and
the great Llandeilo formation of the Shelve and Corndon tract (p. 38).
The steep slopes of the Longmynd which overhang the valley of Church
Stretton exhibit, as already shown (p. 26), the escarpment of the lowest
beds of that enormous mass of ancient sedimentary rocks.    (See Map.)

Immediately to the east of that valley is seen the line of a powerful
fault, the vertical dimensions of which have been estimated (by Professor
Kamsay) at not less than 2000 feet, the place of the intervening strata
being taken by igneous rocks. The latter having been erupted at a period
long after the formation of the original sediments, have altered the schists
into hard clay-slates, and the sandstones into quartz-rock.    These igneous

* See also general section beneath Map, and local sections (pp. 65 & 60).