184 
SILUMA. 
[Chap. VIII. 
of Ross-shire and Sutherland, are on the whole the metamorphosed pro- 
longations of the Lower Silurian rocks of the South of Scotland. 
"We now further know that these crystalline masses of Lower Silurian 
age overlap those red sandstones and conglomerates which, much older 
than the Silurian red conglomerate of Ayrshire (p. 155), are doubtless the 
equivalents of the Longmynd or Cambrian rocks ; whilst the latter repose 
upon ancient Laurentian gneiss, which is nowhere exhibited in England 
and Wales. 
Taken then as a whole, the natural sections of the British Isles sustain 
the view put forth in the earlier pages of this volume, which presented to 
the reader the general order of the primeval rocks*. 
Vertical Dimensions of the Cambrian and Silurian Rocks of the British 
Isles. — We have as yet no means of calculating, even approximately, the 
thickness of the older deposits of Scotland and Ireland which have been 
treated of in this Chapter. In both regions large spaces occur throughout 
which the framework of the subsoil is so obscured and hidden that it has 
not yet been ascertained whether there is a continuous ascending order, 
or if the same strata be repeated by flexures the upper portions of which 
have been cut off. 
In the typical region of Shropshire and the adjacent Welsh counties, as 
described in the preceding Chapters, we can, however, appeal to the esti- 
mates of the Government Surveyors. In Shropshire, the Longmynd rocks 
(the ' Cambrian' of the Survey) are supposed to have the vast thickness of 
26,000 feet, or about three times that of the same strata in North Wales ; 
but even admitting that these rocks are to some extent repetitions (of 
which, however, I see no evidence), their dimensions, according to any . 
estimate, must still be enormous. 
The Lower Silurian strata of Shropshire to the west of the Longmynd, from 
the Stiper Stones upwards, including the Llandeilo formation, exhibit a 
thickness of 14,000 feet ; and if we add to this mass the thickness of the 
Caradoc sandstones, limestones, and shales lying on the eastern slopes 
of Caer Caradoc, which have been described at p. 66, and amount to about 
4000 feet, we ascertain that in the typical tract of Shropshire alone the 
Lower Silurian rocks attain the dimensions of 18,000 feet. These dimen- 
sions are, indeed, nearly equal to those which have been estimated 
for their equivalents in the lofty, slaty, and rugged region between the 
Menai Straits and the Berwyn Mountains, where the Surveyors have 
estimated the total thickness of all the strata of like age at 19,000 feet. 
In the Cambrian or lowest part of these enormous accumulations of 
sedimentary strata, rare traces only of fossils have been found ; but in 
the Lingula-flags (the ' primordial ' of the Silurian zones) organic remains 
increase sensibly, and of late years many new forms have been detected 
* See the generalized diagram at p. 24, in which Laurentian gneiss is represented as the funda- 
mental rod<, though not seen in the Silurian region of England and Wales. 
