42 
SILURIA. 
[Chap. III. 
slates which contained them Silurian *. The classification followed in this 
work, which groups the Lingula- (and Agnostus-) flags of North Wales 
with the Lower Silurian rocks, was first adopted by the late Sir Henry 
De la Beche and his associates of the Geological Survey. M. Barrande has 
identified this band with the 'Zone Primordiale ' of his Silurian basin, 
whilst this view has been adopted by M. de Yerneuil and geological authors 
generally on the continents of Europe and America. 
The preceding sectional diagram, drawn by Professor Ramsay, gives the 
general ascending succession both to Moel Hebog on the flanks of Snowdon 
on the north-west, and to Cader Idris on the south-east. It shows how 
on each side of an axis or dome of the Cambrian rock of Harlech, or the 
equivalent of the upper portion only of the Longmynd, there is a zone of 
Lingula-flags, and how each of these is in its turn overlain by other Lower 
Silurian deposits with inteiiaminated igneous rocks. The order in North 
Wales is therefore similar to that in Shropshire, as exhibited in a prece- 
ding woodcut, p. 38. The conformable passage upwards from the basement 
rock into the Lingula-flags, which is well seen in several other localities, is 
also here represented as observed at Barmouth. 
a. Grits and schists ; Cambrian (Upper Longmynd) rocks. 
b. Lingula-schists, with imperfect transverse slaty cleavage (white lines). 
The Lingula-flags in North Wales are, for the most part, light-grey, 
glossy, arenaceous schists, associated above and below with black and rusty 
slates f. 
The chief fossil, a flat bivalve shell, the Lingula J Davisii, as figured in 
the following woodcut (Foss. 5, fig. 1), has a covering which is very horny, 
and only slightly calcareous, showing that its inhabitant was suited to 
the conditions of a sea-bottom composed of mud and sand and con- 
taining but little lime wherewith to supply the fabric of the thicker 
shell of other mollusca. It is, indeed, a remarkable fact, that in the 
vast thicknesses of the inferior or unfossiliferous greywacke before ad- 
verted to (p. 28), there is little or no lime, and scarcely the trace of a 
shell; whilst in these the lowest strata in which calcareous matter is 
* Quart. Journ. G-eol. Soc. Lond. vol. ii. p. 70. fossils collected in Pembrokeshire by Mr. Hicks, 
t For the results of the recent researches by and named by Mr. Salter, are now placed in the 
Messrs. Hicks and Salter on the west coast of British Museum and the Museum of the School of 
Pembrokeshire, south of St. Davids, see their Mines. They include some new genera and many 
memoirs "On the Fossils of the Lingula-flags" new species, the names of which will be interpo- 
in the Journal of the Geological Society, vol. xx. lated by Mr. Etheridge in the General Table of 
p. 233, and vol. xxi. p. 476. Mr. Salter divides the Organic Eemains. 
Lingula-flags, palaeontologically, into Upper, \ Some palaeontologists have separated this 
Middle, and Lower, the last being his ' Msenevian shell, under the name of ' Lingulella,' with some 
formation,' while the other two are referred by him doubt, from the common Lingulae. See Davidson's 
to the 'Festiniog group ' of Sedgwick (loe. cit., and 'Monograph of British Silurian Brachiopods,' 
Brit, Assoc. Report for 1865, p. 281 &c). The 1866, p. 55. 
>ea. 
