32 
SILUEIA. 
[Chap. II. 
also, as in Scandinavia, the lowest or earliest traces of those forms of ani- 
mal life which characterize the Silurian System occur in what that author 
terms the < Etage C or ' Zone Primordiale,' equivalent to the Lingula- 
flags of the British Isles. 
In the next Chapter we shall open out these interesting pages of the 
history of primordial life, in which we read off with precision how in suc- 
ceeding strata, accumulated beneath the primeval seas, different animals 
were added in each zone. 
In concluding the subject of the Cambrian rocks, however, a few words 
must be offered concerning two natural physical phenomena previously 
adverted to, by which the ancient deposits we are now considering have 
passed, in North Wales, into states very different from their condition in 
the Longmynd. 
Slaty Cleavage. — The lowest rocks of North Wales are often, as before 
mentioned, in the condition of true slates. In this respect they present a 
very different aspect to the strata, of the same age, which constitute the 
Longmynd. For, although it was remarked in the ' Silurian System ' 
(p. 258) that the Cambrian rocks of the Longmynd had more of a slaty 
character than any of the overlying deposits of the Silurian region, I 
pointed out that such cleavage was rude, irregular, and indeed apparent 
upon a large scale only. The materials composing the rocks of the Long- 
mynd and those of the north-western part of Carnarvonshire, however, are 
essentially the same. In both, very fine sand and mud, chiefly purplish, 
greyish, or greenish, have been accumulated in countless thin laminae of 
varying colours, with here and there an interstratified and coarser bed. 
Slaty Cleavage and Bedding. (From Sil. Syst. p. 400.) 
The strata in undulation are represented by the curved lines. The fine oblique 
lines represent the slaty cleavage. (The dark tint represents the portions of a moun- 
tain in which the layers of deposit and lines of cleavage coincide.) 
They are further surmounted in both tracts by the lowest Silurian strata, 
containing many of the same fossils, whether in Shropshire, Carnarvonshire, 
or Pembrokeshire. But still the Welsh rocks differ strikingly from their 
Shropshire equivalents. The latter, when struck by the hammer, usually 
break up along their original lines of stratification, like other ordinary 
