480 
SILURIA. 
[Chap. XX, 
of the Silurian rocks. Thirty-three years have elapsed since these data 
were first indicated, and, after vigilant researches in various regions of 
both hemispheres, these great features remain the same. The labours, 
indeed, of those who followed me have infinitely more sustained the unity 
of that system ; for its lower and upper divisions are now proved to be 
connected by a marked community of generic types and analogous forms. 
Let me here, therefore, and before the superior formations are alluded 
to, recall attention to the Fourteenth and Fifteenth Chapters (on Scan- 
dinavia, Russia, and Germany), in which it has been shown (on the 
authority of de Yerneuil, von Keyserling, Barrande, Kjerulf, Schmidt, 
and others) that the Lower and Upper Silurian rocks of those countries 
constitute one united mass. On this head I cited in the last edition (as 
giving a general conspectus of such comparison) my own conclusions as 
given in a memoir on this subject, read in 1857 before the Geological So- 
ciety of London. I there showed* (and, indeed, the proofs are to be seen 
in previous Chapters) that, in Scandinavia, Eussia, and Germany, the Lower 
and Upper Silurian rocks constitute a natural system, and that often in 
tracts where the deposits are of very small dimensions compared with those 
of Britain, the abundance of characteristic fossils is remarkable. 
Those of my cotemporaries who may still adhere to the belief that the 
' Primordial Zone,' which Barrande has shown to be the base of Silurian 
life (and all the American authorities take the same view), ought to be 
separated from it and placed in the Cambrian system, must be first referred 
back to the natural sections of Norway, Sweden, Bohemia, and North 
America abroad, as well as to those of North and South Wales at home, to 
prove the intimate transition and passage (mineralogical and zoological) of 
that lower zone into the strata of Llandeilo age. No more remarkable 
exhibition of this geological union can be seen than on the sea-cliffs in 
Whitesand Bay, Pembrokeshire. There the Tremadoc Slates of the ' Pri- 
mordial' Silurian zone are seen to be not only symmetrically parallel to, but 
(as in North Wales) gradually passing up into slates of absolutely similar 
structure in which the Lower Llandeilo fossils appear. The lower and 
upper masses of the cliffs are, in short, so intimately united that any 
lithological division between them .is impracticable. 
Whilst such is the physical union of these rocks, I have it now in my 
power to add the evidence produced by the long- continued researches 
of the veteran geologist Dr. Bigsby, who, in his valuable ' Thesaurus 
Siluricus,' shows that, in taking a general view of the distribution of what 
he has classed as Silurian life, no less than twenty-seven forms are found 
to be common to the * Primordial' and overlying Silurian zones f. 
In a broad classification of primeval life, the late eminent naturalist 
* Quart. Journ. Geol. Soc. vol. xiv. p. 36. Grant Committee of the Royal Society. The out- 
t This work of Dr. Bigsby will be shortly pub- line of the ' Thesaurus ' and its chief objects are 
lished ; and the author is, I am happy to learn, to explained in a memoir read before the Royal So- 
be aided in bringing it out by the Government ciety. (See Proc. Roy. Soc. vol. xv. p. 372.) 
