Chap. I.] 
SILUEIAN CLASSIFICATION EXTENDED. 
7 
' Cambrian/ which Professor Sedgwick proposed, and which I applied to 
the Longmynd of Shropshire, was in the year 1836, or a year after the 
introduction of the name ' Silurian/ also applied to the rocks of North 
Wales. All these rocks were supposed by myself and other geologists, in- 
cluding Professor Sedgwick, to be of older date than the Silurian, before 
their true relations, physical and zoological, to the then recognized Silurian 
strata of Shropshire had been ascertained. The assumed inferior position 
of the slaty rocks of North "Wales being considered a fixed point, it was 
naturally thought that such formations, the fossils of which were then un de- 
scribed, would be found to contain a set of organic remains differing, as 
a whole, from those of the classified and published Silurian system. With 
others, therefore, I waited for the production of the fossils which might 
typify such supposed older sediments ; for in obtaining the know- 
ledge I had then acquired, by working down from upper strata whose 
contents were known, to lower and previously unknown rocks, I had in- 
variably found that the inferior masses were characterized by distinct 
organisms. This principle, which had been established in the Tertiary 
and Secondary deposits, was thus proved to be universally applicable, by 
the occurrence of similar distinctions in the Carboniferous, Old Red, and 
Silurian rocks. 
It was, however, in vain that we looked to the production of a pe- 
culiar type of life from the North-Welsh slaty rocks. Professor Sedg- 
wick's collections derived from that region had long remained unex- 
amined ; but as soon as he called palaeontologists to their inspection, Silu- 
rian fossils, already named in my works, were alone found in them ; and 
the reason has since become manifest. The labours of many competent 
observers during the succeeding years proved that the great mass of these 
slaty rocks are not inferior in position, as once supposed, to the Lower 
Silurian strata of Shropshire and the adjacent parts of Montgomeryshire, 
but are merely extensive undulations of the same ; and hence the looked- 
for geological and zoological distinctions could never have been realized. 
Sharpe, De la Beche, Ramsay, E. Forbes, Selwyn, Salter, and other ex- 
plorers have demonstrated that the chief fossil-bearing rocks of North 
Wales are, both in order and contents, the absolute equivalents of the 
strata in Shropshire and Montgomeryshire long previously described and 
named by me ' Lower Silurian.' They therefore used the Silurian nomen- 
clature in all their works and maps relating to North Wales*. 
. But although in 1838, when my large work was really completed f, I 
still held, in common with my associates, the erroneous idea of the infra- 
Silurian position of the slaty rocks of North Wales, I soon saw reason to 
* See also Phillips on the Malvern and Abberley a copious Appendix illustrating the Fossils of that 
Hills ; Memoirs Geol. Surv. vol. ii. pt. 1, 1848. The region, by J. W. Salter, F.G.S. 
student who wishes to obtain a complete acquaint- t Although 1839 is on the title-page, extracts 
ance with the rock-structure of North Wales from published copies of the work were quoted 
should consult the 3rd volume of the Memoirs of by authors in 1838 (see LyelFs ' Elements of Geo- 
the Geological Survey of Great Britain, by A. C. logy ' of that year). 
Ramsay, F.R.S., with Map and Sections, and with 
