r 



Silurian Uocks of North IKales, ^'c. 16^ 



None of my opinions successively put forth from the year 1843 to 

 the present time were objected to, and I naturally therefore believed, 

 judging indeed also from Professor Sedgwick's new map of North 

 Wales,* that the question of palasozoic nomenclature was settled. I 

 firmly thought, that however thick, however diversified in mineral 

 characters all Welsh and British, as well as all foreign strata, in 

 which certain typical forms prevailed, would be included in the Lower 

 Sihu'ian group. From what, I inferred, could it import in a nomen- 

 clature founded on the principle of " strata identified by their fossils," 

 whether the rocks termed Lower Silurian were two or three thou- 

 sand feet thick in the region where they were first described, or 

 many times as thick in the western parts of North Wales, provided 

 they were in both tracts characterised by the same groups of fossils % 

 I knew, for example, that in the very thin but undisturbed bands of 

 llussia, a greater number of species of animals had been found than 

 had then been detected on the same parallel in the greatly expanded 

 North Welsh rocks. It therefore seemed to me to matter little to 

 the geologist occupied with the history of successive races and their 

 apparition on and disappearance from the surface, that he should 

 be told of a tract, in which there was a much grander and more 

 diversified mineral character (with enormous porphyritic masses) 

 than in the region wherein he had previously described similar zoolo- 

 gical types. 



The names of William Smith remained unchanged, although his 

 original mineral types were in aftertimes found to be often of very 

 small persistence. It was not proposed to change the name of Lias, 

 when that formation was found to be three times as thick at Whitby 

 (with numberless new inferior strata and many new fossils), as in 

 the South of England, where it was first named. Modern geology 

 will stand on an insecure basis, if the principle of identifying strata 

 by their fossils be abandoned. But it is now proposed to abrogate 

 the established name of the type, and substitute for it that of a tract, 

 the fossils of which have never been published, long after the Lower 

 Silurian rocks were first named and distinguished by peculiar forms 

 — when the fossils of that age have been so labelled in numerous 

 nmseums on both sides of the Atlantic, and when geologists and palse- 

 ontologists of all countries have adopted the term, and already know 

 (through the previous admissions indeed of Professor Sedgwick him- 

 self), that the greatest portion of the Cambrian is zoologically no- 

 thing more than a downward extension of the Lower Silurian group. 



Let me then ask my brother-labourers to adhere to the use of a 

 name which has been so long current, and which for some years has 

 had a definite meaning attached to it, both by its fossils and position 

 in various parts of the world. Geologists have already honoured me 

 with their approbation, for having worked out certain phenomena, 



* See Geological Journal, vol. i., p. 1, and map. 



