568 LLANDEILO FLAGS. [Ch. XXVII. 



sixty species of fossils (of which, specimens had been examined either 

 by Mr. Salter or Prof. McCoy), all common to the Upper and Lower 

 Silurian strata, or, in other words, which, being found in the Caradoc, 

 are also met with in the "Wenlock formation. The range upwards of 

 so many species from the inferior to the superior group shows that, 

 independently of the link supplied by the Llandovery or Middle 

 Silurian, there is such a connection between the two principal divisions 

 (Upper and Lower Silurian) as makes it natural to assign the whole 

 to one great system. To attempt, therefore, to give a new name to 

 the Llandeilo beds, or to call them Cambrian or Camhro- Silurian, as 

 has been proposed, would be to act in violation of the ordinary rules 

 of classification, and would create much confusion by disturbing a 

 nomenclature long received and originally established, by Sir R. I. 

 Murchison, on well-deiined palseontological and stratigraphical data. 



As to the second question, whether a line should not be drawn 

 between the Llandeilo flags and the subjacent Stiper-stones or Arenig 

 group, more may be said in its favor, since while so many species pass 

 from Lower to Upper Silurian, there are none, according to Mr. 

 Salter, which pass down from the Llandeilo flags or Upper Llandeilo, 

 into the Arenig or Lower Llandeilo beds. But, although the species 

 are distinct, the genera are the same as those which characterize the 

 Silurian rocks above, aud none of the primordial or Cambrian forms, 

 presently to be mentioned, are intermixed. This Arenig group may 

 therefore be conveniently regarded as the base of the great Silurian 

 system, which, by the thickness of its strata and the changes in animal 

 life of which it contains the record, is more than equal in value to the 

 Devonian, or Carboniferous, or other principal divisions, whether of 

 primary or secondary date. 



It would be unsafe to rely on the mere thickness of the strata, con- 

 sidered apart from the great fluctuations in organic life which took 

 place between the era of the Llandeilo and that of the Ludlow forma- 

 tion, especially as the enormous pile of Silurian rocks observed in 

 Great Britain, and, especially in Wales, is derived in great part from 

 igneous action, and is not confined to the ordinary deposition of sedi- 

 ment from rivers or the waste of cliffs. 



In volcanic archipelagoes, such as the Canaries, we see the most 

 active of all known causes, aqueous and igneous, simultaneously at 

 work to produce great results in a comparatively moderate lapse of 

 time. The outpouring of repeated streams of lava — the showering 

 down upon land and sea of volcanic ashes — the sweeping seaward of 

 loose sand and cinders, or of rocks ground down to pebbles and sand, 

 by rivers and torrents descending steeply inclined channels — the 

 undermining and eating away of long lines of sea-cliff* exposed to the 

 swell of a deep and open ocean — these operations combine to pro- 

 duce a considerable volume of superimposed matter, without there 

 being time for any extensive change of species. Nevertheless, there 

 would seem to be a limit to the thickness of stony masses' formed 



