Silurian liocks of North Wales, ^c. 149 



tem,'* thereby to distinguish a series of strata several thousand feet 

 thick, and having a general community of fossil characters within the 

 ancient British kingdom of the Silures. These I divided into two 

 groups, the Upper consisting of the Ludlow and Wenlock, the Lower 

 of the Caradoc and Lhindeilo formations ; the chief or typical fossils 

 of each, being identified or named by Mr James Sowerby and my- 

 self, were then enumerated in tables.* 



At that time the word *' Cambrian" was unknown, and it origi- 

 nated in the following manner .f In a published diagram of suc- 

 cession, the Upper and Lower Silurian rocks were represented as 

 reposing on unconformable strata which were termed ** slaty grey- 

 wacke," which was meant to represent certain inferior unfossiliferous 

 rocks (like those of the Longmynd range of Shropshire). Now, 

 on sending a copy of my new classification to M. Elie de Beaumont, 

 that eminent geologist, wishing to mark strata separated by lines of 

 dislocation by separate names, suggested the propriety of further 

 distinguishing those last-mentioned unconformable and inferior rocks 

 by the term " Hercynian," as taken from the Hartz mountains in 

 Germany, where, as he then believed, the oldest slaty group would 

 prove to be of higher antiquity than the strata to which I had ap- 

 plied the word " Silurian." Unwilling that the name for these 

 infra-Silurian rocks should be taken from a foreign country, in which 

 no precise palaeozoic horizon had then been fixed, I at once urged 

 Professor Sedgwick to apply to his slaty rocks, which were confidently 

 believed to be inferior to my own, some term, on the same geogra- 

 phical principle by which 1 had been governed in proposing *' Silu- 

 rian." I even ventured to suggest the word " Snowdonian," because 

 I knew that my friend then considered the NW. portion of the 

 Welsh chain to be made up of the oldest fossiliferous masses ; but 

 preferring a more comprehensive geographical name, he took that of 



* Even at that early period I guarded against the possible inference that the 

 mineral masses so defined were to be viewed as persistent. I shewed, for ex- 

 ample, that in some tracts tlie lower division exhibited great thickness of sand- 

 stone beneath flags and limestone ; in others vice versa. The same was ex- 

 plained at greater length in the work subsequently published (1839), wherein 

 the evanescence of mineral types being indicated, it was maintained that the 

 fossils alone could truly indicate the age of the strata. This fact is now men- 

 tioned, because, notwithstanding all my caution, it has recently been stated, 

 that flagstone beds, with Asaphus Buchii occasionally lie above sandstone to 

 which the word " Caradoc" may be applied. My classification, I repeat, was 

 never based on such mineral succession ; for whether the lower rocks were flag- 

 like and calcareous, sandy or quartzose, schistose or slaty; whether the upper- 

 most bed was sandstone and the lowest schist, or the reverse ; their age was 

 alone to be determined by their imbedded fossils. And as to the mineral base, I 

 will cite my own words : — " Although the calcareous flags of Llandeilo, with 

 their accompanying schists are considered to form the base of the Silurian sys- 

 tem, their place is sometimes taken, often, indeed, they are underlaid, by sand- 

 stones of considerable thickness." — (Sil. System, p. 8.) 

 t See also Sil. Syst., p. 8. 



