6o 



PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



where there will be found continuous and conformable succession 

 both physically and palaeontologically. We have no visible chrono- 

 logical scale in Geology but such as is afforded by the relative 

 magnitude of zoological changes ; in other words, the geological 

 duration and importance of any system is in strict proportion to the 

 comparative magnitude and distinctness of its collective fauna. 



It would be out of place here to enter upon the question which 

 has been long at issue as to the claims of two, if not three, schools 

 of thought and research in relation to the divisions or classification 

 of the Lower Palaeozoic rocks. 



" To Professor Sedgwick must be conceded the credit of determining 

 and assigning the limits and sequence of the larger subdivisions ; for 

 single-handed he laid down with masterly precision the succession 

 and true stratigraphical arrangement of the Lower Palaeozoic rocks 

 of Wales, from the Bangor beds to the summit of the Bala group." 



" To Sir R. Murchison must also be awarded the high credit of 

 having been the first to distinctively assign to many of these rocks 

 their sequence, distinctive fossils, and palaeontological value " *. Con- 

 flicting views led to the formation of a third school of research, in 

 which Sir Charles Lyell, Mr. Salter, and Dr. Hicks exercised great 

 influence. These last two observers assigned the term Silurian to 

 all the strata ranging from the top of the Ludlow to the base of the 

 Arenig, and restricted the term Cambrian to all between the base 

 of the Lower Arenig and the lowest known beds of the Harlech and 

 Llanberris group. The Tremadoc group, by its fossils, however, has 

 little in common with the underlying Lingula-flags and Mencvians, 

 only sixteen of the Lingula-flag and other forms, out of one hundred 

 and eightj^-two that range below the Tremadoc passing up to the 

 latter horizon. The great and almost total break at the top of the 

 Tremadoc lends strong evidence in favour of the division beiug recog- 

 nized here. 



The recognition of a tripartite grouping of the faunas and 

 strata, between the base of the Old Eed Sandstone and the 

 Harlech scries, cannot be disputed; each is characteristic, and 

 possesses a broadly-marked aspect or facies — the Primordial Cam- 

 brian, or first fauna, the "Ordovician" system, or second fauna, 

 and the Silurian system, or third fauna, according to their succession 

 in time. Sedgwick named his system after; the entire principality 

 in which his rocks were typically developed, his title of Cambrian 

 being comprehensive enough to embrace the whole of the Palaeozoic 

 rocks. Murchison selected the term Silurian, associating the rocks 

 of his system with the tribe of the Silures. 



Mr. C. Lapworth, in his able paper upon the tripartite division of 

 the Lower Palaeozoic rocks, has suggested, and with good reason, 

 the name " Ordovician " for all that group of strata in the Great 

 Bala district called Upper Cambrian of Sedgwick or Lower Silurian 

 of Murchison — from the ancient tribe of the Ordovices, who occupied 

 the country now called Montgomeryshire, Merionethshire, Caernar- 

 vonshire, Denbighshire, and Flintshire. So long, he remarks, as the 

 * Geol. Mag. decade 2, vol. vi. pp. 1-15, 1879. 



