176 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



rock with the Llandeilo flags, as has since been done by the Govern- 

 ment Surveyors ; nor does it ever appear to have occurred to Professor 

 Sedgwick that these were beds of about the same age as those which 

 we had seen together on the east flank of the Berwyns, on the banks 

 of the Twrch, where they were recognized by me as "Llandeilo 

 Flags." But having afterwards ascertained that several species of 

 Orthidse which occur in the Bala rock were common both to the 

 Caradoc and Llandeilo formations, I specially stated, four years after- 

 wards, in publishing the * Silurian System' (1838), "As these shells 

 abound also in the Lower Silurian rocks, it would seem that as yet no 

 defined line of zoological division can be dravra between the Lower 

 Silurian and Upper Cambrian groups, and that, as our knowledge 

 extends, we may probably fix the lowest limit of the Silurian System 

 beneath the line of demarcation which has for the present been 

 assumed*." 



In reference to the argument about a " base-line," as employed by 

 Professor Sedgwick, I must state that in the whole * Silurian System' 

 the term "base-line" is never once applied to these rocks. I had 

 quite labour enough within my own region, without being made re- 

 sponsible for the accuracy of the western boundary of my original 

 map of 1830, that separated Siluria from Cambria; but which ten 

 years ago I abrogated, as being wholly inaccurate. That line was, I 

 maintain, a boundary merely between one region whose contents had 

 been worked out and published, and another whose fossil contents 

 were unpublished and therefore unknown. 



The simple question, then, which every practical geologist has long 

 ago answered in the negative, is this. Was the Cambrian system ever 

 so defined, that a competent observer going into an uninvestigated 

 country could determine whether it existed there ? That it was never 

 so characterized is demonstrated by the successive publications of 

 Professor Sedgwick himself, to say nothing of the inferences of every 

 one of our contemporaries who have examined countries at home or 

 abroad in which such rocks occur. For wherever well-known Lower 

 Silurian fossils occurred in such countries, the tracts so typified have 

 necessarily been called Silurian. 



(1836-38.) I have already stated, that during the establishment of 

 the Silurian System and up to the period when it was so named. 

 Professor Sedgwick pubHshed nothing on the subject ; and, although 

 he refers to a Cambridge Syllabus of Lectures drawn up in 1836, and 

 issued early in 1837, wherein he uses the word "Cambrian" (I have 

 referred above also to the communication made at the Dublin Meet- 

 ing of the British Association), it is only in 1838 that we meet with 

 the first published abstract of a real memoir, his " Synopsis of the 

 English Series of Stratified Rocks inferior to the Old Red Sand- 

 stone "f ; followed in 1841 by the " Supplement to a Synopsis of the 

 English Series of Stratified Rocks inferior to the Old Red Sand- 

 stone;};." For the indistinctness of the author's views on those oc- 

 casions respecting the fossil contents of his Cambrian rocks (two or 



* ' Silurian System,' p. 308. f Proceed. Geol. Soc. vol, ii. p. 675 et seq. 



X Proceed. Geol. Soc. vol. iii. p. 541 et seq. 



